SYNTHETIC STRATEGIES IN CHEMISTRY

NATIONAL CENTRE FOR CATALYSIS RESEARCH INDIAN INSTITUTE OF TECHNOLOGY, MADRAS April 2008

PREFACE

Synthesis has a central role in the development of Science especially in Chemistry. The strategies adopted in synthesis of molecules and materials have undergone considerable changes in recent times and it was felt that it may be necessary to assess and assimilate information on this topic in a single place. Though, no originality is claimed to the contents on this topic, the very fact that the available information on this topic is available in one place itself is considered to be a major step forward. To the best of our knowledge, such an attempt has not yet been made or available in literature, though various comprehensive assessment of various methods have been made and reported in literature.

The contents of this ebook have been generated from the lectures delivered to the summer programme for the Children’s Club during April 14 to April 30, 2008 at the National Centre for Catalysis Research (NCCR). The members of the research community of NCCR are responsible for the compilation of information on each of the topics presented in this ebook.

We have felt the need for a comprehensive compilation on this topic for various reasons and we do hope the scientific community will also feel the same way. It is possible that coverage may not be comprehensive. However, it is our first attempt to tread into an unknown territory in Science and make a compilation. In this sense this can be considered as our humble first attempt. The chapters have been contributed by individuals and hence the language and style may not be uniform though we tried very little to make it uniform.

We are grateful to Mr. Narayanaswamy, Secretary, Children’s Club, Mylapore, Chennai 600 004 for his continuous faith in us and also for providing us opportunity year after year to conduct such programmes. The members of NCCR consider this as a privilege and wish to record their grateful thanks to him. They also acknowledge the support provided by the administration of IIT, Madras to conduct such programmes.

We do hope this ebook will receive the attention it deserves. We shall be grateful for any useful feedback on this exercise, so that we will be able to implement them in our future endeavours.

Chennai 600 036 April 18, 2008 B.Viswanathan

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Content S. No. Title of the Chapter Page No.

1 IntroductiontoSyntheticStrategiesinChemistry 1.11.7

2 SyntheticMethodsbasedonActivatingtheReactant 2.12.24

3 MethodsbasedonActivatingtheReactingSubstance 3.13.28

4 SynthesisofMaterialsbasedonSolubilityPrinciple 4.14.21 5 SolgelTechniques 5.15.18 6 TemplatebasedSynthesis 6.16.18 7 MicroemulsionTechniques 7.17.15 8 SynthesisbySolidStateDecomposition 8.18.11 9 NewerSyntheticStrategiesforNanomaterials 9.19.23 10 TheRoleofSynthesisinMaterialsTechnology 10.110.34 11 ElectrochemicalSynthesis 11.111.10 12 NewerReactionsandProcedures:CatalyticandNoncatalytic 12.112.24 13 SynthesisStrategies–fromLaboratorytoIndustry 13.113.8 14 SynthesisofChemicalsfromCO 2 14.114.14 15 CarbohydratestoChemicals 15.115.21 16 SomeConceptualDevelopmentsinSynthesisinChemistry 16.116.52

17 ComputationalBasicsUnderlyingtheSyntheticStrategies 17.117.23

ii

Chapter - 1 INTRODUCTION TO SYNTHETIC STRATEGIES IN CHEMISTRY Prof.B.Viswanathan INTRODUCTION Oneofthingsthatmakechemistryuniqueamongthesciencesisthesynthesis.Chemists make things, new pharmaceuticals, food additives, materials, agricultural chemicals, coatings,adhesives,andallsortsofusefulnewmolecules. The chemists prepare them fromsimplermorereadilyavailablestartingmaterials.Therearetwoaspectstoorganic synthesisfirstthedevelopmentofasyntheticstrategyorplanofactionandthesecondthe actualimplementationofthatplaninachemicallaboratory. Synthesis of molecules and materials are important aspect of the development of science.Inthepast,synthesisisbasedonsomeoftheNamereactionsinorganicand Inorganic chemistry. The attempts to synthesis new molecules and materials have alwaysdependedontheexperienceandextrapolationoftheexistingknowledgetonew situations and new synthesis of molecules. However in the last two decades, the synthesis has become a well established science going beyond the recollection and adoption of the existing procedures. The normal procedures so far adopted for the synthesisofmaterialsare (i) precipitationbasedonsolubilityprinciple(basedonthermodynamic quantities) (ii) Bondformationandbondbreakingusuallybroughtaboutbyreagents, participatingspeciesandleavingentities(theseareoftenkinetically controlled);processesdependingonthestrengthofbindingalreadyexisting orbeingformed. (iii) simplesolidstatereactionsoftencontrolledbydiffusion (iv) ligatingorbindingspecies These procedures have been satisfying the normal curiosity of the chemists for new molecules. However today, Chemistry is the study of molecules, materials of functionalities and hence they have to be synthesized, built, and architectured and designed

1.2 IntroductiontoSyntheticStrategiesinChemistry

Today the molecules and materials are for many applications.. Every sector of life requires molecules/materials with functionalities, stability, as well as durability under adverseconditions.Moleculesofhighlyactivefunctionalitieswithstability,durability and appropriate stress strain relationship (global properties of dissolution, digestion, attachmenttospeciesandothers)arethedemandsoftoday.Mostoftenthematerials requiredhavetobeinthemetastablestatebutshouldbestableenoughtobeusefulin devices. The exploitable properties must be able to be controlled by Size, shape, orientation and morphology. Over the last thirty years, synthetic chemists have developedanassortmentofroutestoobtainopticallypurecompounds.Manystrategies includetheuseofpurestartingmaterialswithchiralcentres.Inaddition,theutilizationof chiralauxiliarygroupshavebeenimplementedtoachieveanincreaseinstereoselectivity andtosimplifythepurificationprocess.Lastly,asymmetriccatalysisusingenzymesor inorganiccatalystshasalsobeenusedtoaffordthedesiredstereochemistry. Atthisstagetheimportantquestionstobefacedare: 1. Inthecaseoforganicchemistryhowcanonebuildthedesiredcarbonskeleton? 2. Howdoesoneintroducethenecessaryfunctionalgroups? 3. Howdoesonecontroltheregioandstereochemistryofreactions? Thesequestionscanbeexpandedtoanyextentdependingontheneedformolecules andmaterials. Thechemicalsynthesisofcomplexorganicmoleculesisintegraltomanyadvances that enhance the quality of life, such as novel disease treatments, agrochemicals with improved properties and advanced materials for high performance technology and biotechnology. The methods for synthesizing complex organic molecules have traditionallypiecedmoleculestogetherinalinearmanner,graduallybuildingcomplexity intothemolecule.Thiscanbeaverytimeconsumingprocess,andsyntheticroutesto molecules can end up being incredibly long, requiring extensive resources and manpower. Since 2000 pioneering approaches to synthesis that combines twodirectional synthesis and tandem reactions. Linear symmetrical trifunctional compounds are synthesizedthroughuseoftwodirectionalsynthesisandarangeoftandemreactionsare thenappliedtogeneratearangeofdiversestructuresfromthesesimplesubstrates.Two

SyntheticStrategiesinChemistry 1.3 directional synthesis, when used in combination with tandem reactions, can lead to significantlyfasterstrategiesforthesynthesisofcomplexmolecules. Therecognitionofthestrongdimensionalitydependentphysicalchemicalproperties of inorganic matter at the nanoscale has stimulated efforts toward the fabrication of nanostructured materials in a systematic and controlled manner. Surfactantassisted chemical approaches have now advanced to the point of allowing facile access to a varietyoffinelysizeandshapetailoredsemiconductor, oxide and metal nanocrystals (NCs)bybalancingthermodynamicparametersandkineticallylimitedgrowthprocesses inliquidmedia.Whilerefinementofthissyntheticabilityisfarfrombeingexhausted, further efforts are currently made to provide nanocrystals with higher structural complexity as means to increase their functionality. By controlling crystal miscibility, interfacialstrain,andfacetselectivereactivityatthenanoscale,hybridnanocrystalsare currentlybeingengineered,whichconsistoftwoormorechemicallydifferentdomains assembledtogetherinasingleparticlethroughapermanentinorganicjunctions. POROUS MATERIALS Thisisoneclassofmaterialswhichhasseenatremendousadvancementinthesynthetic strategies.Poroussolidshavehighscientificandtechnologicalinterest.Theyareableto interactwithatoms,ionsandmoleculesatsurfacesandthroughoutthebulkofmaterial. Distribution of sizes, shapes and volumes of the void spaces in porous materials are directly related to their ability to perform desired function in particular application. High SA/volume ratio provides a strong driving force to speed up thermodynamic processesthatminimizefreeenergy Inhighsurfaceareamaterialstheactivesitesaremoreisolated.Materialswithuniform porescanseparatemoleculesonbasisofsize.Theyarealsoemployedasadsorbents, catalystsupports,andelectrodematerials. Theporoussolidscanbeclassifiedaccordingtotherangeofporesizesavailableinthem. AsimpleclassificationisshowninScheme1.1.

1.4 IntroductiontoSyntheticStrategiesinChemistry

Porous materials

Microporous mesoporous macroporous

eg: ZSM -5,AlPO MCM-48,SBA-15,CMK-n photonic crystals P.D < 2 nm 2-50 nm > 50 nm SA: 300-500 m 2g-1 1000-3000 m 2g-1 10-500 m 2g-1 Scheme1.1Asimpleclassificationofporoussubstances. Theporousarchitectureisnormallybuiltinvariousways.Onesuchmethodologyis showninFig.1.

Schematic representation

Silica/surfactant = 1/0.27 calcination

As-synthesized MCM-41 (hexagonal structure) MCM-41

Silica/surfactant = 1/0.60 calcination

As-synthesized MCM-48 MCM-48 (cubic structure) J.S.Beck,J.C.Vartuli,W.J.Roth,M.E.Leonowiez,C.T.kresge,K.D.Schmitt,C.Chu,D.H.Oison,andE.W. Sheppard.S.B.McCullen,J.L.Schlenker. J.Am.Chem.Soc. ,114(1992)10834 Fig.1.1.TheformationofmesoporousMobilcompositionofmaterials(MCM)formedas aresultoftemplatemechanism(thedetailswillbediscussedinchapter6)

SyntheticStrategiesinChemistry 1.5

Conventionallythetemplaterouteisunderstoodintermsofspacefillingmechanismas showninFig.2.2bythetemplatemolecules,templatemolecularaggregatesorstructure directingagents.

The role of quaternary directing agents

Smallindividualalkylchainlengthquaternarydirectingagentsgeneratetheformation ofmicroporoussolids

Longalkylchainlengthquaternarydirectingagentsselfassembletosupramolecular Specieswhichcangeneratetheformationofmesoporousmolecularsieves

ThomosJ.Bartonetal.,ChemMater.,11(10)(1999)2633 Fig.1.2.Simplemechanismfortheformationofporoussolids Generally the interactions in formation of materials can be understood in terms of bondinginteractions.OnesuchclassificationisgiveninScheme1.2. ClassicexamplesofsynthesiscanbefoundintheareaoforganicChemistry.Strategies and Tactics in Organic Synthesis provide a forum for investigators to discuss their approachtothescienceandartoforganicsynthesis.Ratherthanasimplepresentationof dataorasecondhandanalysis,oneisprovidedwithstoriesthatvividlydemonstratethe powerofthehumanendeavourknownasorganicsynthesisandthecreativityandtenacity of its practitioners. First hand accounts of each project tell us of the excitement of

1.6 IntroductiontoSyntheticStrategiesinChemistry conception,thefrustrationoffailureandthejoyexperiencedwheneitherrationalthought and/orgoodfortunegiverisetosuccessfulcompletionofaproject.Synthesisisreally doneandareeducated,challengedandinspiredbythesestories,whichportraytheidea thattriumphsdonotcomewithoutchallenges.Wealsolearnthatwecanmeetchallenges tofurtheradvancethescienceandartoforganicsynthesis,drivingitforwardtomeetthe demands of society, in discovering new reactions, creating new designs and building moleculeswithatomandstepeconomiesthatprovidesolutionsthroughfunctiontocreate abetterworld.TheseareusuallytermedasNamereactionsinorganicchemistry.These aspectswillbedealtwithinseparatechapters.

Typeofinteraction Surfactant Inorganicprecursor Notation Examples

+ Ionic Cationic+ Anionic S I M41S, (Direct MMCM41,48 pathways) Anionic+ Cationic SI+ MM41S,

+ + Ionic Cationic+ Cationic S X I SBA,APM (Mediated pathways) Anionic+ Anionic SM +I MetalOxides

Hydrogen Neutral+ Neutral S 0I0 HMS bonding Amine (Neutral) Neutral+ Neutral S 0I0 SBA Polymer Covalent Neutral+ Neutral SI TMS Scheme1.2.PossibleinteractionsintheSyntheticStrategiesforPorousMaterials

META STABLE STATE OF THE MATERIALS Formanydeviceandotherapplications,oneisrequiredtomakematerialsinthemeta stablestate.Metastablestatecanbeobtainedinmaterialsbyavarietyofroutes.The possiblelistofthemethodscanbe: • Organization • Templating–bondingandstructural • Autothermal

SyntheticStrategiesinChemistry 1.7

• TransportCVD,PVD • Fieldinduced(electrodeposition,Tinduced) • Mouldingandblowing • Extrusion(geometrycontrol) Thelistgivenisnotcompletebutitgivesanideaofwhatmethodscangeneratethemeta stableofmaterials. INORGANIC MATERIALS Inorganicmaterialsoccupyauniqueplaceandthey canbesynthesized byvarious methods.Somewellknownmethodsinclude Solgeltechniques Coordinationchemistry Weakinteractions bondinginteractions Thesemethodsaredescribedindetailinsubsequentchapters.Thepurposeofthepresent bookistoassimilateavailableinformationonsyntheticstrategiesinChemistry.Though onecannotclaimcomprehensivenessonthistopic,itistheendeavourtobringsomeof theavailableknowledgeinoneplace.Itishopedfromthatpointofview,thesynthetic chemistsmayfindthisbookuseful.

Chapter – 2

SYNTHETIC METHODS BASED ON ACTIVATING THE REACTANT

P.RamanaMurthy INTRODUCTION: Inorganicsynthesis,activatingthereactanthasacrucialroletoplay.Conventionally,the activatingofthereactantiscarriedoutthermally, photochemically; catalytically,orby changeintheconcentrationofthereactant,and/oracidandbase.Theeffectsofthese parametersonthereactantandalsoreactionconditionshavebeenstudied. HALOGENATION OF BENZENE : Substitution Reactions: Benzenereactswithchlorineorbromineinthepresenceofacatalyst,replacingoneof thehydrogenatomsoftheringbyachlorineorbromineatom.Thereactiontakeplaceat roomtemperature.Thecatalystiseitheraluminium chloride(oraluminiumbromideif oneweretoreactbenzenewithbromine)oriron.Strictlyspeakingironisnotacatalyst, because it gets permanently changed during the reaction. It reacts with some of the chlorineorbrominetoformiron(III)chloride,FeCl 3,oriron(III)bromide,FeBr 3.

2Fe+3Cl 22FeCl 3 2Fe+3Br 22FeBr 3

Thesecompoundsactascatalystandbehaveexactlylikealuminiumchloride,AlCl 3,or aluminiumbromide,AlBr 3,inthesereactions. The reaction with chlorine: Thereactionbetweenbenzeneandchlorineinthepresenceofeitheraluminiumchloride orirongiveschlorobenzene.

(or)C 6H6+Cl 2C 6H5Cl+HCl

2.2 SyntheticMethodsBasedonActivatingtheReactant

The Reaction with Bromine: Thereactionbetweenbenzeneandbromineinthepresenceofeitheraluminiumbromide orirongivesbromobenzene.Ironisusuallyusedbecauseitischeaperandmorereadily available.

(or)C 6H6+Br 2C 6H5Br+HBr

Addition Reactions: Inthepresenceofultravioletlight(butwithoutacatalyst),hotbenzenewillalsoundergo an addition reaction with chlorine or bromine. The ring delocalisation is permanently brokenandachlorineorbromineatomaddsontoeachcarbonatom. Forexample,ifoneweretobubblechlorinegasthroughhotbenzeneexposedtoUV lightforanhour,onegets1,2,3,4,5,6hexachlorocyclohexane.

ss

Brominewouldbehavesimilarly. One of these isomers was once commonly used as an insecticide known variously as BHC, HCH and Gammexane. One of the "chlorinated hydrocarbons" caused much environmentalharm. THE HALOGENATION OF METHYLBENZENE: Substitution Reactions: Itispossibletogettwoquitedifferentsubstitutionreactionsbetweenmethylbenzeneand chlorine or bromine depending on the conditions used. The chlorine or bromine can substituteintoeithertheringorthemethylgroup. Substitution into the Ring:

SyntheticStrategiesinChemistry 2.3

Substitutioninthering happensinthepresenceof aluminium chloride (or aluminium bromide if one were to use bromine) or iron, and in the absence of UV light. The reactionstakesplaceatroomtemperature.Thisisexactlythesameasthereactionwith benzene,exceptthatonehastoworryaboutwherethehalogenatomattachesinthering relativetothepositionofthemethylgroup Methylgroupsare2,4directing,whichmeansthatincominggroupswilltendtogo into the 2 or 4 positions on the ring assuming that the methyl group is in the first position.Inotherwords,thenewgroupwillattachtotheringnextdoortothemethyl group or opposite it. With chlorine, substitution into the ring gives a mixture of 2 chloromethylbenzeneand4chloromethylbenzene.

Substitution into the Methyl group:

Ifchlorineorbrominereactwithboilingmethylbenzeneintheabsenceofacatalystbut inthepresenceofUVlight,substitutiontakesplaceinthemethylgroupratherthanthe ring. Forexample,withchlorine(brominewouldbesimilar):

The organic product is (chloromethyl)benzene or benzyl chloride. The brackets in the nameemphasisethatthechlorineispartoftheattachedmethylgroup,andisnotinthe ring.

2.4 SyntheticMethodsBasedonActivatingtheReactant

Oneofthehydrogenatomsinthemethylgrouphasbeenreplacedbyachlorineatom. However,thereactiondoesnotstopthere,andallthreehydrogensinthemethylgroup caninturnbereplacedbychlorineatoms. That means that one could also get (dichloromethyl)benzene and (trichloromethyl)benzeneastheotherhydrogenatomsinthemethylgrouparereplaced oneatatime.

SUBSTITUTION REACTIONS OF BENZENE AND OTHER AROMATIC COMPOUNDS Theremarkablestabilityoftheunsaturatedhydrocarbonbenzenehasbeendiscussed.The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactionsoccurinpreferencetoadditionreactions,asillustratedinthefollowingdiagram (somecomparablereactionsofcyclohexeneareshowninthegreenbox).

Manyothersubstitutionreactionsofbenzenehavebeenobserved, five most useful are listed below in Table 2.1. Since the reagents and conditions employedinthesereactionsareelectrophilic,thesereactionsarecommonlyreferredtoas

SyntheticStrategiesinChemistry 2.5

ElectrophilicAromaticSubstitution.Thecatalystsandcoreagentsservetogeneratethe strong electrophilic species needed to effect the initial step of the substitution. The specificelectrophilebelievedtofunctionineach typeofreactionislistedintheright handcolumn. Table2.1SubstitutionreactionsofBenzene Electrophile Reaction Type Typical Equation E(+)

(+) (+) Halogenation: C6H6 +Cl 2&heat C6H5Cl+HCl Cl orBr FeCl 3 Chlorobenzene

+ Nitration C6H6 catalyst C6H5NO 2+H 2O NO 2 +HNO 3& Nitrobenzene heat,

(+) Sulfonation : C6H6 H2SO 4 C6H5SO 3H+H 2O; SO 3H Catalyst Benzenesulfonicacid +H 2SO 4+ SO 3, &heat

(+) Alkylation: C6H6 +RCl& C6H5R+HCl R FriedelCrafts heat,AlCl 3 Arene

(+) Acylation: C6H6 +RCOCl& C6H5COR+HCl RCO FriedelCrafts heat,AlCl 3 ArylKetone

1. Mechanism for Electrophilic Substitution Reactions of Benzene - Nitration:

(or)

Theconcentratedsulphuricacidisactingascatalyst.

The formation of the electrophile

2.6 SyntheticMethodsBasedonActivatingtheReactant

+ Theelectrophileisthe"nitroniumion"orthe"nitrylcation",NO 2 .Thisisformedby reactionbetweenthenitricacidandthesulphuricacid.

The electrophilic substitution mechanism

Stage one

Stage two

Atwostepmechanismhasbeenproposedfortheseelectrophilicsubstitutionreactions.In the first, slow or ratedetermining, step the electrophile forms a sigmabond to the benzenering,generatingapositivelychargedbenzenoniumintermediate.Inthesecond, faststep,aprotonisremovedfromthisintermediate,yieldingasubstitutedbenzenering. 2. Substitution Reactions of Benzene Derivatives Whensubstitutedbenzenecompoundsundergoelectrophilicsubstitutionreactionsofthe kinddiscussedabove,tworelatedfeaturesmustbeconsidered: I. The first is the relative reactivity of the compound compared with benzene itself. Experimentshaveshownthatsubstituentsonabenzeneringcaninfluencereactivityina profoundmanner.Forexample,ahydroxyormethoxysubstituentincreasestherateof electrophilicNitration substitutionabouttenthousandfold,asillustratedbythecaseof anisole.Incontrast,anitrosubstituentdecreasesthering'sreactivitybyroughlyamillion. Thisactivationordeactivationofthebenzeneringtowardelectrophilicsubstitutionmay be correlated with the electron donating or electron withdrawing influence of the substituents,asmeasuredbymoleculardipole moments. Inthefollowingdiagramone

SyntheticStrategiesinChemistry 2.7 canseetheelectrondonatingsubstituentsactivatethebenzeneringtowardelectrophilic attack,andelectronwithdrawingsubstituentsdeactivatethering(makeitlessreactiveto electrophilicattack).

Theinfluenceasubstituentexertsonthereactivityofabenzeneringmaybeexplainedby theinteractionoftwoeffects: Thefirstistheinductiveeffectofthesubstituent.Mostelementsotherthanmetalsand carbon have a significantly greater electronegativity than hydrogen. Consequently, substituents in which nitrogen, oxygen and halogen atoms form sigmabonds to the aromatic ring exert an inductive electron withdrawal, which deactivates the ring. The secondeffectistheresultofconjugationofasubstituentfunctionwiththearomaticring. Thisconjugativeinteractionfacilitateselectronpairdonationorwithdrawal,toorfrom thebenzenering,inamannerdifferentfromtheinductiveshift.Iftheatombondedtothe ringhasoneormorenonbondingvalenceshellelectronpairs,asdonitrogen,oxygen and the halogens, electrons may flow into the aromatic ring by pπ conjugation (resonance),asinthemiddlediagram.Finally,polardoubleandtriplebondsconjugated withthebenzeneringmaywithdrawelectrons,asintherighthanddiagram.Inbothcases, the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent. Electrondonationbyresonancedominatestheinductiveeffectandthesecompounds

2.8 SyntheticMethodsBasedonActivatingtheReactant show exceptional reactivity in electrophilic substitution reactions. Although halogen atomshavenonbondingvalenceelectronpairsthatparticipateinpπconjugation,their strong inductive effect predominates, and compounds such as chlorobenzene are less reactivethanbenzene.Thethreeexamplesontheleftofthebottomrow(inthesame diagram) are examples of electron withdrawal by conjugationtopolardoubleortriple bonds,andinthesecasestheinductiveeffectfurther enhances the deactivation of the benzenering.Alkylsubstituentssuchasmethylincreasethenucleophilicityofaromatic ringsinthesamefashionastheyactondoublebonds. II . The second factor that becomes important in reactions of substituted benzenes concerns the site at which electrophilic substitution occurs. Since a monosubstituted benzeneringhastwoequivalentorthosites,twoequivalentmetasitesandauniquepara site, three possible constitutional isomers may be formed in such a substitution. If reaction occurs equally well at all available sites, the expected statistical mixture of isomericproductswouldbe40%ortho,40%metaand20%para.Onecanfindthatthe natureofthesubstituentinfluencestheproductratioinadramaticfashion.Bromination of methoxybenzene (anisole) is fast and gives mainly the parabromo isomer, accompanied by 10% of the orthoisomer and only a trace of the metaisomer. Bromination of nitrobenzene requires strong heating and produces the metabromo isomerasthechiefproduct.

Some additional examples of product isomer distribution in other electrophilic substitutionsaregiveninTable2Itisimportanttonotethatthereactionconditionsfor thesubstitutionreactionsarenotthesame,andmustbeadjustedtofitthereactivityofthe reactantC 6H5Y.Thehighreactivityofanisole,forexample,requiresthatthefirsttwo reactionsbeconductedundermildconditions(lowtemperatureandlittleornocatalyst).

SyntheticStrategiesinChemistry 2.9

The nitrobenzene reactant in the third example is unreactive, so rather harsh reaction conditionsmustbeusedtoaccomplishthatreaction. Table2.2.Examplesofproductisomerdistributioninelectrophilicsubstitution

Y in C 6H5–Y Reaction % Ortho-Product % Meta-Product % Para-Product

–O–CH 3 Nitration 30–40 0–2 60–70

–O–CH 3 FCAcylation 5–10 0–5 90–95

–NO 2 Nitration 5–8 90–95 0–5

–CH 3 Nitration 55–65 1–5 35–45

–CH 3 Sulfonation 30–35 5–10 60–65

–CH 3 FCAcylation 10–15 2–8 85–90 –Br Nitration 35–45 0–4 55–65

–Br Chlorination 40–45 5–10 50–60 These observations, and many others like them, have led chemists to formulate an empirical classification of the various substituent groups commonly encountered in aromaticsubstitutionreactions.Thus,substituentsthatactivatethebenzeneringtoward electrophilic attack generally direct substitution to the ortho and para locations. With someexceptions,suchasthehalogens,deactivatingsubstituentsdirectsubstitutiontothe metalocation.Thefollowingtablesummarizesthisclassification. Table2.3.Summaryoftheclassificationofthesubstituents

Orientation and Reactivity Effects of Ring Substituents

Activating Substituents Deactivating Substituents Deactivating Substituents ortho & para-Orientation meta-Orientation ortho & para-Orientation

(–) –O –NH 2 –NO 2 –CO 2H –F (+) –OH –NR 2 –NR 3 –CO 2R –Cl (+) –OR –NHCOCH 3 –PR 3 –CONH 2 –Br (+) –OC 6H5 –R –SR 2 –CHO –I –OCOCH 3 –C6H5 –SO 3H –COR –CH 2Cl –SO 2R –CN –CH=CHNO 2

2.10 SyntheticMethodsBasedonActivatingtheReactant

TheinformationsummarizedinTable2.3isusefulforrationalizingandpredictingthe courseofaromaticsubstitutionreactions. Nucleophilic Substitution Reactions Thecarbonhalogenbondinalkylhalidesispolarized,placingapartialpositivecharge onthecarbon,andapartialnegativechargeonthehalogen.Thepartialpositivecharged carbonisthereforeelectrophilicandwillbesusceptibletoattackbynucleophiles.Whena suitablenucleophileattacksanalkylhalide,itcandisplacethehalogeninasubstitution reactiontoreleasethehalideanionandformanewbondtothecarbon.Thenucleophile isusuallyneutralornegativelychargedandsomeexamplesareHO ,H 2O,MeOH,EtO , RS .

Withsimpleprimaryalkylhalidesreactingwithsimplenucleophiles,therateatwhich this substitution reaction proceeds is proportional to both the concentration of the nucleophileandtheconcentrationofthereactantalkylhalide,makingthereactionsecond order.Thistypeofsecondorder,nucleophilicdisplacementreactionisthereforetermed an "S N2" reaction (substitution, nucleophilic, bimolecular). The mechanism for this reactionisbestdescribedasconcertedwiththereaction coordinate passing through a single energy maximum with no distinct intermediate. The transition state for this reactionisdescribedbythestructureshownbelowinwhichpartialbondsexistbetween thecentralcarbonandtheattackingnucleophileanddepartinghalogen. Thegeometryofthistransitionstate,withtheplanar carbon in the center, requires that the central carbon undergo a stereochemical inversion ; therefore if the central carbon is chiral, the absolute configuration of the central carbon must change. In the exampleshownR2bromobutanereactswithbromideaniontoformtheenantiomer,S 2bromobutane.

SyntheticStrategiesinChemistry 2.11

Predictingtheproductfromthesetypesofsubstitutionreactionssimplyrequiresthatthe bond to the halogen leaving group be broken and a new bond be made between the nucleophilic atom and the central carbon, inverting the absolute configuration if appropriate.

S N2 Reaction: Kinetics Nucleophilic substitution reactions follow different rate laws, depending on the exact mechanism.TheratelawforanS N2reactionis:Rate= k[RX][Nuc]where kistherate constant,RXisthealkylhalideandNucisthenucleophile.Thereactionrateistherefore secondorderoverall.Thisalsotellsusthatthereactionisbimolecular, i.e.twospecies areinvolvedintheratedeterminingstep.

This proposed mechanism for the S N2 reaction raises an interesting question: If the substitutionoccursatachiralcarbon,doesthereactionproceedwithretention,inversion orlossofstereochemistry?Theanswertothisquestionliesinthedirectionofattackof the incoming nucleophile. Attack on the same side as the halogen would result in retentionofstereochemistry.Attackfromtheoppositesidetothehalogenwouldresultin inversionofstereochemistry.Amixtureofthesetwopossibilitieswouldleadtolossof stereochemicalintegrityatthechiralcarbon.Experimentally,itisfoundthatapurelyS N2 reactionatachiralcarbonproceedswithinversionofstereochemistry. In1896,GermanchemistPaulWaldenreportedtheconversion of enantiopure (+) (R)malicacidintotheenantiomer()(S)malicacid,althoughhedidnotknowatwhich

2.12 SyntheticMethodsBasedonActivatingtheReactant step inversion was occurring. In the 1920s Kenyon and Philips investigated a similar processwith1phenyl2propanol:

From this and several other such cycles, Kenyon and Phillips concluded that the nucleophilicsubstitutionreactionofprimaryandsecondary alkyl halides and tosylates alwaysproceedswithinversionofstereochemistry.Inthecycleshownaboveinversion takesplaceinthenucleophilicsubstitutionoftosylateionbyacetateion.

SN1 Reaction

The S N2 reaction is favoured by basic nucleophiles such as hydroxide ion and disfavouredbysolvents suchasalcoholsand water.Thereactionalsodependsonthe natureofthesubstrate:primarysubstratesreactrapidly,secondarysubstratesreactmore slowlyandtertiarysubstratesarealmostinerttoS N2reaction.Inproticmediawithnon basicnucleophilesunderneutraloracidicconditions,tertiarysubstratescanbeordersof magnitude more reactive than their primary or secondary counterparts. The S N2 mechanism clearly cannot account for this and it can be concluded that a different mechanismcanoperateunderthesecircumstances.Thismechanismiscalled SN1which denotesSubstitutionbyanucleophile,unimolecular. That is, only one species is involved in the ratedetermining step.

SyntheticStrategiesinChemistry 2.13

Kinetics

TheS N1reactionisfirstorderandtheratevariesonlyastheconcentrationofthealkyl halide:Rate= k[RX].Therateofreactionisfoundtobeindependentwithrespecttothe concentrationofthenucleophile.Inotherwords,thenucleophiledoesnottakepartinthe ratedeterminingstep.Anyproposedmechanismforthereactionmustthereforehavethe alkylhalideundergoingsomechange without theaidofthenucleophile.Thefirststep mustthereforebecleavageoftheCXbondtoformacarbocation,followedbyreaction withthenucleophiletogivethesubstitutionproduct.

This mechanism is clearly different from the S N2 pathway and the stereochemical outcome should also differ. Carbocations are sp 2 hybridized, planar species at first glance it would appear that the nucleophile, Y could attack from either face of the carbocation, with an equal probability. One would predict that this should lead to complete racemisation, if the starting alkyl halide were optically pure. In practice, complete racemisation is rarely observed and usually,aminorexcess(upto~20%)of inversionisobserved.OneexplanationforthiswasprovidedbyWinstein,aneminent physicalorganicchemist.Itwasproposedthatanionpair,betweenthecarbocationand theleavinggroupX ispresent,whichpartlyblocksattackofthenucleophilefromone face.Thus,inversionslightlydominates. FactorswhichInfluencetheReactionPathway

SN2

• Steric Effects :ThetransitionstateintheS N2reactioninvolvespartialbonding betweenthenucleophileandthesubstrate.Thebulkierthesubstrate,themore difficultitisforthetransitionstatetobereached.Thereactivityorderis1 o >2 o > 3o.

2.14 SyntheticMethodsBasedonActivatingtheReactant

• The Nucleophile : By definition, a nucleophile must have an unshared pair of electrons,whetheritischargedorneutral.Nucleophilicityfollowsapproximately basicity,sopKavaluescanbeused.Nucleophilicityusuallyincreasesgoingdown a group in the periodic table. The reactivity order of the more common nucleophilesis:CN >I >MeO >HO >Cl >H 2O. • The Leaving Group : The leaving group is normally ejected with a negative charge. Therefore the best leaving groups are those which can best stabilise a negative charge. Weak bases (TsO , I , Br ) are generally good leaving groups, whereasstrongbases(F ,HO ,RO )aregenerallypoorleavinggroups.

• The Solvent : Polar aprotic solvents are best for S N2 reactions. These include

acetonitrile(CH 3CN),dimethylsulphoxide(Me 2SO)and N,Ndimethylformamide

(Me 2NCHO). Proticsolvents tend to form a 'cage' around the nucleophile, decreasingitsreactivity.

SN1: • The Substrate : Substrates which can form relatively stable carbocation o intermediatesfavourS N1reactionsTheorderofstabilityofcarbocationsis:3 > 2o>benzyl>allyl>1 o. • The Nucleophile :Thenucleophileisnotinvolvedintheratedeterminingstepin

an S N1 reaction but the S N1 pathway is more likely to be followed if the

nucleophileispoor, e.g.H 2O. • The Leaving Group :Theleavinggroupisalsoinvolvedintheratedetermining

stepforanS N1reaction,sothesamereactivityorderasforS N2isfollowed.

• The Solvent: ThesolventcanhaveaneffectontherateoftheS N1reaction,but for different reasons. Solvent effects arise from stabilisation of the transition

state andnotthereactantsthemselves.TherateofS N1reactionisincreasedina polarsolventsuchaswateroraqueousethanol. Esterification Reaction: Thisisareactionof formationof estersfromcarboxylic acids and alcohols in the presence of concentrated sulphuric acid acting as thecatalyst. Itusestheformationof ethylethanoatefromethanoicacidandethanolasatypicalexample. Mechanism

SyntheticStrategiesinChemistry 2.15

Ethanoic acid reacts with ethanol in the presence of concentrated sulphuric acid as a catalyst to produce the ester, ethyl ethanoate. The reaction is slow and reversible. To reducethechancesofthereversereactionhappening,theesterisdistilledoffassoonasit isformed.

Step 1 Inthefirststep,theethanoicacidtakesaproton(ahydrogenion)fromtheconcentrated sulphuricacid.Theprotonbecomesattachedtooneofthelonepairsontheoxygenwhich isdoublebondedtothecarbon.

Thetransferoftheprotontotheoxygengivesitapositivecharge.Thepositivecharge is delocalised over the whole of the righthand end of the ion, with a fair amount of positivechargeonthecarbonatom. Inotherwords, one can think of an electron pair shiftingtogivethestructure:

Onecouldalsoimagineanotherelectronpairshiftproducingathirdstructure:

2.16 SyntheticMethodsBasedonActivatingtheReactant

Sowhichoftheseisthecorrectstructureoftheionformed?Noneofthem!Thetruthlies somewhereinbetweenallofthem.Onewayofwritingthedelocalisedstructureofthe ionis:

The double headed arrows means you that each of the individual structures makes a contributiontotherealstructureoftheion.Theydonotmeanthatthebondsareflipping backandforthbetweenonestructureandanother.Thevariousstructuresareknownas resonancestructuresorcanonicalforms. Therewillbesomedegreeofpositivechargeonbothoftheoxygenatoms,andalso onthecarbonatom.Eachofthebondsbetweenthecarbonandthetwooxygenswillbe thesamesomewherebetweenasinglebondandadoublebond. Forthepurposesoftherestofthisdiscussion,wearegoingtousethestructurewherethe positivechargeisonthecarbonatom. Step 2 Thepositivechargeonthecarbonatomisattackedbyoneofthelonepairsontheoxygen oftheethanolmolecule.

Step 3 Whathappensnextisthataproton(ahydrogenion) getstransferredfromthebottom oxygenatomtooneoftheothers.Itgetspickedoffbyoneoftheothersubstancesinthe

SyntheticStrategiesinChemistry 2.17 mixture(forexample,byattachingtoalonepaironanunreactedethanolmolecule),and thendumpedbackontooneoftheoxygensmoreorlessatrandom. Theneteffectis:

Step 4 Nowamoleculeofwaterislostfromtheion.

Thepositivechargeisactuallydelocalisedalloverthatendoftheion,andtherewillalso becontributionsfromstructureswherethechargeisontheeitheroftheoxygens:

Step 5 Thehydrogenisremovedfromtheoxygenbyreactionwiththehydrogensulphateion whichwasformedwaybackinthefirststep.

Andthereweare!Theesterhasbeenformed,andthesulphuricacidcatalysthasbeen regenerated.

2.18 SyntheticMethodsBasedonActivatingtheReactant

Reimer-Tiemann Reaction: TheReimerTiemannreactionisausedfortheorthoformylation ofphenols .Thereaction wasdiscoverdbyKarlLudwigReimer andFerdinandTiemann .

Reaction mechanism: (1) reacts with strong base to form the chloroform carbanion ( 2), which will quickly alphaeliminatetogivedichlorocarbene ( 3).Dichlorocarbenewillreactintheorthoand parapositionofthephenate( 5)togivethedichloromethylsubstitutedphenol( 7).After basichydrolysis,thedesiredproduct( 9)isformed.

SyntheticStrategiesinChemistry 2.19

The Aldol Condensation of Aldehydes :

Reaction type : Nucleophilic addition : Summary

• Reagents:commonlyabasesuchasNaOHorKOHisaddedtothealdehyde. • Thereactioninvolvesanenolatereactingwithanothermoleculeofthealdehyde. • RememberenolatesaregoodnucleophilesandcarbonylCareelectrophiles. • SincethepKaofanaldehydeisclosetothatofNaOH,bothenolateandaldehyde arepresent. • Theproductsofthesereactionsarehydroxyaldehydesor ald ehydealcoh ols = aldols . • Thesimplestaldolreactionisthecondensationofethanal.Thisisshownbelowin 2differentrepresentations.

Step 1: First,anacidbasereaction.Hydroxidefunctionsasabaseandremovestheacidic hydrogengivingthereactiveenolate.

Step 2: Thenucleophilicenolateattacksthealdehydeattheelectrophiliccarbonyl Cina nucleophilicadditiontypeprocess givinganintermediatealkoxide.

Step 3: Anacidbasereaction.Thealkoxidedeprotonatesawatermoleculecreatinghydroxide andthehydroxyaldehydesor aldol product

2.20 SyntheticMethodsBasedonActivatingtheReactant

Benzoin Condensation:

TheBenzoinCondensationisacouplingreactionbetweentwoaldehydesthatallowsthe preparationofαhydroxyketones.Thefirstmethodisonlysuitablefortheconversionof aromaticaldehydes. Mechanism Additionofthecyanideiontocreateacyanohydrineffectsanumpolungofthenormal carbonylchargeaffinity,andtheelectrophilicaldehydecarbonbecomesnucleophilic afterdeprotonation.

SyntheticStrategiesinChemistry 2.21

AstrongbaseisnowabletodeprotonateattheformercarbonylCatom:

Asecondequivalentofaldehydereactswiththiscarbanion;eliminationofthecatalyst regeneratesthecarbonylcompoundattheendofthereaction:

Neighbouring group participation:

The direct interaction of the reaction centre (usually, but not necessarily, an incipient carbeniumcentre)withalonepairofelectronsofanatomorwiththeelectronsofaor bondcontainedwithintheparentmoleculebutnotconjugatedwiththereactioncentre. Adistinctionissometimesmadebetweenn,andparticipation.WhenNGPisin operationitisnormalforthereactionrate tobeincreased. A rate increase due to neighbouring group participation is known as ‘anchimeric assistance’.‘Synarteticacceleration’isthespecialcaseofanchimericassistanceascribed

2.22 SyntheticMethodsBasedonActivatingtheReactant to participation by electrons binding a substituent to a carbon atom in a position relativetotheleavinggroupattachedtothecarbonatom. A classic example of NGP is the reaction of a sulfur or nitrogen mustard with a nucleophile ,therateofreactionishigherforthesulfurmustard andanucleophilethanit wouldbeforaprimaryalkyl chloride withoutaheteroatom .

The π orbitals of an alkene canstabilizeatransitionstate byhelpingtodelocalizethe positivechargeofthecarbocation .Forinstancetheunsaturated tosylate willreactmore quicklywithanucleophilethanthesaturatedtosylate.

Thecarbocationicintermediatewillbestabilizedbyresonance wherethepositivecharge isspreadoverseveralatoms.

SyntheticStrategiesinChemistry 2.23

PHOTOCHEMICAL REACTIONS : Anychemicalreactioncanbecausedbyabsorptionoflight(includingvisible,ultraviolet, andinfrared).Thelightexcitesatomsandmolecules(shiftssomeoftheirelectronstoa higher energy level) and thus makes them more reactive. In comparison to ordinary reactionsusingthermalenergyalone,photochemicalreactionscanfollowdifferentroutes and are more likely to produce free radicals , which can trigger and sustain chain reactions . Some photochemical reactions like photoadditions, photocycloadditions, photoeliminations, photoenolizations, photoFries rearrangements, photoisomerizations, photooxidations,photoreductions,photosubstitutions,etc. The Reaction between hydrogen and chlorine:

H 2(g)+Cl 2(g)2HCl(g) Amixtureofhydrogenandchlorinegaseskeptinthedarkreactsonlyveryslowlyifat all.Nowsubjectittoapulseofultravioletlightandanexplosivereactiontakesplace. Initiation, Propagation and Termination The reaction of hydrogen and chlorine is a typical photochemical chain reaction involving radicals. The reaction involves three stages: initiation, propagation, and termination.Itrequiresphotonsoflightonlytogetitstarted(Initiationofthereaction) after which it rapidly reaches completion. These photons, absorbed by a few of the chlorinemolecules,causetheClClbondstobreakhomolytically.

. Step1 Cl 2+h2Cl Initiation The reaction now has to keep going, or propagate itself. The next two steps in the mechanisminvolve propagation .Apropagationreactioninvolvesthelossofaradical, butalsotheformationofanotherradical.Twopropagationstepsarerequiredotherwise thereactionwouldcometoastopbeforecompletion.

. . Step2 Cl +H 2H +HCl Propagation

. . Step3 H +Cl 2HCl+Cl Propagation

The propagation steps repeat over and over in a chain reaction . Radicals also come togetherformingcovalentbondinterminationsteps.

2.24 SyntheticMethodsBasedonActivatingtheReactant

. . Step4 H +Cl HCl Termination The Chlorination of Methane Thechlorinationofmethaneisanotherphotochemicalradicalchainreaction.Thereaction isasubstitutionreaction;ahydrogenatomofmethaneisswappedforachlorineatom.

CH 4+Cl 2CH3Cl+HCl

. Hereisthemechanismforthereaction..Step1 Cl 2+h2Cl Initiation

. . Step2 CH 4+Cl CH 3+HCl Propagation

. . Step3 CH 3+Cl 2CH3Cl+Cl Propagation

. . Step4 CH 3+Cl CH 3Cl Termination

Inthereactionofmethaneandchlorine,chloromethane(CH 3Cl)isnottheonlyorganic

product. A mixture of organic products (also CH 2Cl 2, CHCl 3, CCl 4) is obtained, corresponding to the substitution of each of the hydrogen atoms of methane. The formation of these arises from steps 2 and 3 above repeating. The formation of the

disubstitutedderivative,dichloromethane(CH 2Cl 2)isshown: . . CH 3Cl+Cl CH 2Cl+HCl . . CH 2Cl+Cl 2CH2Cl 2+Cl Finally,tobemorepreciseaboutthenameofthemechanismforthisreaction:itisa radicalsubstitutionreaction. REFERENCES: 1.O.D.Tyagi,M.Yadav,ATextbookofOrganicReactionMechanism,Anmol PublicationsPvt.Ltd.,2002. 2.O.P.Agarwal,UnifiedCourseinChemistry,VolumeII,JaiPrakashNath&Co., (2003). 3.PureandAppliedChemistry,1994,66,1077. GlossaryoftermsusedinPhysicalOrganicChemistry,(IUPACRecommendations 1994). 4.www.chemguide.co.uk

Chapter - 3 METHODS BASED ON ACTIVATING THE REACTING SUBSTANCE

R.Mahalakshmy 1. INTRODUCTION The art of carrying out efficient chemical transformations is a major concern in modern organicsynthesis.Twoaspectsareofutmostimportancewhenconsideringtheoutcomeofa reaction, viz., selectivity and efficiency (optimization of yields). The new procedures developedshouldalsobecompatiblewithourenvironmentandthusfacilitatepreservingour resources.Therightactivationmodeis,therefore,developmentofmethodsthatpromotean efficientchemicaltransformation.Inthisarticle,activationshouldbeunderstoodinarather wide sense. In onewayit isdescribed asasortof catalysis facilitating the course of the reactionbyloweringtheactivationenergy.Anotherwayitisexplainedinrelationtonon catalyticpath.

Activationmode

Physical Chemical

(i)Microwave

(ii)Ultrasound Noncatalytic Catalytic (iii)HighPressure Aqueous

(i)Solvophobic Ionicliquid (i)Homogeneous

Supercritical (ii)Heterogeneous (ii)Solventfree Micelles (iii)Biocatalysis

(iii)Physicochemical Microemulsion (iv)Photocatalysis

Electrochemical Scheme3.1.Typesofactivationmode Activationisachievedindifferentways,whichmaybeclassifiedintophysical,chemicalor biochemical,physicochemicalmodes.Althoughexhaustivityisnottheaim,thisarticletries to give a survey on the most recent advances in either traditional modes (pressure, light,

3.2 MethodsBasedonActivatingtheReactingSubstance chemical catalysis) or in novel techniques (microwaves, sonication, biocatalysis). The variousactivationmodesdiscussedinthisarticleareshowninscheme3.1. 2. PHYSICAL METHODS FOR THE ACTIVATION OF REACTING SUBSTANCE 2.1. Activation by Microwave Microwavesareaformofelectromagneticwaves(wavelengthbetween1cmand1m).When moleculeswithapermanentdipoleareplacedinanelectricfield,theybecomealignedwith that field. If the electric field oscillates, then the orientations of the molecules will also change in response to each oscillation. Most microwave ovens operate at 2.45 GHz, wavelengthatwhichoscillationsoccur4.9x10 9timespersecond.Moleculessubjectedto thismicrowaveradiationareextremelyagitatedastheyalignandrealignthemselveswiththe oscillating field, creating an intense internal heat that can escalate as quickly as10 °C per second.Thistechniqueprovestobeexcellentincaseswheretraditionalheatinghasalow efficiency because of poor heat transmission and, hence, local overheating is a major inconvenience. Microwave energy is fast becoming the method of choice for both industrial and academicchemistsfordrivingreactionstocompletion,asitoffersthesafest,mosteffective waytoincreasereactionratesandimproveproductyields,whilepromotinggreenchemistry. Reactionsthatpreviouslytookhours,or evendays,tocompletecannowbeperformedin minutes.Decreasingreactiontimesoffersteachingopportunities:studentshavemoretime for design, optimization, characterization and analysis of reaction processes and products. Additionally,microwaveassistedreactionsareoftenperformedinaqueoussolutionsorneat, minimizingthe need fororganic solvents, simplifyingthe workup process,and providing “green”reactionconditions. Is microwave assisted organic synthesis green and safe? It’stimetothinkoftheenvironmentandourimpactonit.Microwaveenergyisaninherently efficientwaytotransferenergytoareaction,asittransferskineticallyratherthanthermally. Becauseofthisquality,itistheidealenergysourcefordrivingreactionsanditalsohasthe followingadvantages. • Usewater,ethanolorotherenvironmentallybenignsolvents • Neatreactions/highconversionshelpeliminatewaste • Nonhazardousreagentshelpstudentsdesignsafersyntheses • Usecatalysts,notstoichiometricreagents

SyntheticStrategiesinChemistry 3.3

Not only is microwaveassisted chemistry is good for the environment, it is also safer for chemists.Microwavesynthesissystemsdesignedforthelaboratoryofferan • Unmatchedlevelofsafety. • Eliminatehotplateburns • Reactionsreturntoroomtemperaturebeforeremovingfrommicrowave Afewrepresentativeexamplesofmicrowaveassistedchemicalconversionsareconsidered. (i) Oxidation of Primary and Secondary Alcohol A fast and facile microwave accelerated oxidation of primary alcohols to carboxylic acids and secondary alcohols to ketones were carried out under organic/aqueous biphasic conditions using 30% aqueous H 2O2 in the presence of sodium tungstate and tetrabutylammonium hydrogen sulfate (TBAHS) as a phasetransfer catalyst[1]. The experimentalprocedureinvolvesasimplemixingofanalcohol,Na 2WO 4, 2H 2OandTBAHS followedbytheadditionof30%aqueousH 2O2in25:1:1:125molarratiosforprimaryand 25:1:1:40molarratioforsecondaryalcoholsinanopenvessel.Thenthereactionmixtures wereplacedinsideamonomodemicrowavereactorandirradiatedunderarefluxcondenser forspecifictime.Thebestresultswereobtainedwhenthetemperaturesofreactionmixtures weresetto90°Cand100°Cforprimaryandsecondaryalcohols,respectively.

O

OH H2O2, MW, 20 min OH

Na2WO4, TBAHS (Yield= 84%)

OH O

H2O2, MW, 10 min

Na2WO4, TBAHS

(Yield= 92%)

(ii) Nucleaphilic Substitution Reaction Thesynthesisofarylaminesthroughdirectnucleophilicsubstitutionofarylhalidestypically requires highly polar solvents such as DMF and DMSO at high temperatures even with highlyactivatedarylhalides.AnovelandefficientsynthesisofNarylaminesbythereaction of activated pbromonitrobenzene with secondary amines (morpholine and Nphenylpi

3.4 MethodsBasedonActivatingtheReactingSubstance

perazine)inthepresenceofbasicAl 2O3undermicrowaveirradiationwascarriedoutunder solventfreeconditionsinasimpledomesticoven[2].

Al2O3, 3 min O2N N O O2N Br + HN O MW (350W) Yield= 80%) Bothaluminaandaminesarepolarsotheyabsorbmicrowaveseffectivelyandconsequently thereactioniscompletedinashorttime. Thus, the microwave irradiation has been successfully applied in organic chemistry. Spectacularaccelerations,higher yieldundermilderreactionconditionsandhigherproduct purities have all been achieved so far. More over, by using this technique, a number of reactionshas been carried out successfullythat do not occur by conventional heating and evenmodificationsofselectivity(chemo,regioandstereoselectivity)areobtained.

2.2. Activation by Ultrasound Ultrasounds are acoustic waves with frequencies rangingfrom20to100MHz.Theyare mechanicalwavesthatarenotabsorbedbysolidand,therefore,theydonotinduceheating. Theyaretransmittedthroughanysubstances–solid,liquidorgas,whichpossesseselastic properties. How does ultrasound accelerate a chemical reaction? The energy of ultrasound is insufficient to cause chemical reactions, but when it travels throughmediaaseriesofcompressionsandrarefactionsarecreated,therarefactionofliquids leading to cavities. During rarefaction, the negative pressure developed by the power of ultrasound is enough to overcome the intermolecular forces binding the fluid and tear it, producingcavitationbubbles.Thesucceedingcompressioncyclecancausethemicrobubbles to collapse almost instantaneously with the release of large amounts of energy. The enormousriseinlocaltemperaturesandpressuresproduces adramatic beneficial effect of reactionacceleration,withrelativelyshorttimesbeingrequiredforcompletingthereaction such that the decomposition of thermally labile products is minimised. A schematic representationofcavitaionalerosionofsolidisgiveninFig.3.1.

SyntheticStrategiesinChemistry 3.5

Fig.3.1.Cavitationalerosionofsolid How to carry out a chemical reaction in an ultrasonic bath? Anumberofcommonreactionsusedinsyntheticorganicchemistrycanbecarriedoutmore efficiently using ultrasound. These reactions can be performed by immersing a reaction vesselintoanultrasonicbath(Fig.3.2)orbyimmersing anultrasonic probe (horn) into a reactionmedium.

Fig.3.2.Apparatusforcarryingoutareactioninanultrasoniccleaningbath (Reproducedfromageneralarticleentitled,“Ultrasound:ABoonintheSynthesisofOrganic Compounds”publishedin Resonance ,1998, 3,56)

Thereareseveraladvantagesofthismethodandtheyare: (i) Itincreasestheyieldandthepercentageofbyproductsdecreases. (ii) Reactionsoccurfaster,sothatlowertemperaturescanbeused. (iii) Reactiontimesdecreasebyafactorfivetofiftyforidenticalisolatedyields.

3.6 MethodsBasedonActivatingtheReactingSubstance

(iv) Itprovidesalternativepathwaysforreactions,duetotheformationofhighenergy intermediates. InthefollowingtextseveralspecificapplicationsofSonochemistryaregiven[3]. (i) Preparation of organometallic reagents Sonochemistryenablesreactionsinvolvingorganometallicreagentstobecarriedoutsafely. Thereactionofmagnesiumwithethyl,butylandphenylbromidesinaqueousdiethyletheror in ndibuty1 ether containing 50% benzene or light petroleum is accelerated by utilising ultrasound.

R-X + Mg R-Mg-X

R = C 2H5 , n–C4H9 , C 6H5 X = Cl, Br (ii) Oxidation of alcohol The oxidation of alcohols by solid potassium permanganate in hexane and benzene is significantlyenhancedbysonicationinanultrasonicbath.

R KMnO4 R

OH O

(iii) Reformatsky reaction TheReformatskyreactioncanbecarriedoutinhighyield(98%)injust30minutesat25–30 o Cascomparedtoconventionalmethod,whichgivesonly50%yieldafter12hoursat80 o C. NaX C4H8C=O + BrCH2COOEt + Zn C4H8C(OH)CH2COOEt

(iv) Breakdown of polymeric organometallic compound Thebreakdownofpolymericorganotinfluoxidesiscarriedoutbysonicationmethod.For thisreaction,thereis900foldincreaseinrateascomparedtoconventionalreflux.

[R SnF] 3 n nR3SnX R=Me, n–Bu or Ph X=Cl, Br, NCO

SyntheticStrategiesinChemistry 3.7

(*Note: In the above examples the symbol represents the sonication method ) Inadditiontothetypesofreactionsmentioned,ultrasoundhasalsobeenusedinthecaseof enzyme catalysed reactions, polmer chemistry and coal liquification. Extension of combination of sonochemistry with other specific methods, such as photochemistry and electrochemistry appear to be promising. Since it is an upcoming and a recent field of interest,thereisagreatdealmoretoexploreinultrasonicsasanimportanttoolinordertotap itsfullpotentialforthediscoveryofnewreactionsutilisinghighly energeticsoundwaves. Thesonochemicalboomisturningouttobearealboonforsyntheticchemistry. 2.3. Activation by high pressure Pressure represents a mild nondestructive activation mode, generally respecting the molecular structure by limiting decomposition or further evolution of the products. The specificeffectsofhighpressurecanbeofimportantvaluefororganicsynthesis.Thekinetic pressure effect is primarily determined by the variation of volume due to changes in the nuclear positions of the reactants during the formation of the transition state. Related to volumerequirementsarestericeffectssincethebulkinessofthemoleculesinvolvedinthe transition state conditions the magnitude of the steric interactions. As a consequence, pressureaffectsvolumechangesandshouldhaveaneffectonstericcongestion. As a mild activation mode, pressure may be considered of value in the synthesis of thermally fragile molecules, permitting a lowering of the temperature. In addition, the selectivity is generally preserved or even improved under such conditions. Highpressure chemistryisnowrecognizedasapowerfulmethodto achieve synthetic organic reactions, which are not readily accessible by usual means. The applications of this technique to organicreactionslikeDielsAlder[4]andCycloaddition[5]arediscussed. (i) Diels-Alder reaction The high pressure (0.8 GPa) DielsAlder reaction of Nmethyl2(1H)pyridones with cyclooctyneat90 °Caffords1:1cycloadductsin6080%Yield.Noadductisrecoveredat normalpressureduetotheextrusionofmethylisocyanate.

H3C O R1 N R1 +

R2 O CH3 R2

3.8 MethodsBasedonActivatingtheReactingSubstance

(ii) Cycloaddition reaction Cycloaddition of mesityl oxide to isoprene is also carried out at high pressure in

LiClO 4/Diethyethermedium.

O

+ 20 MPa, 100 C, 24 h Dimers of + isoprene LiClO4/Diethyether

Yield:52% Yield:19%

Whentraditionalsyntheticstrategiesappearforbiddinglydifficultorfailutterly,thepressure parameter,possiblyassociatedwithotheractivationmethodsmaybeconsidered.Other salientfeaturesofthismethodinclude:

(i) Extremesimplicityofthemethod (ii) Capacitytoinduceionogenesis (iii) Capacitytoremovestericinhibition

Thusthenotableadvantagesofthismethodmaterializeinneworimprovedsyntheticroutes, anditaddsanotherimportantdimensiontotheexistingsyntheticactivationmodes. 3. CHEMICAL METHODS FOR THE ACTIVATION OF REACTING SUBSTANCE 3.1. Non-catalytic activation 3.1.1. Solvophobic activation 3.1.1a. Reaction in water Inthemostrecentdecades,theuseofwaterasareactionsolventorcosolventhasreceived much attention in synthetic organic chemistry, with sometimes surprising and unforeseen results. It plays an essential role in life processes;howeveritsuseasasolventhasbeen limitedinorganicsynthesis.Despitethefactthatitisthecheapest,safestandmostnontoxic solvent in the world, its presence is generally avoided through the dehydrative drying of substratesandsolvents.Butstillitcanbeconsideredasauniquesolvent.Moreover,wateris the‘solventofNature’andthereforetheuseofwaterasamediumfororganicreactionsis oneofthelatestchallengesformodernorganicchemists.

SyntheticStrategiesinChemistry 3.9

Therearemanypotentialreasonstoreplacetheclassicalorganicsolventsbywater.Theyare (i) Cost,safetyandenvironmentalconcern. (ii) Aqueousproceduresareoftenreferredtoasgreen, environmentally friendly, or benign. (iii) The unique solvation properties of water have been shown to have beneficial effectsonmanytypesoforganicreactionsintermsofboththerateandselectivity. (iv) Experimental procedures may be simplified, since isolation of organic products and recycling of watersoluble catalysts and other reagents can be achieved by simplephaseseparation. Selectedfeworganicreactionsruninanaqueousmediumareconsidered. (i) Wittig Reaction Bergdahl and coworkers published the first report in the literature describing that Wittig reactions of stabilised (and poorly watersoluble) ylides with aldehydes are unexpectedly acceleratedinanaqueousmedium[6].

COR2 CHO COR Ph3P 2 R 1 a R1 66-99%

1 R = H, 2-NO2, 4-NO2, 2-CN, 4-OH, 4-OMe, 4-NH2, 2-OBn, 3-OBn R2= Me, OMe, O-Bu, O-Troc, Ph

COR2

Ph3P 2 1 R1 X COR R X CHO a 84-97% X = S, NH 1 R = H, Br, Me, NO2 R2 = OMe, O-Bu a)aldehyde(1mmol),ylide(1.21.5mmol),H 2O(5mL),2090ºC,5min4h.Troc=2,2,2 trichloroethoxycarbonyl. (ii) Mannich-type Reactions Kobayashi and coworkers published an efficient (up to 94% yield) enantio and diastereoselective protocol for Mannichtype reactions of a hydrazono ester with silicon

3.10 MethodsBasedonActivatingtheReactingSubstance enolatesinaqueousmedium.Oneexampleofa syn adductfroman( E)siliconenolateand twoexamplesof anti adductsfrom( Z)siliconenolatesarereported[7].

NHBz R1 OSiMe3 BzHN N a NH O + EtO EtO R2 R3 O R3

R1 R2 O

R1 = H, Me, Et, R2=H, Me, R3= Et, S-tBu, 4-Me-C6H4, 4-OMe-C6H4, 4-Cl-C6H4

Ph Ph

MeO NH HN OMe

b

Where a = acylhydrazonoester(0.4mmol),silylenolether(1.2mmol),ZnF 2(100mol%), b

(10 mol%), CTAB (0.02 mmol), H 2O (1.95 mL), 0 ºC, 20 h. CTAB= cetyltrimethylammoniumbromide. (iii) Deprotection of Functional Groups Methodsforselectivedeprotectionoffunctionalgroupsarekeytoolsfororganicchemists. The following examples, performed in water, open new possibilities for the use of this challenging medium. Konwar and coworkers [8] reported a simple protocol for the deprotections of oximes and imines under neutral conditions (yields up to 90%) using a

I2/surfactant/watersystem

X N a O

R1 R2 R1 R2 X= OH, Ph R1, R2 = alkyl R1,R2= aryl 1 2 R = H, alkyl, R =aryl .

SyntheticStrategiesinChemistry 3.11

where a= oxime or imine (1 mmol), I 2 (20 mmol%), H 2O(15mL),SDS(0.2mmol),25 40°C,3.58h. Themainobstacletotheuseofwaterasreactionsolventisthenegligiblesolubilityofthe majorityoforganiccompoundsinwater.Thisproblemcanbeaddressedbyusingaqueous organicsolventsorphasetransferagents. 3.1.1b. Reaction in ionic liquid Ionicliquidsarelowmeltingpointsaltsthathaveattractedconsiderableattentionrecentlyas greeneralternativestoclassicalenvironmentallydamagingsolvents."Theinterestismainly due to their peculiar properties such as absence of flammability, lack of measurablevapor pressure, and good ability to dissolve organic, organometallic, and even some inorganic compounds. These unique chemical and physical characteristics of ionic liquids are increasinglyenticingchemiststoexploretheiruseasmediafororganicsynthesis. Ionicliquidsoffernumerousadvantagesoverconventionalorganicsolventsforcarryingout organicreactions.Theyare, (i) Easyproductrecovery (ii) Catalystscanberecycled (iii) Ionicliquidscanbereused (iv) Theirthermodynamicandkineticbehaviorisdifferent (v) Ratesofreactionareoftenenhancedand (vi) Selectivityisfrequentlybetter

Examplesfor Ionic Liquids Themostcommonclassesofionicliquidsarealkylammoniumsalts,alkylphosphoniumsalts alkylpyridinium salts, and N, N´dialkylimidazolium salts. Few examples of cationic and anionicionicliquidsaregiveninFig.3.3.

R - - - - 1 R1 BF4 , PF6 , SbF6 , NO3 , - - - CF3SO3 , (CF3SO3)2N , ArSO3 , CF CO -, CH CO -, Al Cl - N R2 P R N 3 2 3 2 2 7 2 R NH N R 1 R2 3 R R3 3 R3 R Fig.3.3.Cationicandanionicionicliquids

3.12 MethodsBasedonActivatingtheReactingSubstance

Synthesis and applications of halide based ionic liquid Halidebased ionic liquids ILX (IL represents cations such as 1, 3dialkylimidazolium, 1 alkylpyridinium and tetraalkylammonium; X represents halide anions) can be used as reagents in nucleophilic substitution for the conversion of alcohols to alkyl halides. This reactionprovidesanalternativewayofpreparingothertypesofionicliquids(ILA)basedon theconjugatebasesofacids(HA)[9].

-H2O ILX + HA + ROH ILA + RX Where, ILX:halidebasedionicliquids;X=Cl,Br,I;HA:acids;ROH:alcohols ILA:newionicliquidswithconjugatebasesofHA These halidebased ionic liquids (ILX) can also be used as reaction media for copper catalyzednucleophilicaromaticsubstitutionreactionforformationofarylnitriles(ArCN). Cu catalyst ArY + NaCN ArCN ILX, -NaY ArY:Arylhalides,Y=IorBr,ILX:halidebasedionicliquid X=Cl,Br,I;Cucatalysts:CuX,X=Cl,Br,I,CN

Trihalidebasedionicliquids(ILX 3)thatcanbeusedasreagentsaswellasreactionmediain halogenationreactions -2H O ILX + 2HX + H O 2 ILX 2 2 3 ILX:halidebasedionicliquids,X=Cl,Br,I HX:hydrogenhalides "Therapidgrowthofinterestinionicliquidsismainly limited to people in academia and national laboratories,” "There is a lot of skepticism among industrial chemists, probably becauseourunderstandingofthesematerialsislimited. Before we see industrial chemists enthusiastically involved in exploring the field, a lot of work has to be done. We need informationonthetoxicityandsafetyofthesematerialsandtheireffectontheenvironment, aswell as an assessmentof their life cycles.Also, we need cost analyses compared with existing technologies. In addition, it is important to develop a good database of all the informationavailableonionicliquids.Unlesswehaveallthisinformation,thegrowthwillbe limitedtoafewsectorsonly.”

SyntheticStrategiesinChemistry 3.13

3.1.1c. Reaction in supercritical media What is a supercritical fluid? Supercriticalfluidsmaybedefinedasthestateofacompound,mixtureorelementaboveits critical pressure (Pc) and critical temperature (Tc), but below the pressure required to condenseitintoasolid.Itpossessesthecharacteristicsofbothfluidandgaseoussubstances: the fluid behavior of dissolving soluble materials, and the gaseous behavior of excellent diffusibility.Theyoccupyapointwherepureandappliedsciencemeetsheadon.Thisisa featurethathasattractedmanyworkerstothefield.A generalphasediagramforcritical fluidisgiveninFig.3.4.

Fig.3.4.Generalphasediagramforsupercriticalfluid (Reproducedfromthewebpage:http://www.ed406.upmc.fr/cours/shaldon.pdf)

Reactionsundersupercriticalconditionshavebeenusedforlargescaleindustrialproduction formostofthetwentiethcentury,buttheapplication of supercritical fluids (SCFs) in the synthesisofcomplexorganicmoleculesisonlyjustemerging.Researchinthisfieldhasbeen particularlyactiveinthelastdecadeofthiscentury,becausethefollowingspecialproperties ofSCFsmakethemattractivesolventsformodernsyntheticchemistry.

3.14 MethodsBasedonActivatingtheReactingSubstance

• Increased reaction rates and selectivities resulting from the high solubility of the reactantgases • Rapiddiffusionofsolvents • Weakeningofthesolvationaroundthereactingspecies andthe localclustering of reactantsorsolvents. • Thesefluidsareeasilyrecycledandallowtheseparationofdissolvedcompoundsbya gradualreleaseofpressure • Sequentialandselectiveprecipitationsofthecatalystandproductwouldbepossible. Example for Carbon–carbon bond formation reactions in supercritical fluids (i) Diels-Alder reaction The Diels–Alder reaction is the most widelyused synthetic method for the synthesis of polycyclic ring compounds. Kolis et al [10] have reported the possibility of performing

Diels–AlderreactionsinsuperheatedandscH 2Oduetotheuniqueproperties ofscH 2O[11]. Thereactionstestedwerethecycloadditionsofcyclopentadiene(1)withdiethylfurmarate(2) anddiethylmaleate(4)usingscH 2Oasthesolvent.Theyobtainedyieldsof10and86%for3 and 5, respectively, after 1 h. Although the yield of the endo/exo2, 3diethyl ester of 5 norbornene3waslow,equalamountsofbothisomersof5wereformedingoodyieldfrom thecisdiene.

CO2Et scH2O + CO2Et 375 oC, 1h EtO2C CO2Et 1 2 3

CO2Et scH2O + CO2Et o CO Et CO2Et 375 C, 1h 2 4 5 1

3.1.2. Solvent Free or Solid State Reaction

SyntheticStrategiesinChemistry 3.15

A solventfree or solidstate reaction may be carried out using the reactants alone or incorporating them in clays, zeolites, silica, alumina or other matrices. Thermal process or irradiationwithUV,microwaveorultrasoundcanbeemployedtobringaboutthereaction. Solventfreereactionsobviouslyreducepollution,andbringdownhandlingcostsdueto simplification of experimental procedure, work up technique and saving in labour. These wouldbeespeciallyimportantduringindustrialproduction. Often,theproductsofsolidstatereactionsturnouttobedifferentfromthoseobtainedin solution phase reactions. This is because of specific spatial orientation or packing of the reacting molecules in the crystalline state. This is true not only of the crystals of single compounds,butalsoofcocrystallizedsolidsoftwoorevenmorereactantmolecules.The hostguestinteractioncomplexesobtainedbysimplymixingthecomponentsintimatelyalso adopt ordered structure. The orientational requirements of the substrate molecules in the crystalline state have provided excellent opportunities to achieve high degree of stereoselectivity in the products. This has made it possible to synthesize chiral molecules fromprochiraloneseitherby complexationwithchiralhostsorformationofintermediates withchiralpartners. Experimental method Iftwoormoresubstratesareinvolvedinthereaction,theyarethoroughlygroundtogetherin aglassmortarorcocrystallized,andallowedtostayatroomtemperatureortransferredtoa suitableapparatusandheatedcarefullyinanoilbathorexposedtoappropriateradiationuntil the reaction is complete. More sophisticated reaction procedures are also adopted, if necessary.TLCcanmonitortheprogressofthereaction.Insomecases,asmallquantityof wateroracatalystmaybeadded.Ifitisasinglecompoundreaction,itissubjectedtoheator radiationdirectly.Careistobetakentocollectthevolatileproducts,iftheyareproduced.In thisarticleillustrativeexamplesrepresentinganumberoforganicsynthesesperformedunder boththermalandphotochemicalconditionsaredescribed[12]. Examples (i)Solidstatereactionsarenotreallyanewconcept.Theyhavebeenreportedevenin undergraduatetextbooks.Infact,thehistoricallysignificantfirstorganicsynthesisof ureabyWöhlerachievedin1828belongstothisclass

Solid NH4NCO NH2-CO-NH2

3.16 MethodsBasedonActivatingtheReactingSubstance

(ii)Pyrolyticdistillationofbariumorcalciumsaltsofcarboxylicacidstoprepareketonesis evennowacommonlyusedprocedure.

(Ph-CH2-COO)2Ba Ph-CH2-CO-CH2-Ph + BaCO3

(iii) Michael Addition Theadditionofanucleophiletoacarboncarbondoublebondwithastrongelectron withdrawinggroupatthevinylicpositionisknownasMichaeladdition.

NO O Al2O3 O 2 + CH3NO2 Ph Ph Ph Ph

(iv) Aldol Reaction Aldol condensation is animportantreaction of aldehydes andketones in forming carbon carbonbonds. Theaddition ofanenol or enolateionof analdehyde oraketonetothe carbonylgroupofanaldehydeoraketoneisaldoladdition,oraldolcondensation,ifwateris eliminated in a subsequent step to produce a, bunsaturated aldehyde or ketone. Many variationsofthisreactionareknownandarecalledbydifferentnames.

NaOH Solid r.t Ar-CHOH-CH -CO-Ar' + Ar-CH=CH-CO-Ar' ArCHO + Ar'-CO-CH3 2 5 min; 97%

( In solution only 11% yield was realized in 5 min)

(v) Oxidations of alcohol to ketone/aldehyde

Cr-Oxides t-Bu OH t-Bu O Solid

Cr-Oxides CH OH CHO 2 Solid

SyntheticStrategiesinChemistry 3.17

3.1.3. Activation by Physicochemical Methods 3.1.3a. Reaction in Micellar media Micelles are dynamic colloidal aggregates formed by amphiphilic surfactant molecules. Thesemoleculescanbeionic,zwitterionic,ornonionic,dependingonthenatureoftheir headgroups,theirmicellesbeingclassifiedinthesameway.Indilutesolutions,amphiphile moleculesexistasindividualspeciesinthemediaandthesesolutionshavecompletelyideal physicalandchemicalproperties.Astheamphiphileconcentrationincreases,aggregationof monomers into micelles occurs and, as a consequence, these properties deviate gradually fromideality.Thisconcentrationiscalledthe criticalmicellisationconcentration.During theformationofmicelles,headgrouprepulsionsarebalancedbyhydrophobicattractionsand forionicmicelles,alsobyattractionsbetweenheadgroupsandcounterions.Hydrogenbonds canbealsoformedbetweenadjacentheadgroups. Itiswellknownthatperformingthereactionsin micellar media instead of pure bulk solventscanaltertheratesandpathwaysofallkindsofchemicalreactions. Micellesare ableto (a) Concentratethereactantswithintheirsmallvolumes, (b) Stabilisesubstrates,intermediatesorproductsand (c) Orientatesubstratessothationizationpotentialsandoxidation–reductionproperties, dissociation constants, physical properties, quantum efficiencies and reactivities are changed. Thus, they can alter the reaction rate, mechanism and the regio and stereochemistry. For manyreactions,rateincrementsof5–100foldoverthereactionsinhomogeneoussolutions havebeenreported.Insomecases,rateincrementsmaybehigherandincrementsintheorder of106foldhavebeenobserved.

Example (i) Ring opening reaction of epoxide TheringopeningreactionofstyreneoxidewithNaCNwasstudiedinthemicellarsolutionof sodium dodecyl sulfate (SDS) as an anionic micelle at different concentrations in the presenceofcatalyticamountsofCe(OTf)[13].

Ph Micelle(SDS) PhCH(OH)CH2CN + PhCH(CN)CH2OH O Ce(OTf) , Cat, rt, NaCN 4 8% 92%

3.18 MethodsBasedonActivatingtheReactingSubstance

TheCMCofSDS=8.1x10 3M Thus,itisestablishedthat,inmanycases,performingthereactionsinmicellarmediainstead oforganicsolventscanalterratesandthepathwaysofthereactions. 3.1.3b. Reaction in Microemulsion Media When water is mixed with an organic liquid immiscible with water and an amphiphile, generallyaturbidmilkyemulsionisobtainedwhich,aftersometime,separatesagainintoan aqueous and an organic phase. On the waterrich side, the mixtures consist of stable dispersionsofoildropletsinwater,whichcoagulatewithrisingtemperature.Aspongelike structureisobtainedifthemixturescontainapproximatelyequalamountsofwaterandoil. On the oilrich side, dispersed water droplets are found, which coagulate with decreasing temperature.Thesizeofthedomainsisafunctionoftheamphiphileconcentrationandthe volume fractions of water and oil. Sincemicroemulsionscontainbothapolarcomponent (water)andanonpolarcomponent(oil),theyarecapableofsolubilisingawidespectrumof substrates. The mechanism of solubilisation is similar to that in micellar solutions. The micelles are replaced by the oil domains, which are capable of solubilising all kinds of hydrophobic substances. The solubilisation of polar substances takes place analogously through the aqueous domains of the microemulsion. The solubilisation capacity of microemulsionsisgenerallysuperiortothatofthemicellarsolutionsandcantherefore,affect therateandcourseofacertainreaction. The use of microemulsions as media for organic reactions is a way to overcome the reagentincompatibilityproblemsthatarefrequentlyencounteredinorganicsynthesis.Inthis sense, microemulsions can be regarded as an alternative to phase transfer catalysis. The microemulsion approach and the phase transfer approaches can also be combined, i.e. the reactioncanbecarriedoutinamicroemulsioninthepresenceofasmallamountofphase transfer agent. A very high reaction rate may then be obtained. The reaction rate in a microemulsionisofteninfluencedbythechargeattheinterfaceandthischargedependson the type of surfactant used. For instance, reactions involving anionic reactants may be acceleratedbycationicsurfactants.Thesurfactantcounterionalsoplaysamajorroleforthe reactionrate.Thehighestreactivityisobtainedwithsmallcounterions,suchasacetate,that are only weakly polarizable. Large polarizable anions, such as iodide, bind strongly to the interfaceandmaypreventotheranionicspeciestoreachthereactionzone. Example

SyntheticStrategiesinChemistry 3.19

(i) Nucleophilic substitution reactions were performed in H 2O/CO 2 (w/c) microemulsions formed with an anionic perfluoropolyether ammonium + carboxylate(PFPECOONH 4 )surfactant.Thesereactionsbetweenhydrophilic nucleophiles and hydrophobic substrates were accomplished in an environmentallybenignmicroemulsionwithoutrequiringtoxicorganicsolventsor phasetransfercatalysts. (ii) The reaction between benzyl chloride and potassium bromide to form benzyl bromideisthefirstorganicreactionperformedinw/cmicroemulsion[14].This

reaction betweenanonaqueous compound soluble in CO 2 and a CO 2insoluble saltmaybeexpectedtotakeplaceatornearthesurfactantinterface.

Cl Br + K+Br- + K+Cl-

3.1.4c. Electrochemical Activation Reactive intermediates such as carbocations, carbanions, radicals and radical ions can be electrochemically generated from various electroactive species. Those intermediates may react chemically (C)or electrochemically(E)accordingtoEC,ECEmechanisms.Anodic oxidationsproduceacidicorelectrophilicspecies,whichcanreactwithnucleophilesor(and) eliminateprotonsorelectrophiles.Cathodicreductionsaffordbasicornucleophilicspecies, whichcanreactwithprotonsorelectrophilesor(and)eliminatenucleophiles.Inthisway, using direct electrolytes can selectively perform functional group conversion, substitution reactions, addition reactions, cleavage reactions and coupling reactions. Activation by transitionmetalcatalystsisrequiredwhentheorganicsubstrateisnotelectroactiveorleadsto nondesiredreactions.Themetalcatalysedproceedsbyadoubleactivation: i) chemical activation of the organic substrate by the electrogenerated active form of a transition metal catalyst that generates an organometallic species more easily reduced than the organic substrate, ii) followed by activation by electron transfer of the organometallic species formed in the previous chemical activation step. This double chemical and electrochemical activation causes new reactions to proceed, which involve either, the classical organic reactive species, produced in any electrochemical steps (carbanions) or organometalliccomplexes(anionicorneutral)asthebasisofnewreactivity.

Example

3.20 MethodsBasedonActivatingtheReactingSubstance

Allylationsofaldehyde Using arecyclable electrochemicalprocess (up tofive cycles with excellent yield), a tin mediatedprotocolfortheallylationofaldehydes(95100%yield)isdeveloped[15].

O Br a OH + R R= Alkyl, aryl

Wherea=graphiteelectrode(2.0V),aldehyde(5mmol),allylbromide(8mmol),SnCl 2(10 mmol),H 2O(10mL),r.t.,610h. 3.2. Catalytic Method of Activation ThewordcatalysiscamefromthetwoGreekwords,theprefix,catameaningdown,andthe verblyseinmeaningtosplitorbreak.Acatalystbreaksdownthenormalforcesthatinhibit thereactionsofthemolecules;awidelyaccepteddefinitionofcatalyst being,‘asubstance thatincreasestherateofapproachtoequilibriumofachemicalreactionwithoutitselfbeing substantiallyconsumedinthereactionprocess’.Catalysisisthephenomenonofacatalystin action,whereinloweringoftheactivationenergyisafundamentalprinciplethatappliestoall formsofcatalysis–homogeneous,heterogeneousorenzymatic. Broadlycatalysiscanbedividedintofivecategories: (i) Homogeneous Catalysis: Boththereactantandcatalystsarepresentinthesame phase (ii) Heterogeneous Catalysis: Reactantandcatalystsarepresentinseparatephase, thecatalystissolidandthereactanteitherliquidorgas (iii) Bio Catalysis: Alsoknownasenzymecatalysis.

(iv) Photo Catalysis: Energy for reactions is from light source (hν) (e.g. TiO 2

photocatalyticpurificationandtreatmentofH 2O) 3.2.1. Homogeneous catalysis Homogeneouscatalysisisachemistrytermwhichdescribescatalysiswherethecatalystisin thesamephase(ie.solid,liquidandgas)asthereactantsandproducts. Thehydrolysisofestersbyacidcatalysisisanexampleofthisallreactantsandcatalystare dissolvedinwater: + CH 3CO 2CH 3(aq)+H 2O(l)↔CH 3CO 2H(aq)+CH 3OH(aq)withH catalyst.

SyntheticStrategiesinChemistry 3.21

Example Hydrogenation of maleic acid to succinic acid O H O O H H2, Pd/C OH HO OH HO EtOH H H O Advantage and drawbacks • Highlyefficientintermsofslectivity(i.e.regioselectivity.enantiomericexcesses)and reactionrates,duetothermonomolecularnature. • Catalyst recovery can be very difficult (due to the homogeneous nature of the solution). • Productcontaminationbyresidualcatalystormetalspeciesisaproblem. 3.2.2. Heterogeneous Catalysis Heterogeneous catalysis isachemistrytermwhichdescribescatalysiswherethecatalystis inadifferentphase(ie.solid,liquidand gas,but also oil and water) to the reactants and products.Heterogeneouscatalystsprovideasurfaceforthechemicalreactiontotakeplace on. Example (i) Synthesis of Ammonia by Haber process

3H 2(g)+N 2(g)↔2NH 3(g)catalysedbyFe(s). IntheHaberprocesstomanufactureammonia,finelydividedironactsasaheterogeneous catalyst.Activesitesonthemetalallowpartialweakbondingtothereactantgases,whichare adsorbedontothemetalsurface.Asaresult,thebondwithinthemoleculeofareactantis weakenedandthereactantmoleculesareheldincloseproximitytoeachother.Inthisway theparticularlystrongtriplebondinnitrogenis weakened and the hydrogen and nitrogen moleculesarebroughtclosertogetherthanwouldbethecaseinthegasphase,sotherateof reactionincreases (ii) Hydrogenation of ethene on a solid support Inorderforthereactiontooccuroneormoreofthe reactantsmustdiffusetothe catalyst surface and adsorb onto it. After reaction, the products must desorb from the surface and diffuseawayfromthesolidsurface.Frequently,thistransportofreactantsandproductsfrom onephasetoanotherplaysadominantroleinlimitingthereactionrate.Understandingthese

3.22 MethodsBasedonActivatingtheReactingSubstance transport phenomena and surface chemistry such as dispersion is an important area of heterogeneous catalyst research. Catalyst surface area may also be considered. A pictorial representationofhydrogenationofethaneonsolidsupportisshowninFig.3.5.

Ni or Pd catalyst CH =CH 2 2 CH3-CH3

Fig.3.5.Pictorialrepresentationofhydrogenationofetheneonsolidsurface (Reproducedfromthewebpage:http://en.wikipedia.org/wiki/Heterogeneous_catalysis) Advantages and drawbacks • Heterogeneouslycatalyzedreactionsalloweasyandefficientseparationofhighvalue productsfromthecatalystandmetalderivatives. • However, selectivity and rates are often limited by the multiphasic nature of this systemand/orvariationsinactivesitedistributionfromthecatalystpreparation. 3.2.3. Enzyme or biocatalysis Biocatalysiscanbedefinedastheutilizationofnaturalcatalysts,calledenzymes,toperform chemicaltransformationsonorganiccompounds.Bothenzymesthathavebeenmoreorless isolatedorenzymestillresidinginsidelivingcellsareemployedforthistask. Themostimportant conversion in thecontextof green chemistry is with the help of enzymes. Enzymes are known as biocatalyst and the transformations are referred to as biocatalytic conversions. Enzymes are now easily available and are an important tool in organic synthesis. Biocatalytic conversions have many advantages in relevance to green chemistry.Someoftheseare: • Mostofthereactionsareperformedinaqueousmediumatambienttemperature andpressure. • Biocatalyticconversionsnormallyinvolveonlyonestep. • Protectionanddeprotectionoffunctionalgroupsisnotnecessary

SyntheticStrategiesinChemistry 3.23

• Reactionsarefastreactions. • Conversionsarestereospecific. • Special advantage of biochemical reaction is that they are chemoselective, regioselectiveandstereoselective. Anumberofdiversereactionsarepossiblebybiocatalyticprocesses,whicharecatalysedby enzymes. The major six classes of enzymes and the type reactions they catalyse are discussed. (i ) Oxidoreductases :Theseenzymescatalystoxidationreductionreactions.Thisclass includesoxidases(directoxidationwithmolecularoxygen)anddehydrogenases (whichcatalysethedirectremovalofhydrogenfromonesubstrateandpassitonto asecondsubstrate. (ii) Transferases: Theseenzymescatalysethetransferofvariousfunctionalgroups.Eg. Transaminase (iii ) Hydrolases: Thisgroupofenzymescatalyseshydrolyticreactions.Eg.Esterase (esters) (iv) Lyases: Theseareoftwotypes,onewhichcatalysesadditiontodoublebondandthe otherwhichcatalysesremovalofdoublebond.Bothadditionandeliminationofsmall moleculesareonsp 3hybridisedcarbon. (v) Isomerases: Thesecatalysevarioustypesofisomerisation,e.g.racemases,epimerases etc. (vi) Ligases: Thesecatalysetheformationorcleavageofsp 3hybridisedcarbon. Theenzymesarespecificintheiraction.Thisspecificityofenzymesmaybemanifestedin oneofthethreeways: a. An enzyme may catalyze a particular type of reaction, e.g. esterases hydrolyses only ester. Such enzymes are called reaction specific. Alternatively,anenzymemaybespecificforaparticularclassofcompounds. Theseenzymes are referredtoassubstratespecific, e.g., Urease hydrolyses onlyureaandphosphataseshydrolyseonlyphosphateesters. b. Anenzymemayexhibitkineticspecificity.Forexample,esterasehydrolyse allestersbutatdifferentrates. c. An enzyme may be stereospecific. For example, maltase hydrolyses alpha glycosidesbutnotbetaglycosides.Ontheotherhandemulsinhydrolysesbeta glycosidesbutnotthealphagluycosides.

3.24 MethodsBasedonActivatingtheReactingSubstance

d. It should be noted that a given enzyme could exhibit more than one specificities. The oxidation accomplished by enzymes or microorganisms excel in regiospecificity, stereospecificity and enantioselectivity. An unbelievably large number of enzymatic oxidationshavebeenaccomplished. Examples: (i)Conversionofalcoholintoaceticacidbybacteriumaceticinpresenceofair(theprocessis nowknownasquickvinegarprocess) Bacterium acetic CH CH OH + O CH COOH + H O 3 2 2 3 2 (ii) Conversion of sucrose into ethyl alcohol by yeast (this process is used for the manufactureofethylalcohol. Invertase C H O + H O 2C6H12O6 12 22 11 2 Yeast

Invertase C6H12O6 2C2H5OH + 2CO2 Yeast (iii) Oxidation of Galactose Galactoseoxidase(GO)isafungalenzymethatcatalyzesthetwoelectronoxidationofD. galactosetothecorrespondingaldehydewiththeconcomitantreductionofmolecularoxygen tohydrogenperoxide. OH OH HO CH OH 2 HO CHO GOase O HO + O + H2O2 HO OH OH

(iv)Hydroxylationofaromaticrings BenzeneundergoesoxidationwithPseudomonusputidainpresenceofoxygenandgivesthe cisdiol.

SyntheticStrategiesinChemistry 3.25

Pseudomonus OH Putida

OH Cis-3,5-cyclohexdiene-1,2-diol The path to new chemical entities often shows the limitations of existing tools both in biocatalysisandorganicchemistry.Organicsyntheticprocedurestoprepareacompoundina targetoriented synthesis can damage other functional parts of the molecule. Protection deprotection schemes can lead to a dead end, when a certain protecting group cannot be cleaved off. In biocatalysis, on the other hand, the required biocatalytic toolbox and methodologymightnotbereadilyavailable,thereforelimitingabiocatalyticapproach.New toolboxes,ingredients,andmethodologiesattheinterfaceofclassicalorganicsynthesisand biocatalytic reactions bridge the gap between these two areas.Since product isolationand purification involves a substantial amount of time in the preparation of chemicals, methodologiestosimplifythesetasksarenecessarytogetthepureproductintothebottle withlessworkuptime. Efficientandsafenewpharmaceuticals,intermediatesandanalyticalreagentsneedtobe prepared under certain safety, health, and environmental and economical boundary conditions. Biocatalytic reactions have been shown to overcome these limitations successfullyandarebecomingincreasinglyimportantinindustrialmanufacturing.Building bridges between biocatalysis and organic synthesis will therefore create roads to new syntheticstrategiesandtechnologicalfrontiersofbothfundamentalandpracticalinterest. 3.2.4. Photocatalysis Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysedphotolysis,lightisabsorbedbyanadsorbedsubstrate.Inphotogeneratedcatalysis thephotocatalyticactivity(PCA)dependsontheabilityofthecatalysttocreateelectron–hole pairs,whichgeneratefreeradicals(hydroxylions;OH)abletoundergosecondaryreactions. Itscomprehensionhasbeenmadepossibleeversincethediscoveryofwaterelectrolysisby meansofthe titaniumdioxide. Commercial application of theprocess is called Advanced OxidationProcess(es)(AOP).ThereareseveralmethodsofachievingAOP’sthatcanbutdo not necessarily involve TiO 2oreventheuseofUV.Generallythedefiningfactor is the productionanduseofthehydroxylion. Example (i) Chlorophyll as photocatalyst

3.26 MethodsBasedonActivatingtheReactingSubstance

Chlorophyllofplantsisatypeofphotocatalyst.Photocatalysiscomparedtophotosynthesis, (Figure 6) in which chlorophyll captures sunlight to turn water and carbon dioxide into oxygenandglucose,photocatalysiscreatesstrongoxidationagenttobreakdownanyorganic mattertocarbondioxideandwaterinthepresenceofphotocatalyst,lightandwater.

Fig.3.6.Comparisonofphotocatalysiswithphotosynthesis (Reproducedfromthewebsite:http://www.mchnanosolutions.com/whatis.html)

(ii)Epoxidationoftransandcis2hexenebyTiO 2

ThereactionwascarriedoutonphotoirradiatedTiO2powderusingtrans2hexeneorcis2 hexeneasthestartingmaterial.Fromtrans2hexene,trans2,3epoxyhexanewasobtained asthemainproductwiththeratiobetweentransandcis2,3epoxyhexanebeing98.4to1.6. Inthecaseofcis2hexene,theratiooftranstocis2,3epoxyhexanewas12.0to88.0.

trans-2-hexene trans-2,3-epoxyhexene cis- 2,3-epoxyhexene

O2 TiO O 2 O 98.4 1.6

cis-2-hexene trans-2,3-epoxyhexene cis- 2,3-epoxyhexene

O2 O TiO2 O 12.0 88.0

Alargenumberofrecentreportsonthephotocatalyticreactionoforganiccompoundshave satisfied the basic requirement of organic synthesis, e.g isolation and identification of product.Hencephotocatalysticreactionisnovelsynthetictoolaswellasactivationmode.

SyntheticStrategiesinChemistry 3.27

4. CONCLUSION Thediversityofactivationmethodsinorganicsynthesishasgrownnoticeablyinrecentyears. Thesemethodshavemanyadvantagescomparedtotraditional techniques. They also have somedrawbacks.Asummaryof advantagesanddrawbacks of a few activation methods, whicharediscussedinthisarticlearesummarizedinTable3.1. Table3.1.Summaryoffeaturesofactivationprocesses Activation mode Advantages Drawbacks

Largevolumereactions Hazardoustemperaturecontrol Fastreactions(Eliminationofvolatile Safety problems (reactions in Microwave products) solution) Noreproducibility

Simplenondestructivemethod Lowvolumereaction Noorlittleworkup Costofequipments Pressure Excellentreproduciblity Limitedtohomogeneous reactions(difficultofmixing)

Simplemethod Nogenerality Largevolumereaction Costofequipments Ultrasound Noorlittleworkup Hazardoustemperaturecontrol Adaptabletoheterogeneousreaction

Considerable acceleration of rate Nogenerality Solvophobic constant, Possibilityofhydrolysis interactions Cheapenvironmentallysafemethod

Lowtemperatureefficientmethod No generality (limited to a few Enzymatic Highlyselectivemethod specificreactions) catalysis Costenzymes Sensitivetotemperature

Thisarticleshowsthatthemethodsdescribedarefastandefficient.Relativelyhighyields are achieved in a very short time. The methods and the results when compared with conventional processes are found to be inexpensive, more ecofriendly and high yielding. Thereforechoosingthecorrectmodeofactivationforaparticularreactiondependsentirely on the knowledge of chemists in this area. In conclusion, this article reports important examplesofactivationmodeshighlightingtheimplementationofnewsyntheticstrategies.It maybeofgreathelptochemistsofpresentandfuturegeneration.

3.28 MethodsBasedonActivatingtheReactingSubstance

5. REFERENCES 1. D.BogdaandM.Ukasiewicz, Synlett, 1(2000)143. 2. J.S.YadavandB.V.SubbaReddy, GreenChemistry , 2(2000)115. 3. V.Singh,K.P.Kaur,A.KhuranaandG.L.Kad, Resonance , 3(1998)56. 4. K.Matsumoto,M.Ciobanu,M.YoshitaandT.Uschida, Heterocycls , 45 (1997)15. 5. G.JennerandR.BenSalem, Tetrahedron , 53 (1997)4637. 6. J. Dambacher, W. Zhao, A. ElBatta, R. Anness, C. Jiang and M, Bergdahl, TetrahedronLett., 46 (2005)4473. 7. T.Hamada,K.ManabeandS.Kobayashi, J.Am.Chem.Soc., 126 (2004)7768. 8. P.Gogoi,P.Hazarika,andD.Konwar, J.Org.Chem., 70 (2005)1934. 9. R.X.Ren,J.X.Wu, Org.Lett., 3(2001)3727. 10. M.B.Korzanski,andJ.W.Kolis, TetrahedronLett ., 38 (1997)5611. 11. J.Gao, J.Am.Chem.Soc.,115 (1993)6893. 12. G.Nagendrappa, Resonance , 7(2002)59. 13. N.Iranpoor, H.Firouzabadi andM.Shekarize, Org.Biomol.Chem. , 1(2003)724. 14. B.Gunilla,C.Jacobson,T.LeeandK.P.Johnston, J.Org.Chem., 64 (1999) 1201. 15. Zha,A.Hui,Y.Zhou,Q.Miao,Z.WangandH.Zhang,Org.Lett., 7(2005)1903. Books

1. A.S.Bommarius,B.R.Riebel, Biocatalysis:FundamentalsandApplication ,Wiley –VCHPublisher,2007. 2.V.K.Ahluwalia,N.Kidwai, NewTrendsinGreenChemistry ,KluwerAcademic PublishersandAnamayaPublishers,2004. 3.M.KanekoandI.Okura, Photocatalysis:ScienceandTechnology ,Biologicaland medicalphysicsseries,SpringerPublishers,2003. Websites 1.http://www.bentham.org/aos/Samples/aos11.htm 2.http://books.google.co.in

Chapter - 4 SYNTHESIS OF MATERIALS BASED ON SOLUBILITY PRINCIPLE C.M.Janet 1. INTRODUCTION

Fundamental to the success of materials science and technology is the availability of highquality materials exhibiting specific tailormade properties together with an appropriate shape and microstructure. Solution based methods especially water based mathods offer numerous advantages. Cheap and easy to handle precursors, low cost, simple equipments, low energy input and the ecofriendly nature are few of them. Moreover they allow the easy tailoring of synthesis parameters throughout the whole process,whichmaybeexploitedtoachieveamoreprecisecontrolofcomposition,shape andsizeoftheresultingmaterial.Sincethesynthesisroutedeterminesthepropertiesof thematerial,thepreparationmethodchosenisveryimportantwhendesigningmaterials for specific applications. Wet chemical routes forthesynthesisofnanostructuresarea valuable alternative to conventional processing and gas phase synthesis, with known commercial applications. Solvothermal methods and hydrothermal methods are extensivelyusedinthesynthesisofmaterialsofnovelshapeandproperties.Themajority ofthemetalorganicframeworksreportedtodatehavebeensynthesizedusingsolution based methods under benchtop conditions (2080 °C, 1 atm). Hydrothermal synthesis especiallyhastheattractionthatitfavorsthecondensationofMOHintoMOMbonds, allowingthepreparationofmaterialswithmultidimensional metaloxygen frameworks [1]. Recognizing that most metal salts are preferentially soluble in polar solvents, for example,water,andthattheoppositeistrueformanyorganicreactants,utilizingahigher temperature and biphasic solvothermal method appears advantageous for synthesizing manyclassesofhybridmaterials.Reactionattheinterfaceoftwoimmisciblesolventsis acommontechniqueforcrystallizingcompoundsatlowtomoderatetemperatures(<100 °C)andiscommonlyusedtopreparehybridinorganicorganicmaterials,inadditionto otherproducts.Theuseofbiphasicsolvothermalsynthesishasanumberofattractions overconventionalhydro/solvothermalmethods.Thebasicconceptthatunderliesinwet

4.2 SynthesisofMaterialsBasedonSolubilityPrinciple chemicalmethodsorsolvothermalmethodsaretheprinciplesofsolubility.Hence,itis importanttohaveaclearunderstandingofthefundamentalsofsolubilitytothatitsusein syntheticprocedures. 2. BASICS OF SOLUBILITY Asolutionisahomogeneousmixtureoftwoormoresubstances.Oneofthesubstancesis calledasolvent(asubstanceinwhichothersubstanceorsubstancesaredissolved).The substancesdissolvedinasolventarecalledsolutes.Asolutioncanexistinasolid,liquid orgasformdependingonmixedsubstancesandexternalconditionssuchastemperature and pressure. According to a chemists’ perspective solubility can be understood as a maximumamountofsolutethatcandissolveinasolvent at so called equilibrium. In chemistry, equilibrium is a state where reactants and products reach a balance which means that no more solute can be dissolved in the solvent in the set conditions (temperature, pressure). Such a solution is called a saturated solution. There are two groupsofsubstancesincaseofwhichsolubilitymeasurecannotbeapplied.Theseare miscibleandimmisciblesubstances.Somesolvents,likewaterandalcohol,canbemixed togetherandcreateahomogenousphaseinanyproportion.Asolubilitymeasurecannot beappliedtosuchtwosubstances.Suchsubstancesarecalledmiscible.Ontheotherhand if two substances cannot be mixed together (like water and oil), they are called immiscible[2]. Intheprocessofdissolving,moleculesofthesoluteareinsertedintoasolventand surrounded by its molecules. For this process to take place, molecular bonds between moleculesofsoluteaswellasthatofsolventhavetobedisrupted.Bothoftheserequire energy.Forexamplewhensugardissolvesinwater,newbondsbetweensugarandwater are created. During this process energy is given off. The amount of this energy is sufficienttobreakbondsbetweenmoleculesofsugarandbetweenmoleculesofwater. Thisexampleisrelevanttoanysoluteandsolvent.Ifthebondsbetweenthesolventand solute are too strong and there is not enough energy provided to break them while dissolving,thesolutewillnotdissolve.Thesameenergyrulecanbeappliedtosalts.Salts composed of positive and negative ions which are bound together by the force of attraction of their opposite charges. In cases where energy neededtobreaktheirionic

SyntheticStrategiesinChemistry 4.3 bondsislower,thedissolutionofsaltscantakeplaceonlyifthesufficientenergyisgiven offbyaninteractionoftheionswithsolvent. + Salts that are considered to be soluble are Group I and ammonium (NH 4 ) compounds,nitrates,acetates,chlorides,bromidesandiodides(except:silver(Ag +),lead 2+ 2+ + (II) (Pb ), mercury (I) (Hg 2 ), copper (Cu ) halides) and sulphates (except: Silver (Ag +),lead(Pb 2+ ),barium(II)(Ba 2+ ),strontium(II)(Sr 2+ )andcalcium(II)(Ca 2+ )).Those which are insoluble are carbonates except Group I, ammonium (NH 4+ ) and uranyl compounds, sulfites except Group I and NH 4+ compounds, phosphates except Group I and NH 4+ compounds, hydroxides and oxides except Group I, NH4+ , barium (Ba 2+ ), strontium (Sr 2+ ) and thallium (Tl +) and sulfides except Group I, Group II and NH 4+ compounds.Thesolubilityprinciplethatholdswellinthecaseoforganiccompoundsis “Likedissolveslike”.

3. SOLUBILITY PRODUCT (K SP ) Solubility product constant is a simplified equilibrium constant (K sp ) defined for equilibriumbetweenasolidanditsrespectiveionsinasolution.Itsvalueindicatesthe degree to which a compound dissociates in water. The higher the solubility product constant, the more soluble the compound is. The expression for K sp for a salt is the productoftheconcentrationsoftheions,witheachconcentrationraisedtoapowerequal to the coefficient of that ion in the balanced equation for the solubility equilibrium. Solubilityproductconstantsareusedtodescribesaturatedsolutionsofioniccompounds of relatively low solubility. A saturated solution is in a state of dynamic equilibrium betweenthedissolved,dissociated,ioniccompoundandtheundissolvedsolid.ForSilver chlorideK cisgiveninthefollowingequation.

(1) Where[Ag +]and[Cl ]reperesentconcentrationsofionsofAg +andCl and[AgCl]isa valuerepresentingtheamountofmolesinaliterofsolidAgCl.[AgCl]isaconstantand therefore,theequationcanbewrittenas

Kc[AgCl]=[Ag +][Cl ](2)

4.4 SynthesisofMaterialsBasedonSolubilityPrinciple

The product of equilibrium concentartions of Ag + and Cl isequaltoaconstant.This constantiscalledassolubilityproductconstantorK sp [3]. 3.1 Significance of Le Chatelier’s Principle in Solubility Concept Onceasystemhasreachedequilibrium,therelativeconcentrationsorpressuresofthe speciesinthereactiondonotchange.However,ifyoudisturbthesysteminsomeway, theequilibriumwilladjustuntilanewequilibriumisestablished.LeChatelier’sprinciple statesthat“Ifasystematequilibriumisdisturbedbyachangeintemperature,pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance”. The ionic product (IP) is simplyameasureoftheionspresentinthesolvent.Thismaysoundtrivialbut,infact,it isnotalwaysstraightforwardandtheconceptopensupanumberofinterestingfeatures ofhowsaltsbehaveinsolution.Theproductofthesolubleionsofasaltinsolutionis called the ionic product. The solubility product (Ksp) is the ionic product when the system is in equilibrium. When the ionic product exceeds the solubility product, precipitationwillhappenaccordingtotheLeChatelier’sprinciple.Fromequation(2)this canbeunderstood.AnykindofprecipitationisthusgovernedbytheLechatlierprinciple. Oneofthedaytodaysignificanceofionicandsolubilityproductsarethattheyarethe important basic chemical phenomena underpinning the tooth mineralisation, demineralisationandstability.Additionofacommonionaffectstheionicproduct.This hasimportantimplicationsinthemouthsincetheconcentrationofcalciumandphosphate ionsinsalivaandplaquefluidcanbeinfluencedbyexternalfactors.Solubilityprinciples inthecaseofgasesarequitedifferentfromthatofsolidsandliquids.Asthematerialsof interestaremostlyinsolidstatethesolubilityprinciplesofgasesarenotdealthere. 3.2 Temperature and Pressure Effects on Solubility

Heat+Solidsugar+Water=Dissolvedsugar(3) The equation (3) represents two processes: dissolution going left to right, and crystallizationgoingrighttoleft.Whenthesugarcrystalsaredissolvingatexactlythe same rate that sugar is crystallizing out of solution, the system is at equilibrium. The balance between dissolution and crystallization can be changed by changing the

SyntheticStrategiesinChemistry 4.5 temperatureofthesolution.Addingheatwillfavordissolution.Coolingthesolutionwill favorcrystallization. ThetemperaturedependenceofsolubilityisalsousuallyexplainedusingLeChatelier's principle.LeChatelier'sprinciplepredictsthatheatingthesolutionmixturewillshiftthe equilibriuminfavorofdissolution,toremovetheaddedheat.Thisexplainswhysugaris moresolubleinhotwaterthanincold.Thesolubilityofasubstanceisitsconcentration inasaturatedsolution.Substanceswithsolubilitiesmuchlessthan1g/100mLofsolvent are usually considered insoluble. The solubility is sometimes called "equilibrium solubility"becausetheratesatwhichsolutedissolvesandisdepositedoutofsolutionare equalatthisconcentration. Thesolubilityofsolutesisdependentontemperature.Whenasoliddissolvesina liquid,achangeinthephysicalstateofthesolidanalogoustomeltingtakesplace.Heatis requiredtobreakthebondsholdingthemoleculesinthesolidtogether.Atthesametime, heatisgivenoffduringtheformationofnewsolutesolventbonds. (a) Decrease in Solubility with Temperature: Iftheheatgivenoffinthedissolvingprocessisgreaterthantheheatrequiredtobreak apartthesolid,thenetdissolvingreactionisexothermic(energygivenoff).Theaddition ofmoreheat(increasestemperature)inhibitsthedissolvingreactionsinceexcessheatis already being produced by the reaction. This situation is not very common where an increaseintemperatureproducesadecreaseinsolubility. (b) Increase in solubility with Temperature Iftheheatgivenoffinthedissolvingreactionislessthantheheatrequiredtobreakapart thesolid,thenetdissolvingreactionisendothermic (energy required). The addition of moreheatfacilitatesthedissolvingreactionbyprovidingenergytobreakbondsinthe solid.Thisisthemostcommonsituationwhereanincreaseintemperatureproducesan increaseinsolubilityforsolids.

Theuseoffirstaidinstantcoldpacksisanapplicationofthissolubilityprinciple.A salt such as ammonium nitrate is dissolved in water after a sharp blow breaks the containersforeach.Thedissolvingreactionisendothermicrequiresheat.Thereforethe heat is drawn from the surroundings, the pack feels cold. The Fig. 4.1 represents the

4.6 SynthesisofMaterialsBasedonSolubilityPrinciple effectoftemperatureonthesolubilityofthreedifferentsalts.Solubilityofnitratesaltof Kismorecomparedtothatofchloridesasthetemperatureincreases.

Fig.4.1.Temperaturedependenceofsolubilityofdifferentsalts(ref.2) 4. HETEROGENEOUS EQUILIBRIA AND PRECIPITATION

Phase transitions such as sublimation, deposition, melting, solidification, vaporization, andcondensationareheterogeneousequilibria,soare the formation of crystals from a saturatedsolution,becauseasolidanditssolutionareseparatedphases.Theequilibrium constantsforsaturatedsolutionandsolidformation(precipitate) are alreadydefined as solubilityproduct,Ksp.Forunsaturatedandsupersaturatedsolutions,thesystemisnotat equilibrium,andionproducts, Qsp ,whichhavethesameexpressionas Ksp isused. Anoversaturatedsolutionbecomesasaturatedsolutionbyformingasolidtoreduce thedissolvedmaterial.Thecrystalsformedarecalled a precipitate. Often, however, a precipitate is formed when two clear solutions are mixed. For example, when a silver nitratesolutionandsodiumchloridesolutionaremixed,silverchloridecrystalsAgCl(s) + (aprecipitate)areformed.Na andNO 3 arebystanderions. Ag +(aq)+Cl (aq) →AgCl(s)(precipitate) Silverchlorideisoneofthefewchloridethathasalimitedsolubility.Aprecipitateisalso formedwhensodiumcarbonateisaddedtoasampleofhardwater, 2+ 2 Ca (aq)+CO 3 (aq) →CaCO 3(s)(precipitate).

SyntheticStrategiesinChemistry 4.7

4.1 Solubility Products, Ksp, and Ion Products Qsp Formationsofprecipitatesarechemicalequilibriaphenomena,andweusuallywritethese heterogeneousequilibriuminthefollowingmanner, andcalltheequilibriumconstants solubilityproducts,Ksp.Ifthesolutionisnotsaturated,noprecipitatewillform.Inthis case,theproductiscalledtheionproduct, Qsp . 4.2 Qsp, Ksp and Saturation

Forsomesubstances,formationofasolidorcrystallizationdoesnotoccurautomatically wheneverasolutionissaturated.Thesesubstanceshaveatendencytoformoversaturated solutions. For example, syrup and honey are oversaturated sugar solutions, containing othersubstancessuchascitricacids.Foroversaturatedsolutions, Qsp isgreaterthan Ksp . Whenaseedcrystalisprovidedorformed,aprecipitatewillformimmediatelydueto equilibriumofrequiring Qsp toapproach Ksp . . Sodiumacetatetrihydrate,NaCH 3COO 3H 2O,whenheatedto370Kwillbecomea liquid.Thesodiumacetateissaidtobedissolvedinitsownwaterofcrystallization.The substancestaysasaliquidwhencooledtoroomtemperatureorevenbelow273K.As soonasaseedcrystalispresent,crystallizationoccurrapidly.Insuchaprocess,heatis released,andtheliquidfeelswarm.Thus,therelationshipamong Qsp , Ksp andsaturation isgivenbelow[3]: Qsp<KspUnsaturatedsolution Qsp=KspSaturatedsolution Qsp>KspOversaturatedsolution 5. THROUGH HOMOGENEOUS NUCLEATION For the formation of nanoparticles by homogeneous nucleation, a supersaturation of growthspeciesmustbecreated.Areductionintemperatureofanequilibriummixture, such as saturated solution would lead to supersaturation. Formation of metal quantum dots in glass matrix by annealing at moderate temperatures is a good example of this approach. Another method is to generate a supersaturation through in situ chemical reactions by converting highly soluble chemicals into less soluble chemicals. For example semiconductor nanoparticles are commonly produced by pyrolysis of organometallic precursors. Nanoparticles can be synthesized through homogeneous

4.8 SynthesisofMaterialsBasedonSolubilityPrinciple nucleation in three mediums: liquid, gas and solid: however, the fundamentals of nucleationandsubsequentgrowthprocessesareessentiallythesame.Beforediscussing thedetailedapproachesforthesynthesisofuniformlysizedmonodispersednanoparticles, it is essential to review the fundamentals of homogeneous nucleation and subsequent growth[4]. 5.1 Fundamentals of Homogeneous Nucleation Whenconcentrationofasoluteinasolventexceedsitsequilibriumsolubilityorwhen temperature decreases below the phase transformation point, a new phase appears. A solutionwithsoluteexceedingthesolubilityorsupersaturationpossessesahighGibbs freeenergy.Theoverallenergyofthesystemwillbereducedbysegregatingsolutefrom thesolution.

Fig.4.2.Nucleationandsubsequentgrowth(ref.4) Whentheconcentrationofasoluteincreasesasafunctionoftimenonucleationwould occur even above the equilibrium solubility. Nucleation occurs only when the supersaturationreachesacertainvalueabovethesolubility.Homogeneousnucleationis importantfortheformationofparticlesofuniformsizeandshape. 5.2 Examples of Nucleation Purewaterfreezesat−42°Cratherthanatitsfreezingtemperatureof0°Cifnocrystal nuclei, such as dust particles, are present to form an ice nucleus. Presence of cloud condensationnucleiisimportantinmeteorologybecausetheyareofteninshortsupplyin theupperatmosphere.

SyntheticStrategiesinChemistry 4.9

Fig.4.3.Nucleationofcarbondioxidebubblesaroundafinger(ref.5)

Nucleationinboilingcanoccurinthebulkliquidifthepressureisreducedsothatthe liquidbecomessuperheatedwithrespecttothepressuredependentboilingpoint.More oftennucleationoccursontheheatingsurface,atnucleationsites.Typically,nucleation sitesaretinycreviceswherefreegasliquidsurfaceismaintainedorspotsontheheating surface with lower wetting properties. Substantial superheating of a liquid can be achievedaftertheliquidisdegassedandiftheheatingsurfacesareclean,smoothand made of materials well wetted by the liquid. The creation of a nucleus implies the formationofaninterfaceattheboundariesofthenewphase.Someenergyisconsumed to form this interface, based on the surface energy of each phase. If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enoughtocreateitssurface,andnucleationdoesnotproceed.Thecriticalnucleussize canbedenotedbyitsradius,anditiswhenr=r*(orrcritical)thatthenucleationproceeds [5]. 6. SOLUBILITY AND CRYSTAL GROWTH Crystal formation is made up of three phases: nucleation, growth, and cessation of growth.Duringnucleation,theslowestandmostdifficult phase in crystal growth, the smallest crystal capable of growth forms. The barrier to formation of these smallest crystalsresultsfromdifferencesinthestabilitybetweenmoleculesatthecoreandthose onthesurfaceofthecrystal.Duringnucleation,theaveragestabilityofamoleculeina crystalisverylowbecausetheattractionbetweenthemoleculeandthecrystalisoften less than that between the molecule and the solvent. Thus, it is significantly easier to

4.10 SynthesisofMaterialsBasedonSolubilityPrinciple successfullystimulatethebirthofacrystalbyintroducingasmallercrystal(seeding)than bymerelysupersaturatingasolution.Oncenucleationhasbeenaccomplished,thegrowth phaseofcrystalformationbegins,aphasecharacterizedbytheadditionofmoleculesto the existing crystal. Two opposing forces impact this process: enthalpy and entropy. Growthofacrystalreducestheentropy(ameasureofrandomness)ofthesystembecause amoleculefreeinsolutionhasgreaterentropythanonetetheredtoacrystal’sgrowing surface.Althoughentropyfavorsdissolutionofcrystals,itisenergeticallyfavorablefora moleculetobeaddedtoacrystal.Aboveacriticalsaturationpoint,enthalpyovercomes entropy,andcrystalformationcanoccur. 6.1 Crystal Kinks and Ledges Inadditiontotemperatureandconcentration,thegeometryofagrowingcrystal’ssurface influences growth. Imagine a spherical molecule approaching a planar surface of a crystal.Inthiscase,theadditionofthemoleculeisnotenergeticallyfavorablebecause there is only one point of contact. Next, consider a spherical molecule approaching a "ledge" surface of a crystal. In this case, the addition of the molecule is more energetically favorable because the molecule is stabilized by two points of contact at whichintermolecularforcesholdthemoleculetothecrystal.Inthethirdcase,the"kink" structureprovidesthreepointsofcontact.Thekinkstructureistheidealsurfacegeometry forcrystalgrowth.Becausekinksaresparseonacrystal’ssurface,theadditionofevena smallamountofimpuritiesthateffectivelyclogthekinksitesiscapableofkillingcrystal growth. In order for crystal growth to be maintained, “ledges” or kinks must remain availableformoleculestobeaddedtothecrystal.Atonepointinthefieldofcrystal research,scientistsobservedthatnucleationoccurstooquicklytobeexplainedsolelyby theadditionofmoleculestoaplanarcrystalsurface.Asaresult,thespiraldislocation (SD)theoryarose,statingthatmoleculesareaddedtothesurfaceofacrystalinamanner resultinginthecreationofnewledgesorkinkssothatcrystalgrowthcancontinue.With thedevelopmentofatomicforcemicroscopy(AFM)forphysicallydetectingchangesin depthontheatomicscale,itwaspossibletostudychangesassociatedwithaledgeona crystal.Toobservecrystalformation,thelabusesanatomicforcemicroscopetogather data on the movement of ledges as organic crystals grow and dissolve. This method allowstheinvestigatortoobservetherateandgeometryofledgemovementatvarious

SyntheticStrategiesinChemistry 4.11 concentrationsofsoluteandinthepresenceofimpurities.Itwasnoticedthatdissolution is not exactly the opposite of growth. During dissolution, the ledges are smooth and regular, while they are rough and irregular during growth. Although there are several theoriesonhowtheirregularitymaybeconducivetocrystalgrowth,thedifferenceshave notbeenfullyexplained. 6.2 Practical Significance of Solubility and Crystal Growth

Understandingthegrowthanddissolutionofcrystalsisvaluableinavarietyofcontexts. Crystallization is an effective and economical purification process, and, in some cases wheresubstancescannotbepurifiedbydistillation,itistheonlypracticalmethod.More importantly, approximately eighty percent of all pharmaceutical agents are pure crystalline solids. Understanding the dissolution of a particular crystalline drug could allow chemists to alter the drug’s dissolution rate. Moreover, understanding the conditions of crystallization may allow chemists to predict whether drugs crystallize withinthebody.In2004,thejournalHeartreportedacaseofalargecrystalformingin the heart of woman given a continuous dose of a drug for ventricular tachycardia. A better understanding of this drug’s crystallization could have prevented the incident. Clearly,muchofthisresearchisnotyetcrystalclear. 7. SOLUTION BASED SYNTHETIC STRATEGIES Solution based synthetic strategies involve mainly sol gel process, hydrothermal synthesis,solvothermalsynthesisandreductioninsolution.Amongwhichsolvothermal synthesisandhydrothermalsynthesisarediscussedindetailinthepresentchapter. 7.1 Solvothermal Synthesis Solvothermal synthesis utilizes a solvent under pressures and temperatures above its criticalpointtoincreasethesolubilityofsolidandtospeedupreactionbetweensolids. Most materials can be made soluble in proper solvent by heating and pressuring the systemclosetoitscriticalpoint.Thismethodallowstheeasycontrolonthesolubilityof a solute. And it leads to lower super saturation state which is necessary for the precipitationtohappen.Thereactionsetupusedforsolvothermalsynthesisisgivenin Fig.4.4.

4.12 SynthesisofMaterialsBasedonSolubilityPrinciple

Fig.4.4.Solvthermalsynthesissetup(ref.6) 7.1.1 Synthesis of Semiconductor Chalcogenides Ingeneral,semiconductorchalcogenidesofdifferentsizesandshapesarepreparedusing solvothermal synthesis. In our laboratory we have synthesized CdS nanostructures especiallynanorodsthroughsolvothermalsynthesisusingasolidprecursorCdC 2O4.The reagentusedtoprecipitateCdSwas(NH 4)2S.Thereactionobservedwasrepresentedin theequation6.Herethesolventusedwasethyleneglycolandthereactiontemperature was60 °Candthereactionwascarriedunderaconstantflowofaninertgas[7].

CdC 2O4+(NH 4)2S CdS+(NH 4)2C2O4(6)

CdC 2O4isaninsolublesolidand(NH 4)2Swasliquid.Butthesolvothermalsynthesisat lowtemperaturesallowedinthiscaseaslowreplacementreactionoftheligandforming thenanostructuresofCdS.TheTEMimagesoftheCdSnanorodsaregivenintheFig. 4.5.Bychangingthesolvents,temperatureandtheprecursorvarietyofsizesandshapes canbeproducedthroughthismethod.Whenethylenediamineisusedthematerialformed wasalsofoundtobenanorodswithhighaspectratio.But,whenpyridinewasusedCdS nanoparticlesareonlyformed.

SyntheticStrategiesinChemistry 4.13

Fig.4.5.TEMimageofCdSpreparedfromCdC 2O4usingsolvothermalmethodin ethyleneglycol(ref.7)

Fig.4.6. TEMimageofCdSpreparedfromCdC 2O4usingsolvothermalmethodin ethylenediamineandpyridine(ref.6)

7.1.2 Synthesis of Metal Nanoparticles PtandPdnanoparticlesweresynthesizedbymicrowaveassistedsolvothermalmethod. Theonlydifferenceistheheatingsourceismicrowave.PVPwithanaveragemolecular weight of 40’000 was used as a capping agent in the experiments. H 2PtCl 6, and Palladium(II)2,4pentanedionatewereusedasmetalprecursors.PVPwasdissolvedin methanolorethanolandthenthemetalsaltswereadded.Thereactantswereheatedfor 60 min at 90 º C when methanol was used as a reducing agent and at 120 ºC when ethanolwasusedasareducingagentfor60minundermicrowaveirradiation[8].

4.14 SynthesisofMaterialsBasedonSolubilityPrinciple

Fig.4.7.TEMimagesoftheobtainedPtandPdnanoparticlesunder microwaveassistedsolvothermalconditions(ref.8). 7.1.3 Synthesis of Metal Nitrides and Oxides InanotherapproachsinglesourceprecursorsofhalidesofAl,GaandInwereprepared with urea complexation and further solvothermal synthesis at higher temperatures resultedintheformationoftherespectivenitrides.Thesenitridesareofspecialrelevance inphotovoltaicapplications[9].

(a)(b)(c) Fig.4.8.TEMimagesof(a)AlN(b)GaN(c)AlNnanowiresformedduringsolvothermal synthesis(ref.9). A new and facile route was developed by Suib et al., to manipulate the growth of hierarchicallyorderedMn 2O3architecturesviaasolvothermalapproach.Varioussolvents are employed to control the product morphologies and structures. Mn 2O3 with unique cuboctahedral, truncatedoctahedral, and octahedral shapes are obtained. In a typical synthesisMn(NO 3)2wasdissolvedinanorganicsolventfollowedbyavigorousstirring at room temperature for half an hour in a Teflon liner. Then the Teflon liner was transferredandsealedinanautoclaveforsolvothermaltreatmentat120°Cfor20h.A

SyntheticStrategiesinChemistry 4.15 variety of different solvents was used to investigate the effect of solvents on the morphologyoftheresultantMn 2O3.InordertoinvestigatethedevelopmentofMn 2O3 crystals,thereactionswerealsoconductedatdifferenttemperaturesusingethanolasthe solvent.FESEMimagesoftheMn 2O3synthesizedindifferentsolventsandfordifferent durationaregiveninFig.4.9andFig.4.10respectively.Theimagesindicatetheshape evolution of Mn 2O3 polyhedra. Mn 2O3 is used in catalytic as well as electrocatalytic applications[10].

Fig.4.9.FESEMimagesofproductssynthesizedindifferent solvents:(a)ethanol,(b)1butanol,(c)2butanol,and(d)acetone(ref.10).

Fig.4.10. FESEMimagesofproductssynthesizedunderdifferent reactionperiods(a)1.5h,(b)2h,and(c)3h(ref.10). 7.2 Hydrothermalsynthesisasthenameindicatesthesolventisalwayswater.Ifliquidwater isplacedinanopencontainer,itstemperaturecannotberaisedabove100°C.But,if water is heated in a sealed container, it can be heated to temperatures above 100 °C which means that supercritical properties of the water can be utilized under this

4.16 SynthesisofMaterialsBasedonSolubilityPrinciple condition.Theadvantagesofinducingsupercriticalbehaviorinhydrothermalsynthesis arethatitwillprovideasinglephasebehaviorand give enhanced permeability, mass transportcapabilityanddissolvingcapacities.Hydrothermalsynthesiscanbedefinedasa methodofsynthesisofsinglecrystalswhichdependsonthesolubilityofmineralsinhot waterunderhighpressure.Hydrothermalsynthesisisacenturyoldsyntheticstrategyand itwasusedforthesynthesisofmineralsingeneral.From80’shydrothermalmethodwas utilizedextensivelyfornewermaterialsynthesis.Advantagesofhydrothermalsynthesis areinthismethodnopostheattreatmentisneededandhenceagglomerationwillbeless. After preparation no milling is required which will reduce impurities. Any complex chemicalcompositionscanbesynthesizedbyusingthismethod.Particlesizeorshapes can be controlled in this approach. It can induce self assembly leading to newer and complex architectures of materials and the precursors used are relatively cheap raw materials. And hydrothermal method crystal growth includes the ability to create crystallinephaseswhicharenotstableatthemeltingpoint.Also,materialswhichhavea highvapourpressureneartheirmeltingpointscanbegrownbythehydrothermalmethod. The method is also particularly suitable for the growth of large goodquality crystals while maintaining good control over their composition. Disadvantages of the method include the need of expensive autoclaves, good quality seeds of a fair size and the impossibilityofobservingthecrystalasitgrows.Fig.4.11showsthereactionsetupfor hydrothermalsynthesis.Thepressuregeneratedinsidethereactorcanbereadfromthe pressuregauge.

Fig.4.11.Hydrothermalreactorsetup(ref.6)

SyntheticStrategiesinChemistry 4.17

7.2.1 Synthesis of Oxides and Mixed Oxides

Bariumtitanate(BaTiO 3)perovskiteiswidelyusedinelectronicindustryinmultilayered ceramic capacitors due to its high dielectric constant. Hydrothermal synthesis route is promising due to homogeneity, exact stoichiometry and spherical morphology of BT powders obtained by this synthesis method at low temperature (<200 °C). Using commercially available titania as Ti precursor (Degussa P25) and Barium hydroxide precursorintheBa:Ti=1:1ratioatatemperatureaslowas120 °Cfor48hyielded

BaTiO 3 crystals[11].TheSEMimageofBaTiO 3preparedisgiveninFig.4.12.

Fig.4.12.SEMimageofBaTiO 3crystals(ref.11).

Alkali treatment of commercially available TiO 2 (Degussa p25) in hydrothermal conditions (130 °C) will result in the formation of nanotubes of TiO2. These tubes obtainedonfurtherhydrothermaltreatmentatahightemperatureofaround175 °Cwill yieldnanorodsofTiO 2[12].Thushydrothermalsynthesisprovidesaroutetoplayaround differentmorphologiesofthesamematerialbysimplychangingthetemperature. Hydrothermal treatment of zinc chloride hydrazene hydrate at 140 ºC for 12 h resulted in the formation of flower like microrod bundles. The solution phase is accelerating the process of self assembly through a dissolutionrecrystallization decompositiongrowth process [13]. The chemical reactions are represented in the followingequations.ZnOhasvarietyofapplicationsinphotonicsandoptics.

ZnCl 2+2N 2H4ZnCl 2(N 2H4)2...... (1) 2+ 4+ Zn +2NH 3. H2O Zn(OH) 2+2NH ...... (2)

Zn(OH) 2ZnO+H 2O...... (3)

4.18 SynthesisofMaterialsBasedonSolubilityPrinciple

Fig.4.13.FESEMimagesof(a)precursorZnCl 2(N 2H4)2obtainedatroomtemperature (b)ZnOsamplesobtainedat140°Cand12h(ref.13). 7.2.2 Synthesis of Noble Metal Architectures through Hydrothermal Synthesis Polymer protected (PDDApoly (diallyl dimethylammonium) chloride) noblemetal (including silver, platinum, palladium, and gold) nanostructures in the absence of any seedsandsurfactantscanbesynthesizedusinghydrothermalmethodinwhichPDDA,an ordinary and watersoluble polyelectrolyte, acts as both a reducing and a stabilizing agent. Under optimal experimental conditions, Ag nanocubes, Pt and Pd nanopolyhedrons,andAunanoplatesareobtained.Intypicalsynthesis,PDDAalongwith therespectiveprecursorssuchasAgNO 3(170°Cfor16h),H 2PtCl 6(140°Cfor40h),

H2PdCl 4(190°Cfor40h)andHAuCl 4(170°Cfor12h)atspecificpHwasused[14].

Fig.4.14.TypicalSEMandTEMimagesofthePDDAprotected(a)Agnanocubes(b)Pt nanopolyhedrons(c)Pdnanopolyhedronsand(d)Aunanoplates(ref.14).

SyntheticStrategiesinChemistry 4.19

Apart from metals and metal oxides hydrothermal synthesis method is used in the preparation of zeolites and mesoporous materials. This method was also found to be effectiveinthesynthesisofdifferentcarbonssuchasnanotubes,fullereneanddiamond [15]. 7.2.2 Synthesis of Polymeric Materials through Hydrothermal Synthesis

Polyaniline (PANI) mesostructures have been synthesized under hydrothermal conditions. The mesostructures show different formsfibers, dendrite fibers, textured plates, featureless plates, and spheres [16]. In a typical synthesis, a complex of

FeCl 36H 2O and methyl orange in hydrochloric acid aqueous solution (pH = 4.0) was stirredandtransferredtoanautoclavewithanilinemonomerandkeptat120◦Cfor24h. ThematerialcollectedwasfoundtobePANInanotubes[17].Theformationmechanism andtheTEMimagesofthetubesformedaregiveninScheme1andFig.4.15.Afibrillar complexofFeCl 3andmethylorange(MO)actingasreactiveselfdegradedtemplatesin hydrothermalconditionswasthedrivingforceforthegrowthofnanotubes.MO,which containsahydrophilicgroup(–SO −3 ),possessesananioniccharacteristicwhendissolved inwater.Itcoulddimerizeataparticularconcentrationtoformhigheroligomers.When anilinemonomerwasaddedintothesolution,polymerizationoccurredonthesurfaceof

MOwheretheoxidantFeCl 3wasadsorbedandMOitselfdegradedautomaticallyduring thepolymerizationprocess.

Scheme4.1.PossiblepolymerizationmechanismfortheformationofPANInanotubes (ref.17)

4.20 SynthesisofMaterialsBasedonSolubilityPrinciple

Fig.4.15.TEMofproductswithpHofelectrolyte(4.0)Synthesisconditions: ◦ temperature,120 C;FeCl 3,1.5mmol;MO,0.075mmol;aniline,1.5mmol(ref.17). CONCLUSIONS Thesolubilityofamaterialishavingamajorrolewhiledesigningthesyntheticstrategy of any new materials. Solution phase synthesis always assisted the self assembly and gradualgrowthofthecrystalsofavarietyofmaterialsrangingfrommetals,metaloxides, chalcogenides,polymers,zeolitesandcarbonmaterials.Easytailoringofthemorphology and properties can be achieved if solubility concepts are suitably exploited in new materialsynthesis.Solutionbasedchemistryisalwaysimportantbecauseultimateutility ofmaterialsisgoingtobeinanyformoflifechemistrywhichisfullybasedonaqueous systems. Drug delivery materials, materials in food processing and preservation, medicines and cosmetics are much trivial cases where the solubility concepts are extremelyimportant.Hence,immensecareshouldbetakenwhiledesigningmaterialsfor daytodayapplicationsthroughsolutionchemistry. REFERENCES 1.P.M.Forster,P.M.ThomasandA.K.Cheetham,Chem.Mater. ,14(2002)17. 2.http://www.elmhurst.edu/~chm/vchembook/174temppres.html 3.http://research.yale.edu/ysm/article.jsp?articleID=354 4.Nanostructuresandnanomaterials,Synthesis,Propertiesandapplications,Cao(Editor) (2004),ImperialCollegepress,London. 5.en.wikipedia.org/wiki/Nucleation 6.D.K.KimNanoparticlelecture,2003. 7.C.M.JanetandR.P.Viswanath, Nanotechnology, 17(2006)5271.

SyntheticStrategiesinChemistry 4.21

8.Q.Lu,F.Gao,D.LiandS.Komarneni, AtoZofJournalofMaterialsonline ,1(2005) 1. 9.K.Sardar,M.Dan,B.SchwenzerandC.N.R.Rao, J.Mater.Chem. ,15(2005)2175. 10.W.N.Li,L.Zhang,S.Sithambaram,J.Yuan,X.F.Shen,M.AindowandS.L.Suib, J.Phys.Chem.C. ,111(2007)1469414697. 11.http://www.isoteccluster.at/internal/uploads 12.J.N.NianandH.Teng, J.Phys.Chem.B 110(2006)4193. 13.C.Jiang,W.Zhang,G.Zou,W.YuandY.Qian,J.Phys.Chem.B. 109(2005)136. 14.H.Chen,Y.Wang,S.Dong, Inorg.Chem. 46(2007)10587. 15.http://www.sawyerresearch.com/Newsletters/May03newsletter.pdf 16.L.J.Pan,L.Pu,Y.Shi,T.Sun,R.ZhangandY.O.Zheng, Adv.Fun.Mater. 16 (2005)1279. 17 . H.Huang,X.FengandJ.J.Zhu, Nanotechnology ,19(2008)145607.

Chapter - 5 SOL-GEL TECHNIQUES L.HimaKumar INTRODUCTION Solgel processing generally refers to the hydrolysis and condensation of alkoxidebased precursors.Itcanproduceceramicandglasseswithbetterpurityandhomogeneitythanhigh temperatureconventionalprocess.Solgelprocesscanbeusedtoproduceawiderangeof oxidesinvariousforms,includingpowders,fibers, coatings and thin films, monoliths and porousmembranes.Organic/inorganichybridscanalsobemade,whereagel,usuallysilica, isimpregnatedwithpolymerororganicdyeswithspecificproperties.Theadvantageofsol gel method is, metal oxides which are difficult to attain by conventional methods can be producedbyusingthesolgelprocess. Anotherbenefitisthatthemixinglevelofthesolution isretainedinthefinalproduct,ofteninthemolecularscale. Solgel processing proved useful in the manufacturing of stained glass and for the preparation of oxide materials from solgel precursors. Solgel derived products have numerousapplications.Oneofthepromisingapplicationareasisforcoatingsandthinfilms usedinelectronics,opticalandelectrooptical componentsanddevices,suchassubstrates, capacitors,memorydevices,infrared(IR)detectorsandwaveguides.Antireflectioncoatings arealsousedforautomotiveandarchitecturalapplications.Submicronparticlesizepowders ofsingleandmulticomponentcompositioncanbemadeforstructural,electronic,dental,and biomedicalapplications.Compositepowderscanalsobeusedasagrochemicalsorherbicides. Optical and refractory fibers are used for fiber optics sensors and thermal insulation, respectively.Inaddition,solgeltechniquescanbeusedtoinfiltratefiberperformstomake composites. Glass monoliths and coating and organic/inorganic hybrids are under development for lenses, mirror substrates, graded index optics, optical filters, chemical sensors,passiveandnonlinearactivewaveguides,andlasers.Membranesforseparationand filtration processes also arebeing investigated, as well as catalysts. Biomolecules (such as proteins,enzymes,antibodies,etc.)areincorporatedintosolgelmatrices,whichcanbeused for the monitoring of biochemical processes, environmental testing, food processing, and drugdeliveryformedicineoragriculture. Solgel process, starting with a metal alkoxide solution and going through various processesandoperationsuntilafinalproductisobtained,suchasadensefilm,anaerogel,a ceramicfiberwasshowninFig.5.1.

5.2 SolGelMethods

Fig. 5.1 Schematic representation of the solgel process (reproduced from http://www.chemat.com/assets/images/Flowchat72.jpg)

DEFINITIONS A colloid isasuspensioninwhichthedispersedphaseisso small (~11000 nm) that the gravitationalforcesarenegligibleandinteractionsaredominatedbyshortrangeforces,such asvanderWaalsattractionandsurfacecharges. Asol isacolloidalsuspensionofsolidparticlesinaliquid. An aerosol isacolloidalsuspensionofparticlesinagas. A gel consistsofathreedimensionalcontinuousnetwork,whichenclosesaliquidphase,ina colloidalgel,thenetworkisbuiltfromagglomerationofcolloidalparticlesandislimitedby thesizeofcontainer.Inapolymergeltheparticleshaveapolymericsubstructuremadeby aggregates of subcolloidal particles. Generally, the sol particles may interact by van der Waalsforcesorhydrogenbonds.Agelmayalsobeformedfromlinkingpolymerchains. SOL-GEL SYNTHESIS Solgelsynthesisisaparticularapproachtothepreparationofglassesandceramicsatlow temperatures,whichmayprecedeeitherbythe metallorganicroute , withmetalalkoxidesin

SyntheticStrategiesinChemistry 5.3 organicsolvents,orbythe inorganicroute , withmetalsalts(chlorides,oxychlorides,nitrates, etc)inaqueoussolution. GENERAL MECHANISM Disregardingthenatureoftheprecursors,thesolgelprocesscanbecharacterizedbyaseries ofdistinctsteps. Step1:Formationofthe‘sol’i.e.stablesolutionsofthealkoxideorsolvatedmetalprecursor.

Fig.5.2.Formationof‘sol’(reproducedformref.3) Step2:Gelationresultingfromtheformationofanoxideoralcoholbridgednetwork(the gel )byapolycondensationorpolyesterificationreactionthatresultsinadramaticincreasein theviscosityofthesolution.Ifsodesired,thegelmaybecastintoamoldduringthisstep. Step3:Agingofthegel,duringwhichthepolycondensationreactionscontinueuntilthegel transformsintoasolidmass,accompaniedbycontractionofthegelnetworkandexpulsionof solvent from the gel pores. Ostwald ripening and phase transformations may occur concurrentlywithsyneresis.Theagingprocessofgelscanexceed7daysandiscriticaltothe preventionofcracksingelsthathavebeencast. Step4:Dryingofthegelis,lossofwater,alcohol and other volatile components, first as syneresis(expulsionoftheliquidasthegelshrinks),thenasevaporationofliquidfromwith intheporestructurewithassociateddevelopmentofcapillarystresswhichfrequentlyleadsto cracking.Thisalsoincludessupercriticaldrying,inwhichcapillarystressisavoidedbythe use of supercritical fluids, such as CO 2, in conditions where there are no liquid/vapor densities.Dryingprocessiscomplicatedduetofundamentalchangesinthestructureofthe gel.Thedryingprocesshasitselfbeenbrokenintofourdistinctsteps:

5.4 SolGelMethods

(i)Constantrateperiod,(ii)thecriticalpoint,(iii)thefirstfallingrateperiod,and(iv)the secondfallingrateperiod.Ifisolatedbythermalevaporation,theresultingmonolithistermed a xerogel .Ifthesolventisextractedundersupercriticalornearsupercriticalconditions,the productisan aerogel .

Fig.5.3.Formationofgel(reproducedformref.3)

Fig.5.4.Agingofgel(reproducedformref.3)

SyntheticStrategiesinChemistry 5.5

Step 5: Dehydration, during which surfacebound MOH groups are removed, thereby stabilizingthegelagainstrehydration.Thisisnormallyachievedbycalciningthemonolithat temperaturesupto800°C. Step6:Densificationanddecompositionofthegelsathightemperatures( T >800°C).The poresofthegelnetworkarecollapsed,andremainingorganicspeciesarevolatilized.This stepisnormallyreservedforthepreparationofdenseceramicsorglasses.

Fig.5.5.Densification(reproducedformref.3) INORGANIC ROUTE Thegrowthofmetaloxopolymers inasolventinorganicpolymerizationreactionsthrough hydrolysisandcondensationofmetalalkoxidesM(OR) Z,whereM=Si,Ti,Zr,Al,Sn,Ce, ORisanalkoxygroupandZisthevalenceortheoxidationstateofthemetal,occurintwo steps Firststep:hydrolysis, Metalcations,M z+ ,areformedbydissolvingmetalsaltsinwater,are solvatedbywatermoleculesasfollows: z+ z+ M + hH2O [M(OH 2)h] (1) where h isthe coordination number ofthe cation. Theelectrontransfer weakens the OH interactions of bound water, and dependingon the pH of the solution, various degrees of hydrolysis(deprotonation)canbeinduced: z+ (z1)+ + (z2)+ + [M(OH 2)] [MOH] +H [M=O] +2H (2) Ingeneral,hydrolysisreactionisfacilitatedbyincreaseinthechargedensityonthemetal,the numberofmetalionsbridgedby ahydroxooroxoligand, and the number of hydrogens containedintheligand.ItalsoaffectedbythepH.ThetypicaleffectsofchargeandpHare showninFig.5.6,wherethreedomainscorrespondingtoaquo,hrdroxyandoxoionsare

5.6 SolGelMethods defined.Acidicconditionsforcetheequilibria(Eq.2)totheleftandfavortheformationof hydroxoligands,whereasbasicconditionswillforcetheequilibriatotherightandfavoroxo ligands. The charge on the metal, however, also plays a significant role in the above equilibria with respect to pH, given that highly positive charges on the metals tend to substantiallyweakentheOHbondsandfavortheformationofoxoligands.

Fig.5.6.Relationshipbetweencharge,pHandhydrolysisequilibriumofcations(reproduced formref.1) Secondstep:polycondensationprocessleadingtotheformationofbranchedoligomersand polymerswithametaloxobasedskeletonandreactiveresidualhydroxoandalkoxygroups MOH+XOM MOM+XOH(3) Thiscondensationstepoccursbyeitherofthefollowingways 1. Olation, isacondensationinwhichahydroxylbridgeisformed.

2. Oxolation ,iscondensationinwhichaoxo(O)bridgeisformed.

SyntheticStrategiesinChemistry 5.7

Chemical Reactivity of Metal Alkoxides The chemical reactivity of metal alkoxides can be examined under acidic and basic conditions,asshowninFig.5.7.

Fig.5.7.Polymerstructuresformedinacidorbasicenvironments(reproducedfromref.1) As with initial hydrolysis, condensation reactions may be either acid catalyzed or base catalyzedandeithercasesthereactionproceedsviaarapidformationofchargedintermediate by a reaction with a proton or hydroxide ion, followed by slow attack of second neutral siliconspecies. Underacidicconditions(e.g.withmineralacids),hydrolysisisfasterthancondensation,as showninthefollowingreaction:

or

5.8 SolGelMethods

ForSi(OR) 4n(OH) n, underacidicconditions(pH<4),therateofhydrolysiswillalwaysbe fasterthantherateofcondensationduetotheabilityofOR groupstobetterstabilizethe transition states. Condensation involves the attack of silicon atoms carrying protonated silanol species on neutral SiOH nucleophiles. A bushy network of weakly branched polymersisobtainedunderacidicconditions.

followedby

Fig.5.8.EffectofpHonparticlemorphologyinsolgelreactions(reproducedfromref.6)

Theresultofbasiccatalysisisanaggregation(monomercluster)thatleadstomorecompact highlybranchedsilicanetworksthatarenotinterpenetrablebeforedryingandthusbehaveas

SyntheticStrategiesinChemistry 5.9 discrete species. The differences between acid and base catalyzed reactions and the consequencesforparticlemorphologyareconceptuallyrepresentedinFig.5.8. Gelation Gelationisfreezinginaparticularstructure(i.e.maybeconsideredasarapidsolidification process).Gelationoccurswhenlinksformbetweensilicasolparticlesproducedbyhydrolysis and condensation, to such an extent that a giant spanning cluster reaches across the containing vessel. In other words, as the sol particles grow and collide, lead to the condensationandthenformingmacroparticles.Thesolbecomesagelwhenitcansupporta stress elastically. This is typically defined as the gelation point or gelation time. All subsequentstagesofprocessingdependontheinitialstructureofthewetgelformedinthe reactionbathduringgelation.Underacidcatalysispolymericgelsaregraduallyformed,as depicted in Fig. 5.8. Under basic conditions and/or with higher additions of water, more highly branched clusters are formed, which behave as discrete species. If the total concentrationofalkoxysilaneislow,e.g.,<~0.3M,gelationleadstoformationofcolloidal silica (Stoeberprocess). Aging Whenagelismaintainedinitsporeliquid,itsstructureandpropertiescontinuetochange longafterthegelpoint.Thisprocessiscalledaging.Duringaging,fourprocessescanoccur, singly or simultaneously, including polycondensation, syneresis, coarsening, and phase transformation. Polycondensation reactions occur continuously, within the gel network as long as neighboringsilanolsarecloseenoughtoreact.Thisincreasestheconnectivityofthenetwork anditsfractaldimension.Usuallyinalkoxidebasedgels,thechemicalhydrolysisreactionis rapid,especially,whenthesolisacidcatalyzed,andiscompletedintheearlystagesofsol preparation. Since the chemical reaction is faster at higher temperatures, aging can be acceleratedbyhydrothermaltreatment,whichincreasestherateofthecondensationreaction. Syneresisisthespontaneousshrinkageofthegelandresultingexpulsionofliquidfrom the pores. Syneresis in alcoholic gel systems is generally attributed to formation of new bonds through condensation reactions, which increases the bridging bonds and causes contraction of the gel network. In aqueous gel systems or colloidal gels, the structure is controlledbythebalancebetweenelectrostaticrepulsionandattractivevanderWaalsforces. Therefore,theextentofshrinkageiscontrolledbyadditionsofelectrolyte.

5.10 SolGelMethods

Coarsening or Ostwald ripening is the irreversible decrease in surface area through dissolution and reprecipitation processes. Structural changes attributed primarily to surface energyeffects.Itiswellknownthatsurfacesexhibitingpositiveradiiofcurvaturedissolve morereadilythansurfacesexhibitingnegativeradiiofcurvature.Ifa gelisimmersedina liquid in which it is soluble, dissolved material will tend to precipitate into regions of negative curvature. Therefore, as the dissolution rate increases, dissolution redeposition resultsinneckformation,causingthegelstructuretobecomefibrillarandtheporeformation. Further,whendissolutionisextensive,thegelnetworkwouldbreakdownandripentoforma colloidalsol. Duringaging,therearechangesinmostphysicalpropertiesofthegel.Time,temperature, solvent and pH conditions affect the aging process and lead to structural modifications. Heatingthegelinwaterat80100 °C canstrengthenthe weak gelstructureand generally bringsaboutreinforcement,butdoesnotmodifytheporestructure.ThepHofthewashwater duringthewashingofporeliquoroutofgeliscriticalinthecaseofgelsmadefromacid catalyzed silicate precursors. The final properties of such gels depend on both the pH at whichthegelwasformedandthepHatwhichitwasagedbeforedrying. Drying Drying is nothing but removing of the solvent phase. The method is influenced by the intendeduseofthedriedmaterial.Ifpowderedceramicsaredesired,nospecialcareneedbe exercisedtopreventfragmentation;ifmonolithsfromcolloidalgelsaredesired,thedrying proceduresarelargelydeterminedbytheneedtominimizeinternalstressesassociatedwith thevolumechangesondryingandthecapillaryforcesinthegelpores. Therearethreestagesofdrying: • During thefirst stageof drying, thevolumechange of the gel equals the volume of evaporated liquid. The gel network is still flexible and can rearrange to accommodate the decreasing volume. The compliant gel network is deformed by the large capillary forces, which causes shrinkage of the object. All pores are filled with solvent and no liquidair interfacesarepresent.Inclassicallargeporesystems,thisfirststageofdryingiscalled“the constant rate period” because the evaporation rate per unit area of the drying surface is independentoftime.Firststageofdryingendswhenshrinkagestops. • Secondstageofdryingbeginswhenthe“criticalpoint”isreached(Fig.8).Thepacking densityofthesolidphaseincreaseswithincreasingthestrengthofthenetwork,andresists further shrinkage. This stage is called critical point.Asdryingproceeds,thegelnetwork

SyntheticStrategiesinChemistry 5.11 becomesmorerestrictedandtheremovalofliquidleadstotheformationofsuchinterfaces andthedevelopmentofcapillarystresses.Theliquidflowstothesurfacewhereevaporation takes place. The flow is driven by the gradient in capillary stress. Because the rate of evaporationdecreasesinsecondstage,classicallythisistermed“thefirstfallingrateperiod’.

Fig. 5.9. Schematic representation of gel surface at the end of first stage (critical point)(reproducedfromref.4) • When the pores have substantially emptied, surface films along the pores cannot be sustained.Thisisthirdstageofdrying,whichisalsocalledas“secondfallingrateperiod”. Theremainingliquidcanescapeonlybyevaporationfromwithintheporesanddiffusionof vaportothesurface.Duringthisstage,therearenofurtherdimensionalchangesbutjusta

5.12 SolGelMethods slow progressive loss of weight until equilibrium is reached, determined by the ambient temperatureandpartialpressureofwater. • Thecapillarypressure,P,developedinacylindricalcapillaryofradiusr,partiallyfilled withaliquidofwettingangle α,canbeexpressedby: 2γ P = cosα r where γ is the surface tension. When evaporation leads to the formation of menisci, the differentradiioftheporescauseunequalcapillarypressurestogeneratedifferentialstresses. If the stress difference locally exceeds the strength of the gel network, a crack will result (dryingfailure).Whenthegelhashadinsufficientagingandstrength,doesnotpossessthe dimensionalstabilitytowithstandtheincreasingcompressivestressandleadstocrackingin thefirststage.Mostfailureofdryingoccursduringtheearlypartofsecondstage,thepointat whichthegelstopsshrinking.Thisisthepointatwhichthemeniscusfallsbelowthesurface. Generally, fractures associated with excessive capillary forces can be eliminated or reducedbyoneormoreofthefollowingproceedings: • strengtheningthegelbyreinforcement • enlargingthepores • reducingthesurfacetensionoftheliquidwithsurfactants • makingtheinteriorsurfaceshydrophobic • evacuatingthesolventbyfreezedrying • Operatingunderhypercriticalconditions. Xerogel and Aerogel Formation As a gel is evaporated to dryness either by thermal evaporation or supercritical solvent extraction,thegelstructurechangesinsomecasessubstantially. During thermaldrying or roomtemperatureevaporation,capillaryforcesinducestressesonthegelthatincreasesthe coordinationnumbersoftheparticlesandinducescollapseofthenetwork.Theincreasein particlecoordinationnumbersresultsintheformationofadditionallinkagesthatstrengthen thestructureagainstfurthercollapse andeventuallyleadstotheformationofarigidpore structure. The structure of the resulting xerogel can therefore be considered a collapsed, highlydistortedformoftheoriginalgelnetwork. Thesupercriticalextractionofsolventfromageldoesnotinducecapillarystressesdueto thelackofsolventvaporinterfaces.As aresult,thecompressive forces exertedonthe gel

SyntheticStrategiesinChemistry 5.13 networkaresignificantlydiminishedrelativetothosecreatedduringformationofaxerogel. Aerogelsconsequentlyretainastrongerresemblancetotheiroriginal gelnetworkstructure comparedtoxerogels(Fig.5.10).

Fig. 5.10. Structural relationship between a solgel precursor, a xerogel, and an aerogel (reproducedfromref.6) Densification Densification is the last treatment process of gels duringwhich thedried gelisheated to convertintoadenseceramic.Thesinteringofgels,wheretheporenetworkiscollapsedand organicspeciesarevolatilized,isacriticalfactorindeterminingthesizeandmorphologyof thesolgelproduct.Forsilicagels,thefollowingreactionsoccurduringthedensification: • desorptionofphysicallyadsorbedsolventandwaterfromthewallsofmicropores(100 200°C)

• decompositionofresidualorganicgroupsintoCO 2+H 2O(300500°C) • collapseofsmallpores(400500°C) • collapseoflargerpores(700900°C) • continuedpolycondensation(100700°C) Ifpowderedceramics aredesired,noneedtotakecaretopreventfragmentation.Attention must,however,bedirectedtotheremovaloforganicstoavoidundesirablebloating,foaming or blackening. Where as, for monoliths preparation, special care must be taken to ensure completeremovalofwater,organic groupsordecomposition products, prior to micropore collapsetoavoidthedevelopmentofstressesleadingtofragmentationfortheproductionof largemonolithsofSiO 2.

5.14 SolGelMethods

Sinteringanddensificationphenomena alsotakeplace,viatypicalsinteringmechanismssuch asevaporationcondensation,surfacediffusion,andgrainboundaryandbulkdiffusion.The small particle size of the powders lead to high reactivities and enhanced sintering and/or coarseningrates(theprincipalprocessinvolvedindensificationisoftenviscoussintering). AsillustratedinFig.5.11,densificationofagelnetworkoccursbetween1000and1700°C dependingupontheradiioftheporesandthesurfacearea.Fig.5.11,describestheoverall development of a solgel process for glass in relative time and temperature scales. This schematicrepresentationsummarizesfromestablishmentofasolconditiontoformationofa densematerial.

Fig.5.11.Timevs.Temperaturedependenceinasolgelprocess (reproducedfromref.4)

APPLICATIONS Monoliths The physical properties of gelderived glasses are usually closely similar to those of glasses obtained from the melt. Gelderived powders are used as batch ingredients for glassmeltingprimarilybecauseofthehighdegreeofchemicalhomogeneitytheyoffer. This leads to shorter melting times and lower melting temperature as well as compositional uniformity. The key to glass formation is the development of an appropriate heat treatment schedule to remove the residual organic groups and achieve porecollapsewithoutinducingcrystallizationinthesample.Themostattractivefeatureis thedevelopmentofnovelglasscompositions:CaOSiO 2 or Na 2OZrO 2SiO 2 with high

ZrO 2content,whichisimpossibletoobtainfromthemelt,becausethecoolingratemust

SyntheticStrategiesinChemistry 5.15 be high to avoid detectable crystallization. Monoliths are defined as bulk gels cast to shapeandprocessedwithoutcracking.Monolithicgelsarepotentiallyinterestingbecause complex shapes may be formed at room temperature and consolidated at rather low temperatureswithoutmelting.Theprincipleapplicationsofmonolithsareopticalones: fiber optic preforms, lenses and other nearnetshape optical components, graded refractive index glasses and transparent foams (Aerogels) used as Cherenkov detectors andassuperinsulation. Fibers Fiberscanbedrawndirectlyfrompolymersolsbycontrollingtheviscosityofacidcatalyzed alkoxidegels.Akeytopreventprematuregelationistomaintainwateratalowerlevel.High viscosity stabilizes the fibers against spheroidization during the drawing operation. Both silicaandtitaniafibershavebeenobtainedatnearroomtemperature,butwithoutsintering, they remain porous and contain some unreacted alkoxide. The porosity decreases with increasingtemperature.Solgelmethodusingmetalalkoxideasprecursorhasbeenappliedto thepreparationofoxideglassfibersotherthanSiO 2.Fibersproducedbysolgelprocessing are an interesting application for the manufacture of optical waveguides (A device that constrainsorguidesthepropagationofelectromagneticradiationalongapathdefinedbythe physical construction of the guide is called waveguide). The dispersioncasting technique produces silicafiberswithopticallosesofabout3.5dB/km. Thin films and coatings Thin films and coatings were the first commercial application of solgel processing technology. Films are applied either by dip coating or spin coating techniques, using polymeric alkoxidesols. By controlling theprecursor chemistry anddeposition conditions, poresize,porosity,surfacearea,andrefractiveindexofthefilmscanbecontrolled. Large substratesmaybeaccommodatedanditispossibletouniformlycoatbothsidesofplanarand axiallysymmetricsubstratessuchaspipes,tubes,rodsandfibers. Generally, ceramics are more resistant than metalstooxidation,corrosion,erosionand wear. Moreover, they can have good thermal and electrical properties that make them particularlyinterestingascoatingmaterials.Ceramiccoatingsareusuallydepositedonmetals for improving their performances in high temperature aggressive environments. Some important applications, include improving resistance against gas, solid, condensed, and moltenphasecorrosion,tolocalizedoverheatingandmelting;decreasingfrettingandwear; decreasingheatlossesand/orreflectingradiationsinhightemperaturesystems.

5.16 SolGelMethods

Amajordevelopmentwastheformationofmultipletransparentlayersthatproduceagradual change in refractive index from the air interface to that of the substrate. Antireflection coatings are also a vast application of the solgel technique, for example, in store front windows,toallowlighttransmissionbutreducedglare.Suchcoatingsarealsousedinsolar cells and laser optics. A head up display is a unique application of these coatings: by controllingthereflectancetotransmissionratio,thespeedoftheautomobilecanbedisplayed at eye level on the windshield, without distorting the field of view. The driver no longer needstoshifthiseyestotheinstrumentpanel. Electrochromic devices Activeelectronicthinfilmsincludehightemperaturesuperconductors,conductiveindiumtin oxide(ITO)andvanadiumpentoxide,andferroelectricbariumandleadtitanates, electrochromictungstenoxide,andtitaniafilmsusedasphtotoanodes.Solidstate electrochromic(EC)devicesforsmartwindows,largeareadisplaysandautomotiverearview mirrorsareofconsiderabletechnologicalandcommercialinterest.

Fig.5.12.SchematicsofanECwindowdevicedepositedononesubstrate.The electrochromiclayerisanode,cathodeorboth(reproducedfromref.7). Controlled modulation of solar radiation through smart windows has a large potential in buildingsforenergysavingsandinautomobilestoincreaseoccupantcomfortwhilereducing airconditioningrequirements.Typicallythesedevicesconsistofmultiplelayerstacksthatare sequentially vacuum deposited or are laminate constructions. Solgel processing offers advantageindepositingfilmscontainingmultiplecationsandcontrollingthemicrostructure. These parameters can influence the kinetics, durability, coloring efficiency and charge storage in the electrochromic films (electrodes). A typical structure of a solid state electrochromicwindowisshowninFig.5.12. Otherapplicationsofsolgeltechniquesinclude  Synthesisofcarbidessulphidesandnitrides

SyntheticStrategiesinChemistry 5.17

 Synthesisofnanocomposites  Synthesisofpowders,grainsandspheres  Catalysis(Mesoporousmaterials)  Preparingsolidelectrolytes  Absorbingcoatings  Filters forlightingandopticalpurposes  Semiconductingcoatings  Protectivelayers (bothchemicalandthermal)  Scratchresistantcoating  Embeddingorganicmolecules(hybridorganicinorganicmaterials)  Flammablegassensors  Porousgelsandmembranes REFERENCES 1.C.J.Brinker,andG.W.Scherer,SolGelScience:ThePhysicsandChemistryofSolGel Processing,AcademicPressInc.,NewYork,1989. 2.J.D.WrightandN.A.J.MSommerdijk,Solgelmaterialschemistryandapplications,vol.4, Taylor&FrancisBooksLtd.,London,2001. 3.R.D.Gonzalez,T.LopezandR.Gomez,SolGelpreparationofsupportedmetalcatalysts. CatalysisToday,35(1997)293. 4.L.C.Klein,(Ed.),SolGelOptics:ProcessingandApplications,KluwerAcademic Publishers,Boston,1993. 5.L.L.HenchandJ.K.West,Thesolgelprocess.Chem.Rev.,90(1990)33. 6.B.L.Cushing,V.L.KolesnichenkoandC.J.O’Connor,RecentAdvancesintheLiquid PhaseSynthesesofInorganicNanoparticles.Chem.Rev.,104(2004)3893. 7.A.Agrawal,J.P.CroninandR.Zhang,Reviewofsolidstateelectrochromiccoatings producedusingsolgeltechniques.SolarEnergyMaterialsandSolarCells,31(1993)9.

Chapter - 6 TEMPLATE BASED SYNTHESIS B.Kuppan INTRODUCTION Tomakeajar,apieceofwoodofthedesiredshapeisfirstfabricated,andlayerof clay is applied to the wood. Heat treatment of the clay/wood composite at high temperaturegeneratesaceramicjar.Duringtheheattreatment,clayistransformedto a ceramic material and the wood is burned off leaving the empty space in the resultingjar.Whenthisprocessisscaledowntonanometerregime,itisbasicallythe templatesynthesisprocess[1]. Fig.6.1.Schematicrepresentationoftemplatesynthesis(reproducedfromref.1) Oneclassoftemplatesaresurfactantsthatareusedtoproducemesoporousmaterials. Itistruetosayoneofthemostexcitingdiscoveriesinthefieldofmaterialsynthesis overthelast15 yearsistheformationofmesoporoussilicate andaluminosilicate molecularsieveswithliquidcrystaltemplates.Thisfamilymaterialsgenerallycalled as M41S family (MCM41, MCM48 and MCM50) [2]. These M41S family materialscanbepreparedbyusingcationicsurfactantsasthetemplates,andother kindofmesoporousmaterialsalsocanbepreparedbyusingnonionicsurfactantsas thetemplates.ThesefamilymaterialsarecalledasSBAnfamily[3]. Theabovementionedmesoporousmolecularsievesalsocanbeusedastemplates to prepare ordered mesoporous carbon by using sucrose as the carbon precursors. These mesoporous silicates, aluminosilicates and mesoporous carbon materials are

6.2 TemplateBasedSynthesis moreimportantinthefieldofcatalysisbecauseofitshighsurfacearea,narrowpore sizedistributionandlargenumberofsurfacefunctionalgroups.Thesurfacesofthese materialscanalsobetuneddependingontheapplications. Anotherkindoftemplatebasedsynthesisistopreparefreestanding,nonoriented andorientednanowires,nanorodsornanotubes.Thefabricationofmetalnanowires had potential applications in the microelectronic industry and in particular, for interconnectioninelectroniccircuits.Theprocedureisbasedonmetaldisplacement reaction leading to the growth of metal nanowires into the pores. The galvanic displacement reaction for the synthesis of core /sheet nanostructured materials has beeninvestigatedintheliterature[4]. Nanostructured materials have attracted much interest because of their unique properties.Infact,duetotheirstructurefeaturesandsizeeffects,theyshowphysical propertiesthataredifferentfrombulkmaterials.Manymethodshavebeendeveloped for the fabrication of nanowires. Among these methods template synthesis is considered as quite useful, because it can be used for the preparation of different typesofnanostructures.Thehighorderdegreeofporousstructureofanodicalumina membrane(AAM)(consistinginaclosepackedarrayofcolumnarhexagonalcells, eachcontainingacentralcylindricalporenormalto the surface), makes it an ideal template for the fabrication of nanostructured materials,suitableforapplicationsin optoelectronics,sensors,magneticmemoriesandelectroniccircuits[5]. Synthesis, Mechanism and Pathway A large number of studies have been carried out to investigate the formation and assemblyofmesostructuresonthebasisofsurfactantselfassembly.Theinitialliquid crystaltemplatemechanismfirstproposedbyMobil’sscientistsisessentiallyalways “true” because the pathways basically include almost all possibilities. Two main pathways that is cooperative self assembly and “true” liquid –crystal templating process,seemstobeeffectiveinthesynthesisoforderedmesostructures. Surfactants for the formation of templates The term surfactant is a blend of “surface acting agent” surfactants are usually organiccompoundsthatareamphophilicinnature.Amphophilicmeanstheycontain

SyntheticStrategiesinChemistry 6.3 bothhydrophobicgroups(tails)andhydrophilicgroups (heads). Therefore they are solubleinbothorganicsolventsandwater. Generallyaclearhomogeneoussolutionofsurfactantsinwaterisrequiredtoget ordered mesostructures. Frequently used surfactants can be classified into cationic, anionicandnonionicsurfactants. Table6.1.Cationicsurfactants (Reproducedfromref.7)

6.4 TemplateBasedSynthesis

Quaternarycationicsurfactants,C nH2n+1 N(CH 3)3Br(n=822),aregenerallyefficient for the synthesis of ordered mesoporous silicate materials. Commercially available CTAB (cetyltrimethylammonium bromide) is often used. Gemini surfactants, multiheadgroupsurfactants,andrecentlyreportedcationicfluorinatedsurfactantscan alsobeusedastemplatestopreparevariousmesostructures.Frequentlyusedcationic quaternaryammoniumsurfactantsareshowninthetablebelow First reports of mesoporous silica from Mobil’s company, cationic surfactants were used as structure directing agents (SDAs). Cationic surfactants have excellent solubility, have high critical micelle temperature (CMT) values and can be widely usedinacidicandbasicmedia.Buttheyaretoxicandexpensive. Anionic surfactants include carboxilates, sulphates, sulfonates, and phosphates; recently a kind of labmade anionic surfactant terminalcarboxylicacids isusedto templatethesynthesisofmesoporoussilicaswiththeassistanceof aminosilanesor quaternary aminosilanes such as 3aminoprpoyltrimethoxysilane (APS) N trimethoxylsilylpropylN, N, Ntrimethylammonium chloride (TMAPS) as co structuredirectingagents(CSDAs). Anionic surfactants

Fig.6.2.Representationofanionicsurfactants(reproducedfromref.7)

SyntheticStrategiesinChemistry 6.5

Table6.2.Nonionicsurfactants(reproducedfromref.7)

Nonionicsurfactantsareavailableinawidevarietyofdifferentchemicalstructures. Theyarewidelyusedinindustrybecauseofattractivecharacteristicslikelowprice, nontoxicity, and biodegradability. In addition the self assembling of nonionic surfactants products mesophase with different geometries and arrangements. They becomemoreandmorepopularandpowerfulinthesynthesesofmesoporoussolids. The syntheses that largely promote the development of mesoporous materials are simpleandreproducible.Afamilyofmesoporoussilicamaterialshasbeenprepared withvariousmesoporouspackingsymmetriesandwelldefinedporeconnectivity.

6.6 TemplateBasedSynthesis

Cooperative Self-Assembly of Surfactant and Silica Source to Form Mesostructure This pathway is established on the basis of the interaction between the silicates surfactantstofrominorganicorganicmesostructurecomposites. Cooperative Self-Assembly Mechanism

Fig.6.3.schematicdiagramofthecooperativeselfassemblyofsilicatesurfactant mesophase.(reproducedfromref.6) Alayertohexagonalmechanism(foldedsheetmechanism)waspostulatedbyKuroda and coworkers, according to which the mesostructure is created from a layer kanemite precursors. Silicate polyanions such as silicate oligomers interact with positively charged groups in cationic surfactants driven by Coulomb forces. The silicate species at the interface polymerized and crosslink and further change the

SyntheticStrategiesinChemistry 6.7 charge density of the inorganic layers. With the proceeding of the reaction, the arrangementofthesurfactantsandthechargedensitybetweeninorganicandorganic species influence each other. Hence the compositions of the inorganicorganic hybrids differ to some degree. It is the matching of charge density at the surfactants/inorganic species interfaces that govern the assembly process. The final mesosphere is the ordered 3D arrangement with the lowest interface energy. The transformationoftheisotropicmicellarsolutionsofCTABintohexagonalorlamellar phasewhenmixedwithanionicsilicateoligomersinhighlyalkalinesolutionswere indeeddetectedthroughacombinationofcorrelatedsolutionstate 2H, 13 C,and 29 Si nuclear magnetic resonance (NMR) spectroscopy, small angle xray scattering (SAXS), and polarized optical microscopic measurements. The mechanism in differentsurfactantsystemshasbeenstudiedusingNMRtechnique. This cooperative formation mechanism in nonionic surfactant system was investigated by insitu techniques. Goldfarb and coworkers investigated the formationmechanismofmesoporoussilicaSBA15,whicharetemplatedbytriblock copolymer P123 (EO 20 PO 20 EO 20 ) by using direct imaging and freezefracture replication cryoTEM techniques, in situ electron paramagnetic resonance (EPR) spectroscopy, and electron spinecho envelope modulation (ESEEM) experiments. They found a continuous transformation from speriodial micelles into threadlike micelles.Bundleswerethenformedwithdimensionthataresimilartothosefoundin thefinalmaterials.Theelongationofmicellesinaconsequenceofthereductionof polarity and water contact within the micelles due to the adsorption and polymerizationofsilicatespecies.Beforethehydrothermaltreatment,themajorityof PEOchainsinsertintosilicateframeworks,whichgeneratesmicroporesafterremoval oftemplates.MoreovertheyfoundthattheextentofthePEOchainslocatedwithin thesilicamicroporesdependedonthehydrothermalagingtemperatureandSi/P123 molarratio.TheformationdynamicsofSBA15studiedbyFlodstrometal.onthe basisoftimeresolved insitu 1HNMRandTEMinvestigations.Theyobservedfour stages during the cooperative assembly, which are the adsorption of silicates on globularmicelles,theassociationofglobularmicellesintofloes,theprecipitationof floes,andthemicellemicellecoalescence.Khodakovetal.proposedastructurewith

6.8 TemplateBasedSynthesis a hydrophobic PPO core and a PEO –water –silicate corona in the first stage. The cylindricalmicellespackintothedomains.Atthesametime,solventsarereplacedby condensedsilicatespecies. Thesemechanismsconsidertheinteractionsonthesurfactants/inorganicspecies interfaces. Monnier and Huo et al. gave a formula of the free energy in the whole process.

G= Ginter + Gwall + Gintra + Gsol

In which Ginter associated with interaction between the inorganic walls and surfactantmicelles, Gwall isthestructuralfreeenergyfortheinorganicframeworks,

Gintra isthevanderWaalsforceandconformationalenergyofthesurfactant,and

Gsol isthechemicalpotentialassociatedwiththespeciesinsolutionphase.

For surfactanttemplating assembly mesostructured silicates, Gsol can be regardedas aconstantina givensolutionsystem. Therefore, the key factor is the interactionbetweensurfactantandinorganicspecies,suchasthematchingofcharge density.Themorenegative Ginter is,themoreeasilytheassemblyprocesscanbe proceeded. Elaborate investigations on mesoporous materials have been focused on understanding and utilizing the inorganicorganic interactions. Table 6.3 lists the mainsynthesisroutesandthecorrespondingsurfactantsandclassicalproducts. Stuckyandcoworkersproposedfourgeneralsyntheticroutes,whichareS +I,S I+,S +XI+,andS X+I (S +=surfactantcations,S =surfactantanions,I +=inorganic precursorscations,I =inorganicprecursorsanions,X +=cationiccounterions,and X=anioniccounterions).Toyieldmesoporousmaterials,itisimportanttoadjustthe chemistry of the surfactants headgroups, which can fit the requirement of the inorganic components. Under basic conditions, silicate anions (I ) match with the surfactantcations(S +)throughCoulombforces(S +I).Theassemblyofthepolyacid anions and surfactant cations to “salt”like mesostructures also belongs to S +I interaction.Incontrasttothis,oneoftheexamplesofS +Iinteractionoccursbetween 7+ cationic keggin ion (Al 13 ) and anionic surfactants like dodecyl benzenesulfonate salt.

SyntheticStrategiesinChemistry 6.9

Theorganicinorganicassemblyofsurfactantsandinorganicprecursorswiththesame charge is also possible. However, counter ion is necessary. For example, in the syntheses of mesoporous silicates by the S +XI+ interaction, S + and I are cationic surfactantsandinorganicanionicprecursors,andXcanbehalogenions(Cl ,Br ,and 2 + + I ),SO 4 ,NO 3 ,etc.Instronglyacidicmedium,theinitialS X I interactionthrough Coulomb forces or more exactly, doublelayer hydrogen bonding interaction, gradually transforms to the (IX) S+ one. It was the first time that the mesoporous silica was synthesized under a strongly acidic condition. Here anions affect the structures,regularity,morphologies,thermalstability,andpropertiesofmesoporous silicas. The Hofmeister series of the anions are one of the possible reasons that changethehydrolysisratesofthesilicateprecursorsandthemicellarstructures. SchematicillustrationofthetwotypesofinteractionsbetweenAPS(A)orTMAPS (B)andanionicsurfactantheadgroups. Fig.6.4.interactionsbetweenAPS(A)orTMAPS(B)andanionicsurfactant headgroups(reproducedfromref.7) Comparedwiththosecationicsurfactants,therepulsiveinteractionbetweenanionic surfactantsandsilicatespeciesfailstoorganizeorderedmesostructures.Concerning the charge matching effect, Che et al. demonstrated a synthetic route to create a familyofmesoporoussilicastructuresunderbasicconditionsbyemployinganionic

6.10 TemplateBasedSynthesis surfactantsasSDAsAPSorTMAPSasCSDAs.Thisroutecanbedescribedasan“S N+I“pathway,whereN +arecationicaminogroupsoforganoalkoxysilanes.Fig.6.4 gives the schematic illustration of interactions between amino groups and anionic surfactantheadgroups.Thenegativelychargedheadgroupsoftheanionicsurfactants interactwiththepositivelychargedammoniumsitesofAPSorTMAPSelectrostatic ally through neutralization. The most efficient surfactant is possibly terminal carboxylicacid.Thecocondensationoftetraethylorthosillane(TEOS)withAPSor TMAPSandassemblywithsurfactantsoccurtoformthesilicaframework. Table6.3.SynthesisRoutestoMesoporousMaterialswiththeEmphasisonSilicates (reproducedfromref.7) routeinteractionssymbolsconditionsclassicalproducts S+IelectrostaticS +,cationicsurfactantsbasicMCM41,MCM48 CoulombI ,anionicsilicatespecies MCM50, SBA6 forceSBA2,SBA8 FDU2,FDU11 FDU 13 SI+electrostaticS ,anionicsurfactantsaqueousmesoporousalumina CoulombI +,Transitionmetalions Force S+XI+electrostaticS +,cationicsurfactantsacidicSBA1,SBA2 CoulombI +,cationicsilicatespeciesSBA3 force,doubleX ,Cl ,Br ,I ,SO 4 ,NO 3 layerHbond SN+IelectrostaticS ,anionicsurfactantsbasicASMn CoulombN +,cationicaminogroups

SyntheticStrategiesinChemistry 6.11

ForceI ,anionicsilicatespecies SX+IelectrostaticS ,anionicphosphatebasicW,Mooxides Coulombsurfactants force,doubleI ,transitionmetalions, layerHbondX +,Na +,K +,Cr3 +,Ni 2+ SoIoHbondS o,nonionicsurfactantsneutralHMS,MSU, disordered (N oIo)N o,organicaminesWormlike I o,Silicatespecies,aluminatemesoporous speciessilicates SoH+XI+electrostaticS o,nonionicsurfactantsacidicSBAn(n=11, Coulomb12,15,16) Force,I +,silicatespeciesFDUn(n=1,5) 2 1 double,X ,Cl ,Br ,I ,SO 4 ,NO 3 KIT5andKIT6 layer Hbond No...I +coordinationN o,organicaminesacidicNb,Taoxides bondI +,transitionmetal S+IcovalentS +,cationicsurfactantsbasicmesoporous bondI ,silicatespeciessilica

Liquid Crystal Template Pathway Inthispathwaytrueorsemiliquidcrystalmesophasesareinvolvedinthesurfactant assembly to synthesize ordered mesoporous solids. Attard and coworkers synthesizedmesoporoussilicasusinghighconcentrationsofnonionicsurfactantsas

6.12 TemplateBasedSynthesis templates. The condensation of inorganic precursors is improved owing to the confinedgrowtharoundthesurfactantsandthusceramiclikeframeworksareformed. After the condensation, the organic templates can be removed by calciation, extraction, etc. The inorganic materials “cast” the mesostuructures, pore sizes, and symmetries from the liquid crystal scaffolds. Direct templating of microemulsion liquidcrystalmesophaseswereusedtosynthesismesoporous silicas from butanol watercopolymer–silicaternarysystem[6,7]. Silica/surfactant=1/0.27 calcination AssynthesizedMCM41 (hexagonalstructure) MCM-41 Silica/surfactant= 1/0.60 calcination AssynthesizedMCM48 MCM-48 (cubicstructure) Fig.6.4.possiblemechanismpathwaysfortheformationofM41Sfamilybyliquid crystaltemplatemethod(reproducedfromref.2). Evaporation Induced Self Assembling Techniques (EISA) Evaporation induced self-assembly mechanism Theevaporationinducedselfassemblingtechniqueisoneofthebesttechniquesto preparethenanomaterials.Thesuccessfulsynthesisofmesoporoussilicafilms,the EISA method is engaged to prepare ordered mesoporous polymer and carbon materials. The EISA method is strategy that avoids the cooperative assembling

SyntheticStrategiesinChemistry 6.13 process between the precursors and the surfactant template. Therefore, the cross linkingandthermopolymerizationprocessoftheresolsseparatefromtheassembly.

Fig. 6.5. Scheme for the preparation of ordered mesoporous polymer resins and carbonframeworksfromthesurfactanttemplatingprocessofEISA.(reproducedfrom ref.8) ComparetohydrothermalsynthesistheEISAmethodiseasierandcanproducethe mesoporous resins and carbon in a wider synthetic range, including pH values, surfactantandphenol/templateratio. ThechoiceoforganicprecursorsisessentialfortheEISAoftheorganic–organic templatingprocess.Thepolymerizationofinorganicprecursorsshouldbelowenough to form a moldable inorganicorganic framework at the initial assembly stage of inorganic species with organic surfactants. Highly ordered mesostructures can be formed. The inorganic framework is rigid. Therefore, the mesophase can be

6.14 TemplateBasedSynthesis solidified,andthesurfactantcanbeeasilyremovedbycalcinationsorextractedwith ethanol. The synthesis procedure includes five major steps (Fig. 6.5), which are the preparation of resol precursors, the formation of ordered hybrid mesophases by organicorganicselfassemblyduringthesolventevaporation,thermopolymerization oftheresolsaroundthetemplatestosolidifytheorderedmesophases,theremovalof thetemplates,andcarbonizationoftheresinpolymerframeworkstothehomologous carbons[8]. Template Synthesis of Metal Nanostructures Fig.6.6.Schemeofthearrangementusedforthefabrication of copper nanowires intotheAAMtemplate(reproducedfromref.5) Copper nanowires are prepared by using aluminium foil as active metal having a thicknessof1.5mm.Inordertodepositcopperinsidethechannelsofanodicalumina membrane(AAM),priortocementationathinconductivelayerofAuwassputtered on one side of the AAM using a conventional sputter coater to make this surface electricallyconductive.AportionofAAMwasmountedontothealuminumsupport by means of a conductive paste and delimited by an insulating film. Since the displacement deposition process needs an electrolytic contact of the active metal

SyntheticStrategiesinChemistry 6.15 surface,asmallareaofthealuminumsupportwasexposedtothedepositionsolution. Aschemeofthearrangementusedforthefabricationofcoppernanowiresisreported inFig.6.5.Thisarrangementwasplacedhorizontallyinabeakerandcoveredwith 25mlofa0.2Mcoppersulphateand0.1MboricacidsolutionhavingpH3.The experiments were conducted at room temperature. The surface area of the AAM exposedtothedepositionsolutionwasoftheorderof1cm 2.Afreshsolutionwas used for each experiment. Experiments were carried out for different times of deposition(from7hto7days)[5]. Preparation of Carbon Nanotubes Fig.6.7.Schematicrepresentationofcarbonnanotubes(reproducedfromref.9) Aluminamembraneisusedastemplateforthepreparationofcarbonnanotubes;here polyphenyl acetylene is used as a carbon source for the preparation of carbon nanotubes. It contains only carbonhydrogen bonds. The polyphenyl acetylene/aluminacompositewaspreparedbyadding10mlof5%w/wpolyphenyl acetylene in dichloromethane to the alumina membrane applying vacuum from the bottom.Theentirepolymersolutionpenetratesinsidetheporesofthemembranewas dried in vacuum at 373 K for 10 min. the composite was then polished with fine neutralaluminapowdertoremovethesurfacelayersandultrasonicatedfor20minto

6.16 TemplateBasedSynthesis remove the residual alumina powder used for polishing. The composite was carbonizedbyheatinginAratmosphereat1173Kfor6hatheatingrateof10K/min. Thisresultedinthedepositionofcarbononthechannelwallsofthemembrane.The carbon/aluminacompositewasthenplacedin48%HFtofreethenanotubes.The nanotubeswerewashedwithdistilledwatertoremoveHF[9]. Preparation of Porous Solids by Template Method Ordered mesoporous carbon has been prepared by using mesoporous silica as the template, and this mesoporous silica can be prepared by using CTAB, P123, surfactantsasthetemplates. MesoporoussilicaSBA15ispreparedbythefollowingmethod,P123nonionic surfactantwasdissolvedin2MHClsolution,anditwasstirredfor1hthenTEOS wasaddedasthesilicasource,thismixturewasstirreduntilTEOSwerecompletely dissolved.Themixturewasplacedinovenat373Kfor48h.Theproductwasfiltered andwashedwithdistilledwateranddriedat333K for6handthenSBA15was calcinedat823Kfor6hinairatmosphere. Fig.6.8.XRDpatternsofSBA15andCMK3(reproducedfromref.10)

SyntheticStrategiesinChemistry 6.17

The calcined SBA15 was impregnated with aqueous solution sucrose containing sulfuric acid, this mixture was heated to 373 k to 433 K for 24 h. then the carbonizationwascompletedbypyrolysiswithheatingtotypically1173KunderN 2 atmospherefor6hattherateof5K/min.thecarbonsilicacompositeobtainedwas washedwith1MNaOHsolutionor5Wt%hydrofluoricacidatroomtemperature,to remove the silica template. The template free carbon product thus obtained was filtered,washedwithethanolanddriedat393K. InFig.6.9thelowangleXRDconformsthemesoporousstructureandtosupportthis mesoporousstructurethetransitionelectronmicroscopyimagesshowsthepresence oforderedmesoporousstructure[10]. Fig.6.9TEMandselectedareaelectrondiffraction(SAED)imagesofCMK3 (reproducedfromref.10) SUMMARY Strategyaimedatthecontrollablesynthesishasbeenfocusedonthecontrolofmicro, meso and macroscale, including synthetic methods, architecture concepts, and fundamentalprinciplesthatgoverntherationaldesignandsynthesis.Inthischapter, synthesismechanismsandthecorrespondingpathwaysarefirstdemonstratedforthe synthesisofmesoporoussilicatesfromthesurfactanttemplatingapproach.Virtually all mesoporous silicates begin with an understanding of the interaction between organicsurfactantsandinorganicspecies,aswellasamongthemselves.

6.18 TemplateBasedSynthesis

Soft templating approach is one of the most general strategies now available for cratingnanostructures.Theassemblyofsurfactantsandsilicatesspeciesisnormally carried in solutions or the interface to allow the required driving force for the formationofnanostructures.Relyingonsolgel,solutionandsurfacechemistry,there isgreatpotentialtoexplorenovelstrategiesformesostructures,especiallyastrategy thatcanutilizeinterfacialtension.Itemsthatattractattentionalsoincludethecontrol of weak interaction such as the hydrophobic interaction between the assembling components. In view of the fact that surfactant selfassembly can occur with componentslargerthan1nmcontinuoustobechallengeandaninterestincondensed matterscience. REFERENCES 1.J.Lee,J.Kim,T.Hyeon, Adv.Mater .,18(2006)2073. 2.C.T.Kresge,M.E.Leonowicz,W.J.Roth,J.C.Vartuli,J.S.Beck, Nature, 359 (1992)710. 3.D.Zhao,J.Feng,Q.Huo,N.Elosh,G.H.Fredirckson,B.F.Chemlka, G.D.Stucky, Science , 279 (1998)548. 4.G.Cao,D.Liu, Adv.Colloid.Interface.Sci. 136(2008)45 5.R.Inguanta,S.Piazza,C.Sunseri, Eleectrochem.Commun., 10(2008)506 6.A.Corma, Chem.Rev. 97 (1997)2373. 7.Y.Wan,D.Zhao, Chem.Rev .107(2007)2821. 8.Y.Meng,D.Gu,F.Zhang,Y.Shi,L.Cheng,D.Feng,Z.Wu,Z.Chen, Y.Wan,A.Stein,D.Zhao, Chem.Mater. ,18(2006)4447. 9.M.Sankaran,Ph.DThesis ,IndianInstituteofTechnoloygMadras, 2007. 10.S.Jun,S.Joo,R.Ryoo,M.Kruk,M.Jaroniec,Z.Liu,T.Ohsuna, O.Terasaki, J.Am.Chem.Soc. , 122(2000) 712.

Chapter - 7 MICROEMULSION TECHNIQUES Ch.VenkateswaraRao The main focus of this chapter is to give brief introduction to the microemulsion systems, formation of microemulsions, how those microemulsions can be used for the preparation of various types of nanoparticles and the dominant factors that influence the nanoparticles preparationinthemicroemulsion. INTRODUCTION In1943,HoarandSchulmanfirstreportedthatoilcanbedissolvedinbulkwaterorwaterin bulkoilwiththeaidofsurfactanttoproduceaclearhomogeneoussolution.Theoilphasesare simple longchain hydrocarbons and the surfactants are longchain organic molecules with a hydrophilichead(usuallyanionicsulfateorquarternaryamine)andlipophilictail.Thegenerally usedoilphasesarecyclohexane,decane,heptaneandsurfactantsarecetyltrimethylammonium bromide (CTAB), sodium bis(2ethylhexyl) sulfosuccinate (AOT). The clear homogeneous solution is called as microemulsion. According to the nature of the bulk solvent used in the microemulsion, they are designated as waterinoil or oilinwater microemulsions. The microemulsionisprimarilydistinguishedfromtheemulsionnotbybeingcomposedofsmaller dropletsbutbybeingsubjectedtoarestrictiveconditionthatitisthermodynamicallystable. MICROEMULSION FORMATION AND NANOPARTICLES PREPARATION Ingeneral,waterandoilarenotmiscible.Buttheamphiphilicnatureofthesurfactantssuchas CTAB makes them miscible. The surfactant molecules form a monolayer at the interface betweentheoilandwater,withthehydrophobictailsofthesurfactantmoleculesdissolvedinthe oilphaseandthehydrophilicheadgroupsintheaqueous phase. It leads to the formation of microemulsion.Sothemicroemulsionisdefinedasasystemofwater,oilandsurfactant.This system is an optically isotropic and thermodynamically stable. At macroscopic scale, a microemulsion looks like a homogeneous solution but at molecular scale, it appears to be heterogeneous.Eventhoughitisopticallyisotropic;itcannotbeproperlydescribedasasolution. Theinternalstructureofthemicroemulsionatagiventemperatureisdeterminedbytheratioof its constituents. The structure consists either of nanospherical monosized droplets or a

7.2 MicroemulsionTechniques bicontinuous phase. The different structures of a microemulsion at a given concentration of surfactantareshowninFig1.Itindicatesthat (i) Athighconcentrationofwater,theinternalstructureofthemicroemulsionconsistsof small oil droplets in a continuous water phase (micelles), known as o/w microemulsion. (ii) Withincreasedoilconcentration,abicontinuousphasewithoutanydefinedshapeis formed. (iii) Athighoilconcentration,thebicontinuousphaseistransformedinto astructureof small water droplets in a continuous oil phase (reverse micelles), known as a w/o microemulsion. Depending on the type of surfactant used to form microemulsion, size of the different dropletswillbevariedfrom5100nm.Itisalsoevidentthatthemicroemulsionsystemis sensitive to temperature. It can be seen in Fig. 7.1 that the increase in temperature will destroytheoildropletswhilethedecreaseintemperaturewilldestroythewaterdroplets.

Fig.7.1Themicroscopicstructureofamicroemulsionatagivenconcentrationofsurfactantas functionoftemperatureandwaterconcentration(reproducedfromref.20). Significance of Packing Parameter Theshapeofmicellaraggregatesandtheformationofmicroemulsioncanbeunderstoodfrom the packing parameter of surfactant molecule used for the microemulsion formation. Packing

TheEvolvingSyntheticStrategiesinChemistry 7.3 parameterisdefinedas v/a.l,where v isthevolumeofhydrocarbonofthesurfactant, a isthe polarheadareaand l isthefullyextendedchainlengthofthesurfactant.Thepackingparameter valuecanbe1,<1and>1.Whenthepackingparametervalue(orratio v/a.l)isgreaterthan1,the aggregate curvature will be toward the water. Thiscorrespondstoasituationwheretheoilis penetratingthesurfactanttailsand/ortheelectrostaticrepulsionbetweenthechargedheadgroup is low. When the packing parameter value (or ratio v/a.l) is less than 1, it corresponds to a situationwheretheelectrostaticrepulsionislargerand/ortheoilisnotpenetratingthesurfactant tails. It implies that (i) when oil is solubilized in hydrophilic micelles, one can observe the formation of o/w microemulsions for v/a.l< 1; (ii) when water is solubilized in hydrophobic micelles, one can observe the formation of w/o microemulsions for v/a.l>1 .; and (iii) When v/a.l≈1,lamellarphasesorbicontinuousmicroemulsionsareobserved. Basedontheexperimentalresults,itisobservedthatthemicelleswillbeinsphericalshape whenthepackingparameterislessthan1/3.Thepackingparametersforcylindersandplanar bilayers are 0.5 and 1, respectively. In the case of reverse micellar structures, the packing parameter is greater than 2 for cylinders as well as spherical micelles. It was observed that reversemicelleswillbeincylindershapeupto v/a.l≤2andsphericalshapewhen v/a.l> 3. Oilinwater (o/w) microemulsions are monodisperse. Waterinoil (w/o) microemulsion solutionsaremostlytransparent,isotropicliquidmediawithnanosizedwaterdropletsthatare dispersed in the continuous oil phase and stabilized by surfactant molecules at the water/oil interface. These surfactantcovered water pools offer a unique microenvironment for the formationofnanoparticles.Theynotonlyactasmicro/nanoreactorsforprocessingreactions butalsoexhibittheprocessaggregationofparticlesbecausethesurfactantscouldadsorbonthe particle surface when the particle size approaches to that of the water pool. As a result, the particlesobtainedinsuchamediumaregenerallyuniforminsizeandshape(i.e.,monodisperse). APPLICATIONS OF MICROEMULSIONS 1. Synthesis of Metal Nanoparticles Precipitation of metal particles in the Waterinoil (w/o) microemulsion solutions (reverse micellar system) has been found to be the simple methodology for the preparation of nanoparticles.

7.4 MicroemulsionTechniques

Themethodofpreparationconsistsinmixingoftwomicroemulsionscarryingthe appropriate reactants (generally metal precursor and reducing agent) in order to obtain the desiredparticles.ItisrepresentedinScheme7.1.

Scheme 7.1. Proposed mechanism for the formation of metal particles by the microemulsion approach(reproducedfromref.24) Atthebeginningofreaction,theattractivevanderWaalsforceandtherepulsiveosmoticforce between reverse micelles lead to collisions of micelles. It results in the interchange of the reactants (in general, metal ions and reducing species) solubilized in two different reverse micellesrespectively.Asaresult,theinitialmonomericmetalnucleibegintoformandgrow. Whentheexchangeofreactantsisfastinthewaterdroplets,metalionsreduceandthemetal nuclei grow quickly. It remains unchanged when the particle size reaches a certain size. It correspondstothethermodynamicallystabilizedspeciesinthepresenceofmicroemulsion.Due

TheEvolvingSyntheticStrategiesinChemistry 7.5 tothepossibleaggregation,thefinalsizeofsilverparticlesisgenerallylargerthanthatofthe watercores.Oncetheparticlesattainthefinalsize,thesurfactantmoleculesareattachedtothe surfaceofparticlesandstabilizeandprotectthemagainstfurthergrowth.Thedynamicexchange ofreactantssuchasmetallicsaltsandreducingagentsbetweendropletsviathecontinuousoil phaseisstronglydepressedduetotherestrictedsolubilityofmetalsaltsintheoilphase.Thisisa reasonwhytheattractiveinteractions(percolation)betweendropletsplayadominantroleinthe particlenucleationandgrowthinthewaterinoil(w/o)microemulsionreactionmedium. 2. Synthesis of Metal Alloys Variousmetalalloyscanbepreparedinthesimilarwayasdescribedinscheme1.Itconsistsof mixingthereversemicellecontainingtwoorthreemetalprecursorsandanotherreversemicelle containreducingspecies.Example:formationofPtRu(1:1)alloy.Inordertopreparethealloy nanoparticles,bothPt 4+ andRu 3+ willbetakenin1:1atomicratiotoformreversemicroemulsion

I.ReversemicroemulsionIIwillbepreparedbyusingNaBH 4reducingagent.Whenboththe 4+ 3+ reversemicroemulsionsaremixedtogether,reductionofPt andRu takesplace byBH 4 ions andresultsintheformationofPtRu(1:1)nanoparticles(XiongandManthiram,2005).Inthe similarway,PtFenanoparticlesindifferentratios(1:1,1:2and1:3)canbeprepared(Carpenter etal .,2000).Inthesamemanner,variousmetalchalcogenidesalsocanbepreparedbytaking twomicellesolutionscontainingthedesiredionspreparedseparatelyandthenrapidlymixed. Inconventionalmethodsofpreparation,thereactiontemperatureshouldbehightopromote alloyformation.Asaresult,theformedparticleswillbelargeinsize.Moreover,theshapeofthe particlecannotbecontrolled.Sincethereactiontakesplaceinsidethemicellarcores, (i)theenormousamountofheatthatisgeneratedwithinthemicellarcoresduringthereduction processisenoughtoformalloyofdesiredcomposition. and(ii)bothsize,shapeandcompositioncanbecontrolled. 3. Synthesis of Metal Oxides Thesynthesisofoxidesfromreversemicroemulsionreliesonthecoprecipitationofoneormore metalions. Itisalmost similarinmany respectsto the precipitation of oxides from aqueous solutions.Buttheprecipitationoccurswithinthemicellarcoressothatparticlesizeaswellas shapecanbecontrolled.Inatypicalprocess,precipitationofhydroxidesisinducedbyaddition ofareversemicellesolutioncontainingdiluteNH 4OHtoareversemicellesolutioncontaining aqueous metal ions at the micellar cores. Alternatively, dilute NH 4OH can simply be added

7.6 MicroemulsionTechniques directly to a micelle solution of the metal ions. The precipitation of the metal hydroxides is typically followed by centrifugation and heating in the presence of oxygen to remove water and/orimprovecrystallinity.Ingeneral,itisrepresentedasfollows: 2+ 2+ A +2B +OH (excess)→AB 2O4+ xH2O↑ WhereAandBaremetals. Inthisway,simplemetaloxidesandmulticomponentmetaloxidescanbeprepared.Someofthe examplesofnanomaterialssynthesizedinw/omicroemulsionsaregiveninTable7.1.

Table7.1.Nanomaterialsformedinw/omicroemulsions Nanoparticlesystem Example Reference Metals/Metalalloys Pt Rojas etal .,2005 Pd Boutonnet etal .,1982 Ag Taleb etal .,1997 Au Herrera etal .,2005 PtRu XiongandManthiram,2005 FePt Carpenter etal .,2002 PdCoAu Raghuveer etal .,2006 Metalchalcogenides ZnS Khiew etal .,2005 PbS Eastoe etal .,1995 RuSe VenkateswaraRaoandViswanathan,2007

Metaloxides Pb 2Ru 2O7 Raghuveer etal .,2002

CeO 2 Bumajdad etal .,2004

CoFe 2O4 Liu etal .,2003

LiNi 0.8 Co 0.2 O2 Lu etal .,2000

Coreshellnanoparticles Fe 3O4/SiO 2 Tago etal .,2002 Fe/Au Zhou etal .,2000

TheEvolvingSyntheticStrategiesinChemistry 7.7

4. Synthesis of Core-Shell Nanoparticles Some of the metallic nanoparticles like Fe, Co, Ni are susceptible to rapid oxidation. This problem can be largely circumvented by coating the nanoparticles with gold or other inert metals. The technique for applying gold coatings on metal nanoparticles is reasonably straightforward and simply adds an additional step to the reverse micelle synthesis. In this process, a watersoluble gold salt (HAuCl 4) is dissolved and dispersed in a separate reverse micelle solution that is then added to the metalcontaining reverse micelle solution that has already been reduced with an excess of BH 4 .TheaqueousAuCl 4 ionsencapsulatethemetal particles and are subsequently reduced by forming a metallic gold shell around the metal particles. + 8AuCl 4 +3BH 4 +9H 2O→8Au+3B(OH) 3+21H +32Cl The preparation of coreshell type structures is not limited to metals as the core or shell materials.Combinationsofprecipitation,reduction and hydrolysis reactions can be performed sequentiallytoproduceoxidescoatedwithmetals,oxidescoatedwithoxides,andsoforth. EFFECTS OF THE PARAMETERS ON THE FORMATION OF NANOPARTICLES IN MICROEMULSION The main parameters that influence the size of nanoparticle are molar ratio, W = [water]/[surfactant], type of solvent employed, surfactant or cosurfactants used and concentration of reagents. Recent investigations suggest that the particle shape also can be affected by the influence of micellar template, added anions and molecular adsorption. But a generalmethodforcontrollingnanocrystalshapesthroughsoftchemistryhasnotyetbeenfound. The major factors that influence the formation of nanoparticles are explained with examples below. Water-to-Surfactant Ratio, W Thesizeofthemetallicparticlewilldependonthesizeofthedropletsinthemicroemulsion.The dropletsizewillbeinfluencedbythewatertosurfactantratio,W. Ingeneral,thevolumeofwaterinmicroemulsionsisproportionaltothecubicradiusofthe watercore.Andalsotheamountofsurfactantwhichispresentasafilmaroundthewatercoresis proportional to the surface area of the micro/nanodroplets. So the molar ratio of water to surfactant (W) has a linear relationship with the radii of the water cores (R w). It has been observed in AOT microemulsion system that the waterpool radius increases with the water

7.8 MicroemulsionTechniques content.Anincreaseofmolarratio,Watconstantconcentrationofsurfactantwillincreasethe averagediameterofthedroplets.Consequently,theobtainednanoparticleswillbelargeinsize.

Table7.2.Characteristicofsilverparticlesobtainedinw/omicroemulsion(datatakenfromref. 23) W Reducingagent Conc.ofreducing Particlesize(nm) agent(M) 4 2 NaBH 4 2.5x10 4 5 NaBH 4 2.5x10 2.7 5 7.5 NaBH 4 1.0x10 4 7.5 NaBH 4 1.0x10 4.5 4 7.5 NaBH 4 2.5x10 5.5 4 15 NaBH 4 1.0x10 6.0 4 15 NaBH 4 2.5x10 7.0 Forexample,bydynamiclightscatteringtechnique,ithasbeenconfirmedthatthewatercore radius,R w,iscoincidentwiththeequationR w(nm)=0.18W+1.5inawiderangeofalkanes.For example in the AOTwaterisooctane microemulsion system, it was found that the linear relationshipbetweenR wandWwasR w (Å)=1.5W.Thusthelinearrelationshipmaybedifferent in various systems. Generally, low water content is favorable to form smaller microemulsion droplets(reversemicelles)whichyieldfinenanoparticleswithanarrowsizedistribution.Onthe contrary,highwatercontentisfavorabletoformbiggermicroemulsiondropletswhichareeasy tofabricatelargerparticles.Thereasonforthebehaviourisexplainedasfollows:Actually,at lowwatercontent,thewatersolubilizedinthepolarcoreisboundbythesurfactantmolecules, whichincreasestheboundarystrengthanddecreasestheintermicellarexchangerate.Asaresult, adecreaseinwatercontentinducesformationofmonodispersenanoparticleswithsmallparticle size.However,theboundwaterwouldturnintobulkwaterwithanincreaseinwatercontent, which is benefit for the water pools to exchange their contents by collisions and makes the chemical reaction or coprecipitation between compounds solubilized in two different reverse micellescompletemorequickly.Sincethenatureofbulkwaterisdrasticallydifferentfromthat ofboundwater,reactantswouldberapidlytransferredfromonewatercoretoanother,andthus

TheEvolvingSyntheticStrategiesinChemistry 7.9 theresultantparticlesizeisrelativelybigandthesizedistributionbecomesrelativelywide.The variationofsilverandFenanoparticlessizewithWisgiveninTables7.2and7.3respectively. Table7.3.Characteristicofironparticlesobtainedinw/omicroemulsion(Datatakenfromref. 24)

4 3 S.No [AOT] W [FeCl 2]x10 [NaBH 4]x10 Particlesize (mol/dm 3) (mol/dm 3) (nm) 1 0.1 22 7.5 3.5 3.9 2 0.1 22 1.6 0.68 4.1 3 0.05 10 0.93 0.35 2.5 4 0.025 5 48 24 1.9 5 0.025 9 48 24 2.5 6 0.025 13 48 24 3.3 7 0.025 18 48 24 4.3 8 0.025 22 48 24 4.7 9 0.025 26 48 24 5.4 10 0.025 31 48 24 5.8

Solvent Effect Particlesizeisaffectedbysolventtype.ThiswasshowninitiallybyPileni(2003)inastudyon silvernanoparticles,inwhichlargerparticleswereseen(byTEM)tobeformedinisooctanethan incyclohexane.Thisisprobablyduetothesignificantdifferenceinintermicellarexchangerate constantbetweenthetwosolventsafactorof10. Thechangeingrowthratehasbeenexplainedinthefollowingway: (i) Smaller and less bulky solvent molecules with lower molecular volumes such as cyclohexane, can penetrate between surfactant tails and increase the surfactant curvature and rigidity. The increased rigidity at theinterfacemayleadtoaslower growthrate.

7.10 MicroemulsionTechniques

(ii) Isooctane, being bulkier with a larger molecular volume, cannot penetrate the surfactanttailssoefficiently,therebyleadingtoamorefluidinterfaceandthusfaster growthrates. Although these ideas provide plausible explanations of the phenomena they remain controversial. Measurementofthesurfactantfilmrigiditiesinmicroemulsions show that the solvent type has only a minor effect. Solvent molecular volume may also explain the observed changeinfinalparticlesize.Itwasproposedthatamorestablemicellesystemarisesfrom greater interactions between the solvent and surfactant tails which in turn leads to an enhancedabilitytostabilizelargerparticles.Anyincreaseinrateofintermicellarexchange willresultinahigherrateofgrowthcomparabletonucleation,henceislikelytogenerate systemswithlowerpolydispersity.EffectofsolventonthesizeoftheAgnanoparticlesis giveninTable7.4. Table 7.4. Effect of solvent on the absorption spectra of silver nanoparticles synthesized in reversemicellesofAOT(datatakenfromref.22) System Particlesize(nm) AOT/decane 6.0 AOT/heptane 22.0 AOT/cyclohexane 5.4

Surfactants and co-surfactants Variousstudiesshowedthatthechoiceofsurfactantiscriticaltothesize,shapeandstabilityof the particles. The most commonly used surfactant is the anionic AOT, although a variety of common cationic surfactants are also frequently employed, such as CTAB or DDAB (din didodecyldimethylammoniumbromide) and nonionics Triton X100, polyoxyethylene (5) nonylphenylether(NP5)orpolyoxyethylene(9)nonylphenylether(NP9).Forsomesystems cosurfactants (intermediate chain length alcohols, such as nbutanol or nhexanol) are also employed.

TheEvolvingSyntheticStrategiesinChemistry 7.11

Among the anionic surfactants that form reverse micelles, the best known are the systems derived from AOT (sodium 1,4bis2ethylhexylsulfosuccinate) in different nonpolar media. Thereasonsareasfollows: (i) AOThasawellknown Vshapedmoleculargeometry, givingrisetostablereverse micelleswithoutcosurfactant. (ii) AOT has the remarkable ability to solubilize water with values of W (W =

[H 2O]/[AOT])aslargeas4060dependingonthesurroundingnonpolarmedium,the solute and the temperature; however the droplet size depends only on the water amount,W.Thebulkpropertiesofwater(polarity,viscosity,hydrogenbondability, etc.)eitherinsidethepool(free)orattheinterface(bound)changewithW. It is also known that addition of cosurfactant can reduce the surfactant concentration in microemulsionpreparation.Normally,lowmolecularweightalcohols,suchasnbutanolcanbe usedforthispurpose.Theirshorthydrophobicchainandterminalhydroxylgroupisknownto enhance the interaction with surfactant monolayers at the interface, which can influence the curvatureoftheinterfaceandinternalenergy.Theamphiphilicnatureofcosurfactants could alsoenablethemtodistributebetweentheaqueousandoilphase. Ingeneral,itwasconcludedthattheadditionofacosurfactantleadstoahigherfluidityof the interfacial film, thus increasing the rate of intermicellar exchange (but also leading to a highercurvatureofthedroplets),sosmallerparticles. Surfactant Concentration Whentheamountofwaterandoiliskeptatfixedvalues,anincreaseoftheamountofsurfactant willincreasethenumberofdroplets. Itmeansthatthenumberofmetalionsperdropletwill decrease and consequently the size of the particles. The morphology of reverse micelles is differentwiththesurfactantconcentration.Atdifferentconcentrations,thesurfactantmolecules canformvariousmoleculeaggregations(Fig.7.2).

7.12 MicroemulsionTechniques

Fig.7.2.Structuresofdifferentmicelles(reproducedfromref.26) Itwasfoundthatthechangeofmicellestructureshadanenergeticbarrier.Themainpointsare givenasfollows: (i) Atalowsurfactantconcentration,onlysphericalmicellesappearinsolution. (ii) Whenthesurfactantconcentrationinthesolutionreaches a welldefined saturation value,i.e.,secondcriticalmicelleconcentration(secondCMC),theenergeticbarrier willbeovercomeandthemicellestructurewillchange from asphericalmicelleto other special structure, and then the micelle will be steady again at a new concentration range. For example, if the concentration of surfactant attains to 40– 50%, the spherical micelle turns into rodshaped or columnshaped micelle. Furthermore, the micelle is also able to selfassemble into layer or liquid crystal structure. The special micelles formed at different concentrations of surfactants can be usually used as effectivestructuredirectingagentstopreparenanoparticleswithdesiredmorphologies.Thus,the micelle formed by the surfactant with a proper concentration can offer an appropriate growth conditionfornanoparticles.

TheEvolvingSyntheticStrategiesinChemistry 7.13

Nature of the Precipitating Agent (reducing agent) The main point that should be followed in the selection of suitable reducing agent in the preparationofnanoparticlesisitmustbestableinanaqueousenvironmentanddoesnotreact withtheothercomponentsofthereversemicellesystem. As a general rule, a fast nucleation processwillresultintheproductionofsmallparticles.

Inmostofthecases,watersolublesodiumborohydrideandhydrazine(usuallyN 2H4.2HClor

N2H4.H 2O) are used as effective reducing agents. Eventhough bubbled H 2 gas results in the reductionofmetalparticles,thekineticsisnotdesirable,particularlyatroomtemperature.Asit was mentioned earlier, faster exchange between the reactantsandfastnucleationprocesswill givesmallerparticles.Sobothsodiumborohydride(NaBH 4)andhydrazineareefficientreducing agents for most of the transition metal salts. The reduction process is in this case completed instantlyandisveryfastincomparisontopurehydrogen. When increasing the concentration of hydrazine while the concentration of metal salt is kept constant,adecreaseintheparticlesizeisobserved.ThiswasshownwhenNiparticles were prepared in a microemulsion containing cetyltriammonium bromide (CTAB) as surfactant, n hexanolasoilphase,waterasaqueousphaseandhydrazineasreducingagentatatemperatureof 350 K (Capek, 2004). The diameter of the nickel particles decreases when the ratio of the hydrazine to nickel chloride concentrations increases. The diameter of the particles reaches a constantvaluewhenthisratioisabove10. Effect of the Micellar Template It was claimed that particle shape can be controlled by using micellar templates. A simple surfactant/water/oil system can produce many different selfassembly structures: by changing composition, one can obtain spheres (reverse micelles or micelles), cylinders, interconnected cylinders and planes, termed lamellar phases, which also can reorganise into oniontype structures.Henceintheorymanypossiblenanoparticlestructurescouldbegrowninsidethese differentshapedtemplates.Despiteplentyofwork,thereisstillmuchcontroversialdebateon theseaspects. Influence of ion/molecular adsorption Anion species added as electrolyte is important for generating different shapes of metal nanocrystals.Theinitialmicellarshapeisshowntobelargelyunaffectedbytheseadditives.For example,inthecaseofcoppernanoparticlesystems,alargeexcessofhydrazinefavoursdisk

7.14 MicroemulsionTechniques oversphericalparticles.Inbothcases,Pileni etal .(2003)postulatethatselectiveadsorptionof moleculesorionsontofacetsofthenanocrystalaffectsgrowthincertaindirections,explaining theapparentpreferenceforcertainshapes. ItwasalsoknownthatpHaffectstheshapeofnanocrystals, for example, nanostructured NiZn ferrites (Uskokovic et al ., 2005). When the pH is lower, needlelike nanocrystals are formed,whereasotherspheresareobservedathigherpH.Onepossiblereasonforthisisdueto anincreasednumberofhydroxylionsathigher pHwhicheliminatethesulphateandbromide ions,hamperingtheirabilitytopromoteuniaxialgrowth. SUMMARY The most remarkable features of the microemulsion technique are: (i) particle size and compositioncanbecontrolledtoagreatextentand a narrow particle size distribution can be obtainedand(ii)Bimetallicparticlescanbeobtainedatroomtemperature. A large number of different nanomaterials can be synthesisedinwaterinoilmicroemulsions and reverse micelles. Particle growth has shown to be strongly dependent on intermicellar exchange rates. The resultant particle size appears to be dependent on dominant parameters, namelysolventtype,surfactant/cosurfactanttype,concentrationofthereagentsandcomposition via[water]:[surfactant]ratio,W. However,thegeneralityofthefactorsthataffectstheshapeofparticleremainstobeestablished. REFERENCES 1. K.TapasandA.Maitra, Adv.ColloidInterfaceSci.,59(1995)95. 2. J.J.Silber,U.A.Biasuttia,E.AbuinbandE.Lissi, Adv.ColloidInterfaceSci .,82(1999) 189. 3. T.Tadros,P.Izquierdo,J.EsquenaandC.Solans,Adv.ColloidInterfaceSci .,108109 (2004)303. 4. S.Eriksson,UlfNylén,S.RojasandM.Boutonnet , AppliedCatalysisA:General 265 (2004)207. 5. T.P.HoarandJ.H.Schulman, Nature 152(1943)102. 6. M.P.Pileni, Nature 2(2003)145. 7. T.Tago,T.Hatsuta,K.Miyajima,M.Kishida,S.TashiroandK.Wakabayashi, J.Am. Ceram.Soc. 85(2002)2188.

TheEvolvingSyntheticStrategiesinChemistry 7.15

8. S.Rojas,F.J.GarcıaGarcıa,S.Jaras,M.V.MartınezHuerta,J.L.GarcıaFierroandM. Boutonnet, AppliedCatalysisA:General 285(2005)24. 9. M.Boutonnet,J.KitzlingandP.Stenius, Colloids.Surf. 5(1982)209. 10. A.P.Herrera,O.Resto,J.G.BrianoandC.Rinaldi, Nanotechnology 16(2005)S618. 11. L.XiongandA.Manthiram, SolidStateIonics 176(2005)385. 12. V.Raghuveer,P.J.Ferreira,A.Manthiram, Electrochem.Commun. 8(2006)807. 13. P.S.Khiew,S.Radiman,N.M.Huang,MdSootAhmadandK.Nadarajah, Mater.Lett. 59(2005)989. 14. J.EastoeandA.R.Cox, ColloidsSurfAPhysicochem.Eng.Asp. 101(1995)63. 15. Ch.VenkateswaraRaoandB.Viswanathan , J.Phys.Chem.C 111(2007) 16538. 16. V.Raghuveer,Keshav,KumarandB.Viswanathan, IndianJ.Eng.Mater.Sci. 9(2002) 137. 17. A.Bumajdad,M.I.Zaki,J.EastoeandL.Pasupulety, Langmuir 20(2004)11223. 18. S.Xing,Y.Chu,X.SuiandZ.Wu, J.Mater.Sci. 40(2005)215. 19. E.Carpenter,J.A.Sims,J.A.Wienmann,W.L.ZhouandC.J.O’Connor, J.Appl.Phys. 87 (2000)5615. 20. R.Schomäcker,M.J.SchwugerandK.Stickdorn, Chem.Rev. 95(1995)849. 21. V.UskokovicandM.Drofenik, ColloidsSurf.APhysicochem.Eng.Asp. 266(2005)168. 22. R.P.BagweandK.C.Khilar, Langmuir 16(2000)905. 23. C.Petit,P.LixonandM.P.Pileni, J.Phys.Chem. 97(1993)12974. 24. I.Capek, Adv.ColloidInterfaceSci .,110(2004)49. 25. Taleb,C.PetitandM.P.Pileni, Chem.Mater. 9(1997) 950. 26. J.H.Fendler, Chem.Rev .87(1987)877. 27. W.L.Zhou,E.E.Carpenter,J.Sims,A.KumbharandC.J.O’Connor, Mater.Res.Soc. Symp.Proc. 581(2000)107. 28. C.H.LuandH.C.Wang, J.Mater.Chem. 13(2003)428. 29. C.Liu,A.J.RondinoneandZ.J.Zhang, PureAppl.Chem. 72(2000)37.

Chapter - 8 SYNTHESIS BY SOLID STATE DECOMPOSITION S.Navaladian INTRODUCTION Synthesisofmaterialsisanimportantpartinmaterialsscienceandchemistrybecausethe propertiesofmaterialsvarydrasticallybasedontheirsyntheticmethod.Eventhestability ofthematerialvariesbasedonmethodbywhichitissynthesized.Byvaryingsynthetic methods, surface area, pore size, crysatallite size, allotropes, morphology, presence of impurities,defectsandotheroxidationstateofthemetalcanalsobevaried.Hence,their potentialtocertainapplicationsvarydrastically.Forexample,incatalysis,catalystwith highsurfaceareaispreferredtoachievehighconversion.So,syntheticmethodthatyield highsurfaceofthecatalystisadopted.Alotofmethodsareknowninthesynthesisof materials of various applications in different fields, still efficient and costeffective methodsarebeingdeveloped. DECOMPOSITION METHODS Decompositionisoneofthemethodsknowninthesynthesisofvariousmaterials . Inthis method,asolidorliquidisheatedtoitsdecompositiontemperaturestoobtainthesolidof interest.Forexample,sugarcanbedecomposedinaninertatmospheretogetthecarbon material (solidtosolid). Decomposition of titanium isopropoxide (a liquid) in air atmosphereisusedtogetthetitaniumdioxide(TiO2)(liquidtosolid).Ni(CO) 4, avolatile complexcanbedecomposedtogetNimetal(vapourtosolid).Inthisparticularchapter, solidstatedecompositionisconsidered. Solid-State Decomposition Decompositionofanymaterialoccursduetoitsinstabilitywhentheparticularconditions areapplied.Also,itdependsupontheactivatingsource.Theseactivatingsourcescanbe thermal, photochemical, microwave and γradiation. For example chemical compound whichneedhightemperaturestodecomposecanbephotochemicallydecomposedwith easeatroomtemperature.Particularly,thedecompositionofAgBrtoAgcanbeachieved bythermalmodesonlyat1330 °Cbutitcanbeeasilydecomposedatroomtemperature undervisiblelight.Asweknow,thisprincipleisusedinphotography.Thus,basedonthe activating sources, solidstate decomposition can be classified into the following

8.2 SynthesisbySolidStateDecomposition decomposition methods (1) thermal decomposition, (2) Microwaveassisted and (3) Photochemicaldecomposition. Thermal Decomposition Methods In general, thermal decomposition methods are chosen for synthesis of metal oxides. Decomposableprecursorswhichposseslowdecompositiontemperaturearepreferredfor thesynthesisbecauseathightemperatures,sinteringisalsohigh.Somaterialsformed willhavepoorsurfaceareaandlargecrystallitesize.Decompositiontemperatureofany compoundsismainlydecidedbytheredoxpropertiesofthecompound.Ingeneral,salts of the silver with easily utilizable organic moiety can be decomposed at lower temperatures. Various materials such as metal, metal oxides, mixed metal oxides and metalschalogenideshavebeensynthesizedusingdecompositionmethod. Synthesis of Metals 100 a b 95 90 85 80 Weight loss (%) loss Weight 75 70 40 60 80 100 120 140 160 180 200 Temperature ( οοοC ) Fig. 8.1. TGA profile of silver oxalate and (b) silver nanoparticles synthesized by decompositionofsilveroxalate[1]. Synthesisofmetallicsilvercanbeachievedusingsilverformate,silveroxalate,oleate, maleateandfumarateandtheirdecompositiontemperatureis93,140,287,170and280 °Crespectively(1and2).Recentlysilverdodecanate,myristateandpalmitatealsohave been utilized for the synthesis of silver nanopartices. Decomposition of these silver compounds yield various gaseous products like CO, CO 2 and organic residues. In the caseofsilveroxalatedecompositiontoAgmetalandCO 2areproduced.Hence,thisis oneofthemethodsknownfortheproductionpureCO2. In the case of silver oxalate

SyntheticStrategiesinChemistry 8.3 decomposition,reactionwascarriedoutinawatermediumunderrefluxingconditionsat underN 2atmospherewithacappingagent(poly(vinylalcohol))(Fig.8.1).TGAofthe silveroxalate,giveninFig.8.1(a)showsthesharpdecompositionataround140 °C.The correspondingweightlossisduetothelossoftwo moles of CO 2. TEM image of Ag nanoparticlessynthesizedbydecompositionofsilveroxalateisgiveninFig.8.1(b).The averageparticleisaround7nm. Metal Oxides Metal oxides can also be synthesized by decomposition of certain precursors like hydroxides,oxalates,hydroxylcarbonates,hydroxylsulphatesandcarbonates.Ingeneral, the direct calcination of metal salt such as nitrates and sulphates is not carried out to synthesis the metal oxides in order to avoid the impurities and heterogeity of the materials. SynthesisofMagnesia

The decomposition of magnesium hydroxycarbonate (Mg5(CO 3)4(OH) 2) yields magnesium oxide (MgO). In this particular case, the morphology of the precursor is flower like particle of 3 m. After calcinations at 400 °C, MgO with tubelike morphologyisobtainedasshowninFig.8.2.

400ο C Mg5 (CO3)4 (OH)2  → 5 MgO+ 4CO2 + H2O

(a) (b) Fig.8.2SEMimageof(a)magnesiumhydroxycarbonateand(a)MgO[3].

8.4 SynthesisbySolidStateDecomposition

Similarly MgSO 4. 5Mg(OH) 2. 2H 2O also yields the MgO by decomposition, but the completedecompositionneedstemperatureabove800 °C. Inthiscase,morphologyof theparticlesisstriplike.Ingeneral,MgOissynthesizeddecomposingtheMg(OH) 2 in theair.ButMgOsynthesizedinthepresenceofairshowslesssurfaceareathanthatof MgOsynthesizedinnitrogenatmosphere.Thisisduetothehighamountofsinteringin presenceofoxygen. SynthesisofMatelOxidesfromOxalatePrecursors Duetothelowtemperaturedecompositionofmetaloxalates,oxalateprecursorsaremost widelyusedforsynthesesofmetaloxides. Nickel,copper,ironandzincoxidescanbe synthesizeddirectlydecomposingtheiroxalateprecursor.Nickeloxalateprecursorhas yieldednickeloxidewithsizesaround9nmat450ºC.Fe 2O3hasbeensynthesizedby thermal decomposition of ferrous oxalate at 415 °C. When copper oxalate has been decomposedinairat300°Cfor1hmesoporousCuOmicrosphereshavebeenobtained (Fig.8.3).ZincoxalatehasyieldedZnOparticlesofsizearound55nm.Inallthesecases, the formation of CO and CO 2 are the by products. Zinc acetate also decomposes and yieldsthezincoxideat600 °C.Stoichiometricbariumtitanate(BaTiO 3)issynthesized bythermaldecompositionofbariumtitanyloxalateat600 °C.

Fig.8.3.Copperoxidemicropshereobtainedbythedecompositionofcopperoxalate[4].

SyntheticStrategiesinChemistry 8.5

MetalChalcogenides Cd(II)complexofthiosemicarbazideandselenosemicarbazidearethermallydecomposed toobtainCdSandCdSe.Intypicalprocedure500mgofcadmiumtetramethylthiourea complexwasmixedwith182mgofthiosemicarbazideligand,dispersedwellin3mltri noctylphosphineoxide(TOPO).ThisprecursormixturewasinjectedintoTOPO(5g)at 300 °C.Theresultingorangeyellowsolutionwasmaintainedat300 °Cforabout1hand thencooledto70 °C.TEMimageofCdSnanorodsformedareshowninFig.8.4.Inthis casethethicknessesoftherodsformedare15nmandaspectratiois34.

Cadmiumthiosemicarbazide

Fig.8.4.CdSnanorodssynthesizedbydecompositionofcadmiumthiosemicarbazide[5]

8.6 SynthesisbySolidStateDecomposition

Microwave-assisted Decomposition In general, microwave is used to quicken the reactions. The main advantages of microwaveassistedprocessesare:(i)therateofthereactionisincreasedbyordersof magnitude, (ii) the initial heating process is rapid, and (iii) microwaves induce the generationoflocalizedhightemperaturesatreactionsites,whichenhancesthereaction rates.Moreover,microwavebasedsynthesesare energy efficient.Hence, microwaveis usedinthesynthesisofmaterialsaswellasorganiccompounds. Synthesis of Metals Decompositionofsilveroxalatehasbeencarriedoutbyusingmicrowaveradiationin ethylene glycol and diethylene glycol. Formation of silver nanoparticles has been observedin60sinthecaseofethyleneglycol.Inthecaseofdiethyeleneglycol,only after 75 s of irradiation, the formation of silver nanoparticles has been observed. Ag nanoparticle formed in ethylene glycol medium is given in Fig. 8.5 (a). Formation of anisotropicnanoparticleisobservedinthecaseof75sofmicrowaveirradiation(seefor exampleFig.8.5(a)).

a b

Fig.8.5.TEMimagesofAgnanoparticlesformedin(a)60sand(b)75sofmicrowave irradiation[6].

Synthesis of Metal Oxides

CaMoO 4hasbeensynthesizedatlowtemperaturesbyamodifiedcitratecomplexmethod usingmicrowaveirradiation.Synthesizingmixedmetaloxidesatlowtemperaturesisa

SyntheticStrategiesinChemistry 8.7 tediousprocessanitneedshightemperatures.Asaresult,theresultingmaterialhaspoor surfaceareaandlargercrystallitesize.Forthiskindofpurpose,microwavecanbeused to effectively bring down the temperature required for the formation of mixed metal oxides.AflowchartforthesynthesisofCaMoO 4isgivenScheme8.1.Inthiscase,citric acidmakesa complexwiththemetalionsanddueto the presence of the citrate, the formation of the nanoparticles is observed. This method is also known as citric acid combustionmethod.ThecorrespondingnanoparticlesofCaMoO 4 formed areshownin Fig.8.6.

Scheme8.1.FlowchartofthesynthesisofCaMoO 4bymicrowaveassistedroute[9]

8.8 SynthesisbySolidStateDecomposition

Fig.8.6.TEMimageofCaMoO 4 nanoparticlesynthesizedat600 °Cfor3h[9] Photochemical Decomposition Method Synthesis of Metals Silvernanowireshavebeensynthesizedusingphotographicprinciples.Inthiscase,AgBr isreducedbyinpresenceofafluorescentlightandfollowedbythedevelopmentofthe film.TEMimagesAgBrandAgnanowiresformedfromthisprocessareshowninFig 8.7. a b

Fig.8.7.TEMimagesof(a)AgBrand(b)Agnanowire[10]

SyntheticStrategiesinChemistry 8.9

Synthesis of Metal Oxides Iron oxide Nanoparticles

Nanoparticles of iron oxide have been synthesized by decomposing Fe(CO) 5 under ultraviolet light. During the photolysis, decomposition of Fe(CO) 5 takes place to form nanoparticlesofironmetalandfurtheritturnsintoironoxideduetotheinstabilityof ironnanoparticles. Synthesis of EuO nanocrystals

Inaquartzvessel,Eu(NO 3)3(37.5mM)andurea(112.5mM)weredissolvedinmethanol

(400 ml) under an N 2atmosphere,andthesolutionwasirradiatedwitha500Whigh pressuremercuryarclampat25 °C.Ayellowishpowderprecipitatedafter30min.After 24 h of irradiation, the powder was separated by centrifugation and washed with methanolseveraltimes. Synthesis of CdS nanorod

Fig.8.8.TEMimageofCdSnanowiressynthesizedbyphotochemicalmethod[11]. CdS nanorods have been synthesized using cadmium salt, DNA base pairs and thioacetamide (TAA). Photoirradiation was performed by directly placing the cuvette containing the mixture of DNA, Cd salt, and TAA solutions under 260 nm UV light

8.10 SynthesisbySolidStateDecomposition irradiation for six hours. The initially occurring DNACd 2+ complex formation was confirmedbyashiftintheUVvisspectrumofthemixedsolutioncomparedtothepure DNA. After photoirradiation the UVvis spectrum was again recorded showing the formationofCdSnanoparticles.Thioacetamide acts assulphursourceinthisreaction. ThecorrespondingCdSnanowiresformedareshowninTEMimageinFig.8.8. SUMMARY Solidstatedecompositionmethodsareimportantinthesynthesisofthematerialbothin nano size ranges as well as in bulk. In the case of thermal decomposition route, the compound having lower decomposition temperature is preferred. The decomposition temperatureofthecompoundismainlybasedontheredoxpotentialofcationandanion of the compound. Microwaveassisted syntheses are more advantageous than conventionalheatingduetotheincreasedrateofthereaction.Photochemicalsynthetic methods are also advantageous due to fact that the material is formed at ambient conditionssothatthesinteringofthematerialsisreduced. REFERENCES 1. S. Navaladian, B. Viswanathan, R. P. Viswanath and T. K. Varadarajan, NanoscaleResearchLetter , 2(2007)44. 2. I.K.Shim,Y.L.Lee,K.J.Lee,J.Joung, Materials Chemistry andPhysics, (2008)(inpress). 3. C.M.Janet,B.Viswanathan,R.P.ViswanathandT.K.Varadarajan, Journalof PhysicalChemistryC ,111(2007)10267. 4. R. N. Nickolov, B. V. Donkova, K. I. Milenova1 and D. R. Mehandjiev, AdsorptionScience&Technology ,24(2006)497. 5. P. S. Nair, T. Radhakrishnan, N. Revaprasadu, G. A. Kolawole and P. O’Brien ,ChemiaclCommunication (2002)564. 6. S. Navaladian, C. M. Janet, B. Viswanathan, R. P. Viswanath and T. K. Varadarajan, Nanotechnology,19 (2008)045603. 7. X.Wang,J.Song,L.Gao,J.Jin,H.ZhengandZ.Zhang, Nanotechnology 16 (2005)37. 8. Z. Zhou, Q. Sun, Z. Hu, and Y. Deng, Journal of Physical Chemistry C, 110 (2006) 13387.

SyntheticStrategiesinChemistry 8.11

9. J.H.Ryu,J.W.Yoon,C.S.Lim,W.C.OhandK.B.Shim, JournalofAlloys andCompounds, 39 0(2005)245. 10. C.Liewhiran,S.Seraphin,S.Phanichphant,CurrentAppliedPhysics,6(2006) 499. 11. S. Liu, R. J. Wehmschulte, G. Lian and C. M. Burba, Journal of Solid State Chemistry ,179(2006)696.

Chapter – 9

NEWER SYNTHETIC STRATERGIES FOR NANOMATERIALS

P.Sangeetha

1. INTRODUCTION The emerging science in the world today mainly focuses on nanomaterials and its applications.Nanoisinterrelatedtochemistry,physicsandbiologyandithasattracted theattentionofalleminentscientiststoitsside. Nanochemistry can be described as a special discipline of inorganic or solid state chemistry. It focuses on the synthesis of nanoparticulatesystems. Thenanochemistcanbeconsideredtoworktowardsthisgoal fromthe atom“up”,whereasthenanophysicisttendstooperatefromthebulk“down”.It isschematicallyrepresentedinFig.9.1

Fig.9.1Schematicdiagram 1.1 What is Nano? Thetermnanoisameasurementofsize.Ananometre(nm)isamillionthofamillimetre. Bywayofillustration,ananometerisabout1/50,000ththewidthofahumanhair,anda sheetofnormalofficepaperisabout100,000nmthick.Ananomaterialorananoparticle isusuallyconsideredtobeastructurebetween0.1and100nm(1/1,000,000mm).The origin of the prefix "nano" is from the Greek word "nanos"’ meaning dwarf. Nanotechnologydealswithascalethatisover10,000timessmallerthanamillimetre.It

9.2 NewerSyntheticStrategiesforNanomaterials involves investigating, producing and applying structures that are smaller than 100 nanometres(nm).Atthenanoscale,thephysical,chemical,andbiologicalpropertiesof materialsdifferinfundamentalandoftenvaluablewaysfromthepropertiesofindividual atomsandmoleculesorbulkmatter.Researchanddevelopmentinnanotechnologiesis directedtowardunderstandingandcreatingimprovedmaterials,devices,andsystemsthat exploitthesenewproperties.Thesepropertieshavebeenfoundtobeveryusefulforan increasingnumberofcommercialapplications,forexample: protective coatings, light weight materials, selfcleaning clothing, to name but a few. The main classes of nanoscalestructuresandtunablepropertiesaresummarizedinTable9.1and9.2. Table9.1Mainclassesofnanoscale: Dimension

3dimensions<100nm Particles,quantumdots,hollowspheres,etc.

2dimensions<100nm Tubes,fibres,wires,platelets,etc.

1dimension<100nm Films,coatings,multilayer,etc.

Phase composition

Singlephasesolids Crystalline,amorphousparticlesandlayers,etc.

Multiphasesolids Matrixcomposites,coatedparticles,etc.

Multiphasesystems Colloids,aerogels,Ferrofluids,etc.

Manufacturing process

Gasphasereaction Flamesynthesis,condensation,CVD,etc.

LiquidphasereactionSolgel,precipitation,hydrothermalprocessingetc.

MechanicalpropertiesBallmilling,plasticdeformationetc.

SyntheticStrategiesinChemistry 9.3

Table9.2.Tunablepropertiesbynanoscalesurfacedesignandtheirapplicationpotentials

Mechanicalproperties(e.g.,hardness, Wearprotectionofmachineryandequipment, scratchresistance,tribology). mechanicalprotectionofsoftmaterials (polymers,wood,textiles,etc.)

Wettingproperties(e.g.antiadhesive, Antigraffiting,antifouling,lotuseffect,self hydrophobic,hydrophilic). cleaningsurfacefortextilesandceramics,etc. Thermalandchemicalproperties(e.g. Corrosionprotectionformachineryand heatresistanceandinsulation,corrosion equipment,heatresistanceforturbinesand resistance). engines,thermalinsulationequipmentand buildingmaterialsetc. Biologicalproperties(biocompatibility, Biocompatibleimplants,medicaltoolsand antiinfective). wounddressingsetc. Electronicalandmagneticproperties Ultrathindielectricsforfieldeffecttransistors, (e.g.magnetoresistance,dielectric) magnetoresistivesensorsanddatamemoryetc. Opticalproperties(e.g.antireflection, Photoandelectrochromicwindows,anti photoandelectrochromatic) reflectivescreensandsolarcellsetc.

1.2 Nanotechnology: Nanotechnologyisoneofthekeytechnologiesofthe21 st century.Nanotechnologyrefers broadlytoafieldofappliedscienceandtechnologywhoseunifyingthemeisthecontrol of matter on the atomic and molecular scale as shown in Fig. 9.2, generally 100 nanometers or smaller, and the fabrication of devices with critical dimensions that lie withinthissizerange.

Fig.9.2.Schematicrepresentationforatomandmolecule

9.4 NewerSyntheticStrategiesforNanomaterials

1.3 Nanomaterials: It is the study of how materials behave when their dimensions are reduced to the nanoscale.Itcanalsorefertothematerialsthemselvesthatareusedinnanotechnology. Uniquechangesoccurintheelectronicstructureandchemicalpropertiesofsolidswhen their dimensions are reduced to the nanoscale, including development of discrete electronic structure, quantum confinement, structural strain and distortion, and altered interfacial/surface reactivity. These changes translate into new physical and chemical behaviorwhichisnotobservedinthe‘bulk’formofthematerial. Giventhatprocessesatthegas/solidandliquid/solidinterfacesarelargelycontrolled bylocalphenomena(e.g.,theavailability,reactivityanddensityofsurfacesites),changes in the local electronic and physical structures of a solid brought about by ‘nanostructuring’ which shouldhaveatremendousimpact on heterogeneous chemistry (catalysis).Examples:Auforselectiveoxidation,areneandalkenehydrogenationbyIr,

Rh,photocatalyticactivityofTiO 2anduniquereactivityofoxidenanoparticles. 2. NEWER SYNTHETIC STRATEGIES TO BUILD PROTEIN BASED NANOMATERIALS: Recent progress in nanotechnology has yielded new device components with unprecedented capabilities. However, the small size of these building blocks makes it difficulttopositionthemintofunctionalassembliesusingexistingpatterningtechniques.

(a) (b)

Fig.9.3(a)Arrysofchromophoresforlightharvesting and (b) Spherical carriers for PETimagingapplications

SyntheticStrategiesinChemistry 9.5

3. SYNTHESIS AND CHARACTERIZATION OF INORGANIC NANOMATERIALS As one solution to this problem, the protein shells of two viruses are converted into scaffoldsthatcanpositionnanoscaleobjectswithexcellentspatialresolution.Inonecase, this strategy has been used to synthesize arrays of fluorescent molecules, providing efficientmimicsofthelightharvestingsystempresentinphotosyntheticorganisms.Ina second research area, welldefined core/shell materials have been prepared for applicationsindiagnosticimaging.Thecornerstoneoftheseeffortshasbeenaseriesof newsyntheticreactionsthatcanmodifybiomolecules(Fig.9.3)withhighsiteselectivity and yield. The synthetic route will focus on the development of the methods and the applicationsofthenewmaterialsthathavebeenbuiltthroughtheiruse. Thestudyofnanoscaleinorganicmaterialsisanexcitingandrapidlygrowingareaof researchwhichoffersmultipleopportunitiesforinnovationandcreativity.Ourresearch focusesprimarilyonthedevelopmentofnewsyntheticapproachestonanoparticleswith tunableproperties.Oneaspectofourworkissynthesisofnewprecursorsaswellasthe applicationofestablishedmolecularprecursorstosynthesizenanometersizemetalalloy, metal phosphide, metal oxide, and metal chalcogenide materials. Some new wet chemistryprocedurestoproducesizeandmorphologycontrollednanoparticlesare:

 Singlesourceprecursorstoheterometallicoxidematerials

 Singlesourceprecursorstophosphidenanoparticles

 Singlesourceprecursorstometalchalcogenidematerials

 Synthesisofnanoalloysfromorganometallicprecursors

 Shapecontrolsynthesesofironandmanganeseoxides

3.1 Single-Source Precursors to Heterometallic Oxide Materials Thedevelopmentofefficientphotocatalyticsystemshasbeenofvitalinterestsincethese can contribute to the reduction of pollution and solve some energyrelated problems. Howeverthesynthesisofenvironmentallyfriendlybinaryoxidesofdesiredmorphology still remains a challenge. In the synthesis described so far, the problems encountered were phase inhomogenities and the inclusion of carbon originating from the ligands.

Developingnewheterometallic(singlesource)precursorstosynthesizeAMO 3(A=Li,

Na,K,Rb;M=Nb,Ta)andMBiO 4(M=V,Nb,Ta)nanoparticles,whichareknownas

9.6 NewerSyntheticStrategiesforNanomaterials stable and efficient photocatalysts. The precursors to be synthesized are composed of salicylateandalkoxidecomplexesthatcontainthemetalelementsinthedesiredratios.A number of heterometallic complexes were synthesized (Fig. 9.4) and their utility as singlesource precursors was tested. Pyrolysis and wetchemistry routes were successfullyusedtoproducemixedoxideofmicronandnanosizedparticles.

Fig.9.4Bi 2Ta 2(sal) 4(Hsal) 4(OEt) 2 (left)andSEMimagehydrolysisproduct(right) (sal=O 2CC 6H4O,Hsal=O 2CC 6H4OH)[12]

3.2 Single-Source Precursors to Phosphide Nanoparticles

Magnetic materials can be made in a variety of forms including nanoparticles, thin molecularfilms,andbulkmaterials.

Fig.9.5.Ironphosphidenanorods[13]

SyntheticStrategiesinChemistry 9.7

Improvingthe currenttechnologiesthatutilizemagneticcompoundsdependsuponthe abilitytoproducepure materials;ofconcerninthesynthesisofthese materialsisthe ability to produce materials with a controlled stoichiometry. While most studies of magnetic nanoparticles have been on magnetic metals,alloys,andoxides,thefocusof thisresearchisonbinarymagneticmaterialscomposedoftransitionmetalsandpblock elements.

3.3 Single-Source Precursors to Metal Chalcogenide Materials The initial research will involve the synthesis of iron phosphide nanomaterials from known heterometallic precursors; there are a few binaryphasesofironphosphidethat possessmagneticproperties(FePpossessesantiferromagneticbehavior,whileFe 3Pand

Fe 2P exhibit ferromagnetic properties). Fig. 9.5 shows iron phosphide nanorods synthesized,utilizingaheterometallicironphosphoruscluster.

5 0 n m

Fig.9.6.NanoparticlesofPbSandBi 2S3[14]

The design of chalcogenide materials in discrete formsisamajorproblemformodern solidstatechemistsandmaterialsscientists.Manyofthesechalcogenidematerialshave potentialapplicationsinelectronicdevices.Itismainlyfocusedonthedesign,syntheses, anddecompositionofnewmolecularprecursorstosuchchalcogenidematerialsinforms, such as rods, wires, and more complex shapes that have been unattainable with conventional solidstate methods. Developing the chemistry of metal complexes with chalcogencontainingligandsasmolecularprecursorstoanumberofmetalchalcogenide materials,includingBi 2S3,Bi 2Se 3,PbS,PbSe,CdS,CdSe(Fig.9.6).Amajoremphasis ofthesynthesisistocontroltheorganizationofthematerialsatthenanoscaleandtofind

9.8 NewerSyntheticStrategiesforNanomaterials arelationshipbetweenthenatureoftheprecursorsandthequalityofthenanomaterials obtained.Toaccomplishthis,aninterdisciplinaryapproach,withexperienceinsynthetic and materials chemistry, as well as crystallography, will provide a plethora of opportunitiestoinvestigatetheseinterestingproblemsinnanoscience.Variousmethods are being developed to synthesize, re shape and study the assembly characteristics of chalcogenidemetalnanoparticles. 3.4 Synthesis of Nanoalloys from Organometallic Precursors Anotherresearchintheareaofnanoscalematerialsconcernsthesynthesisofalloyand intermetallic nanoparticles from the corresponding organometallic precursors. The preparationofaseriesofBiRu,BiPd,andBiPtintermetallicnanocrystalsusingwet chemistry procedures has been achieved (Fig. 9.7). These methods are based on the relatively lowtemperature reaction between two organometallic precursors in the presence or absence of stabilizing agents. The most advantageous points of these procedures consist in overcoming the need of chemical reducing agents and high temperaturedecomposition.Inaddition,regularsphericalagglomeratesareformedand theirsizecouldbetunedbyvaryingtemperatureandsurfactants . Theefficiencyofthis procedure is shown by the ability of the particles to be dried and redispersed, while maintaining their morphology. These nanoalloys have recently attracted attention because of their activity and selectivity toward numerous catalytic reactions such as dehalogenation,oxidationofalcoholsandaldehydes,aswellasforapplicationsinfuel cells.

Fig.9.7BiPdnanoparticlespreparedattwodifferenttemperatures[13 ]

SyntheticStrategiesinChemistry 9.9

3.5 Shape Control Synthesis of Iron and Manganese oxides Bulkironoxidesandmanganeseoxideshavemanytechnical applicationsduetotheir magneticandcatalyticproperties.Thesepropertiescanbeenhancedbytuningparticle sizes within the nanometer scale. Magnetic nanoparticles (NPs), for example, have potentialapplicationsininformationstorage,medicalimaging,drugdelivery,andwater remediation. Many applications, such as heterogeneous catalysis, are enhanced by the high surface area that nanoparticles possess. However, enhancement of physical properties is not only limited to size effects. There are a number of publications on nanoparticleswithshapedependentmagnetic,electronic,andopticalproperties.Mostof these studies are on NPs such as spheres, rods, wires, belts, and disks, which can be classifiedas‘traditional’shapes.Thesynthesis,thephysicalproperties,andthegrowth mechanisms of ‘traditional’ shapes, including iron and manganese oxides, have been widely investigated and are wellunderstood. However, the formation mechanism and physicalpropertiesofnontraditionalmultibranchedNPssuchastetrapods,hexapods, tripods, stars, and dumbbells, are not completely understood and have led to investigations on shape controlled synthesis and to the development of nontraditional growthmechanism.

A B C D

Fig.9.8.MnO(AandB)andFeO(CandD)NPs[13] The synthesis of new crossshaped, hexapod, and concavefaces cubic nanoparticles

(NPs)ofMnO,FeO,andFe 1xMn xOhavebeenreported.Themaingoalsofthesestudies are: 1. Todevelopsimpleandinexpensivesynthesistogrownanoparticleswithnewshapes.

9.10 NewerSyntheticStrategiesforNanomaterials

2. Tostudythegrowthmechanismsbyobservationofthenanoparticlemorphologiesas a function of the reaction parameters: surfactant ratio, water concentration, time, temperature,andprecursor. SomeofthemorphologiesobtainedareshowninFig.9.8 Amotivationinnanoscienceistotrytounderstandhowmaterialsbehavewhen sample sizes are close to atomic dimensions. Fig. 9.8 for example shows a picture of nanofibrilsthatare10to100timessmallerindiameterthanconventionaltextilefibers.In comparisontoahumanhairwhichisca.80,000nmindiameter,thenanofibersare1,000 timessmallerindiameter.Whenthecharacteristiclengthscaleofthemicrostructureisin the1100nmrange,itbecomescomparablewiththecriticallengthscalesofphysical phenomena, resulting in the socalled "size and shape effects." This leads to unique propertiesandtheopportunitytousesuchnanostructuredmaterialsinnovelapplications and devices. Phenomena occurring on this length scale are of interest to physicists, chemists, biologists, electrical and mechanical engineers, and computer scientists, makingresearchinnanotechnologyafrontieractivityinmaterialsscience.

Fig.9.9 Apictureofnanofibrilsshownwithahumanhairforreference[17] On tracking the nano evolution, it has been stated that no matter what the market outcomesinthenearorlongterm,nanosciencewillneverbeanindustryuntoitselfbuta scienceofmanyavenuesofapplication,andpossibilitythatcouldredefinethedirection ofseveralindustries.Thisinsightallowsonetorecognizethatnanotechnologyisnot"a

SyntheticStrategiesinChemistry 9.11 technology"but"asetoftechnologies,"yieldingasetoftechnicalbreakthroughsthat will sweep into many different markets. Within such a framework, the world of nanotechnologymaybedividedintothreebroadcategories: nanostructured materials, nanotools,andnanodevices. 4. Nanotools Theseincludefabricationtechniques;analysisandmetrologyinstruments;andsoftware fornanotechnologyresearchanddevelopment.Theyareusedinlithography,chemical vapordeposition(CVD),3Dprinting,andnanofluidics.Nanofluidics,thestudyof nanoscalefluidbehavior,forexample,thestudyofdynamicsofdropletsadsorbedonto surfacesundershearing,ismostlyusedinareassuchasmedicaldiagnosticsand biosensors. 5. Nanostructured Materials • NanocrystallineMaterials • Fullerenes/CarbonNanotubes • Dendrimers(OrganicNanoparticles) • PolyhedralSilsesquioxanes(InorganicOrganicHybridNanoparticles) • NanoIntermediates • Nanocomposites Nanostructured (NsM) materials are materials with a microstructure the characteristic length scale of which is on the order of a few (typically 1100) nanometers. The microstructurereferstothechemicalcomposition,thearrangementoftheatoms,andthe size of a solid in one, two, or three dimensions. Effects controlling the properties of nanostructured materials include size effects (where critical length scales of physical phenomenabecomecomparablewiththecharacteristicsizeofthebuildingblocksofthe microstructure), changes of the dimensionality of the system, changes of the atomic structure,andalloyingofcomponents(e.g.,elements)thatarenotmiscibleinthesolid and/orthemoltenstate. Thesynthesis,characterizationandprocessingofnanostructuredmaterials arepartof an emerging and rapidly growing field. Research and developement in this field emphasizes scientific discoveries in the generation of materials with controlled microstructural characteristics, research on their processing into bulk materials with

9.12 NewerSyntheticStrategiesforNanomaterials engineered properties and technological functions, and introduction of new device conceptsandmanufacturingmethods. Nanostructuredmaterialsmaybegroupedundernanoparticles(thebuildingblocks), nanointermediates, and nanocomposites. They may be in or far away from thermodynamic equilibrium. For example, nanostructured materials consisting of nanometersized crystallites of Au or NaCl with different crystallographic orientations and/or chemical compositions vary greatly from their thermodynamic equilibrium. Nanostructured materials synthesized by supramolecular chemistry yielding nanoassembliesareexamplesofthoseinthermodynamicequilibrium.Inthesubsequent paragraphs, the various classes of nanoparticles that serve as the building blocks of nanomaterialsanddeviceswillbediscussed.Theyincludenanocrystallinematerialssuch as ceramic, metal and metal oxide nanoparticles; fullerenes, nanotubes and related structures;nanofibersandwires,andpreciseorganicaswellashybridorganicinorganic nanoarchitechturessuchasdendrimersandpolyhedralsilsesquioxanes,respectively. 5.1 Nanocrystalline Materials Ceramics, metals, and metal oxide nanoparticles fall in this category. In the last two decades a class of materials with a nanometersized microstructure have been synthesizedandstudied.Thesematerialsareassembledfromnanometersizedbuilding blocks, mostly crystallites. The building blocks may differ in their atomic structure, crystallographic orientation, or chemical composition. In cases where the building blocksarecrystallites,incoherentorcoherentinterfacesmaybeformedbetweenthem, depending on the atomic structure, the crystallographic orientation, and the chemical compositionofadjacentcrystallites.Inotherwords,materialsassembledofnanometer sized building blocks are microstructurally heterogeneous, consisting of the building blocks (e.g. crystallites) and the regions between adjacent building blocks (e.g. grain boundaries).Itisthisinherentlyheterogeneousstructureonananometerscalethatis crucialformanyoftheirpropertiesanddistinguishesthemfromglasses,gelsthatare microstructurallyhomogeneous. Grainboundariesmakeupamajorportionofthematerialatnanoscales,andstrongly affectproperties andprocessing.ThepropertiesofNsMdeviatefromthoseofsingle crystals (or coarsegrained polycrystals) and glasses with the same average chemical

SyntheticStrategiesinChemistry 9.13 composition. This deviation results from the reduced size and dimensionality of the nanometersizedcrystallitesaswellasfromthenumerousinterfacesbetweenadjacent crystallites. An attempt is made to summarize the basic physical concepts and the microstructural features of equilibrium and nonequilibrium NsM. Nanocrystallites of bulk inorganic solids have been shown to exhibit size dependent properties, such as lower melting points, higher energy gaps, and nonthermodynamic structures. In comparison to macroscale powders, increased ductility has been observed in nanopowders of metal alloys. In addition, quantum effects from boundary values becomesignificantleadingtosuchphenomenaasquantumdotslasers. Oneoftheprimaryapplicationsofmetalsinchemistryistheiruseasheterogeneous catalystsinavarietyofreactions.Ingeneral,heterogeneouscatalystactivityissurface dependent. Due to their vastly increased surface area over macroscale materials, nanometalsandoxidesareultrahighactivitycatalysts.Theyarealsousedasdesirable startingmaterialsforavarietyofreactions.Nanometalsandoxidesarealsowidelyused intheformationofnanocomposites.Asidefromtheirsyntheticutility,theyhavemany usefulanduniquemagnetic,electric,andopticalproperties. 5.2 a. Fullerenes: Thediscoveryoffullerenesin1985byCurl,Kroto, and Smalley culminated in their NobelPrizein1996.Fullerenes,orBuckminsterfullerenes,arenamedafterBuckminster Fullerthearchitectanddesignerofthegeodesicdomeandaresometimescalledbucky balls.Thenamesderivefromthebasicshapethatdefinesfullerenes;anelongatedsphere ofcarbonatomsformedbyinterconnectingsixmemberringsandtwelveisolatedfive member rings forming hexagonal and pentagonal faces. The first isolated and characterizedfullerene, C 60 ,contains20hexagonalfacesand12pentagonalfacesjust likeasoccerballandpossessesperfecticosahedralsymmetry. Fullerenechemistrycontinuestobeanexcitingfieldgeneratingmany articleswith promising new applications every year. Magnetic nanoparticles (nanomagnetic materials)showgreatpotentialforhighdensitymagneticstoragemedia.Recent work hasshownthatC 60 dispersedintoferromagneticmaterialssuchasiron,cobalt,orcobalt iron alloy can form thin films with promising magnetic properties. A number of organometallicfullerenecompoundshaverecentlybeensynthesized.Ofparticularnote

9.14 NewerSyntheticStrategiesforNanomaterials

areaferrocenelikeC 60 derivativeandpairoffullerenesbridgedbyarhodiumcluster. Some fullerene derivatives even exhibit superconducting character. There has been a reportofafullerenecontaining,superconductingfieldeffectdevicewitha Tcashighas 117K. 5.2 b. Carbon Nanotubes: Carbonnanotubes(CNTs)arehollowcylindersofcarbon atoms.Their appearanceis thatofrolledtubesofgraphitesuchthattheirwallsarehexagonalcarbonringsandare oftenformedinlargebundles.TheendsofCNTsaredomedstructuresofsixmembered ringscappedbyafivememberedring.Generallyspeaking,therearetwotypesofCNTs: singlewalled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs) (Fig.10). As their names imply, SWNTs consist of a single, cylindrical graphenelayer,whereasMWNTsconsistofmultiplegraphenelayerstelescopedabout oneanother.Carbonnanotubes(CNTs)werefirstisolatedandcharacterizedbyIjimain 1991.Sincethennumerousresearcharticleshavebeenpublished,andnewapplications forCNTshavebeenproposedeveryyear.Theuniquephysicalandchemicalproperties of CNTs, such as structural rigidity and flexibility continue to generate considerable interest. Additionally, CNTs are extremely strong, about 100 times stronger (stress resistant)thansteelatonesixththeweight.CNTscanalsoactaseitherconductorsor semiconductorsdependingontheirchirality,possessanintrinsicsuperconductivity,are idealthermalconductors,andcanalsobehaveasfieldemitters.

Fig.9.10.Carbonnanotubes

SyntheticStrategiesinChemistry 9.15

Currently,thephysicalpropertiesarestillbeingdiscoveredanddisputed.Whatmakesit so difficult is that nanotubes have a very broad range of electonic, thermal, and structuralpropertiesthatchangedependingonthedifferentkindsofnanotube(defined byitsdiameter,length,andchirality,ortwist).Tomakethingsmoreinteresting,besides having a single cylindrical wall (SWNTs), nanotubes can have multiple walls (MWNTs)cylindersinsidetheothercylinders. (i) Properties of Carbon Nanotubes :Withgraphenetubesparalleltothefilamentaxis, nanotubes would inherit several important properties of ‘intraplane’ graphite. This impartsaveryuniquecombinationofpropertiesonthismaterial,namely:  High aspect ratio structures with diameters in nanometers, lengths in microns  Highmechanicalstrength(tensilestrength60GPa)andmodulus(Young’s modulus1TPa)  High electrical conductivity (10 6 ohm m typically), and for well crystallizednanotubesballistic transportisobserved  Highthermalconductivity(17505800W/mK)  Being covalently bonded, as electrical conductors they do not suffer from electromigration or atomic diffusion and thus can carry high currentdensities(10 710 9A/cm 2)  Singlewallnanotubescanbemetallicorsemiconducting  Chemicallyinert,notattackedbystrongacidsoralkali  Collectively,nanotubescanexhibitextremelyhighsurfacearea (ii) CarbonNnanotubes Developed by Arry Group Arryhasdonealotofresearchanddevelopmentworkinthecarbonnanotubessynthesis andapplication,anddevelopedascalableCVDmethodtoproducehighpuritysingle walledcarbonnanotubes(SWNTs)andmultiwalledcarbonnanotubes(MWNTs)with variousdiametersandnarrowdiameterdistribution.Thetechnologyforproducing5kg 2040nmMWNTsperdaywasevaluatedasanadvanced technologyintheworldby CASscientists.Theycanproduce1.5kgSWCNTsperdayand10kgMWNTswith8 15nmindiameterperday.Theproductionfacilitiescanbeenlargedeasily.InFig.11,the proportionofusageasconceivedbyArraygroupisshown.

9.16 NewerSyntheticStrategiesforNanomaterials

Fig.9.11.IllustrationgivenbyArraygroup[15] Nanotechnology develops the basis for increasingly smaller data memories with increasinglylargerstoragecapacity,forhighlyefficientfiltersforsewagetreatment,for photovoltaicwindows,formaterialswhichcanbeusedtobuildultralightenginesand bodypartsintheautomobileindustryorforartificialjointswhicharebettertoleratedby thehumanbodyduetoorganicnanosurfaces.Inthefuture,asnanoscalemolecularself assembly becomes a commercial reality, nanotech will move into conventional manufacturing.Whilenanotechnologyoffersopportunitiesforsociety,italsoinvolves profoundsocialandenvironmentalrisks,notonlybecauseitisanenablingtechnology tothebiotechindustry,butalsobecauseitinvolvesatomicmanipulationandwillmake possiblethefusingofthebiologicalworldwiththemechanicalworld.Thereisacritical needtoevaluatethesocialimplicationsofallnanotechnologies;inthe meantime,the Arry Group believes that a moratorium should be placed on research involving molecularselfassemblyandselfreplication. (iii) Carbon Nanotube-Based Nanodevices Carbonnanotubesareahotresearchareaatthemoment.Theexcitementhasbeenfueled by experimental breakthroughs that have led to realistic possibilities of using them commercially.Applicationscouldincludefieldemissionbasedflatpaneldisplays,novel semiconducting devices, chemical sensors, and ultrasensitive electromechanical sensors. The utility of carbon nanotubes for molecular electronics or computers, first predicted by theory and simulations, is now being explored through experiments to

SyntheticStrategiesinChemistry 9.17 fabricate and conceptualize new devices based on simulations. Carbon nanotubes are nowthetopcandidatetoreplacesiliconwhencurrentchipfeaturescannotbemadeany smaller in 1015 year’s time. Calculations show that nanotubes can have metallic or variablesemiconductingpropertieswithenergygapsrangingfromafewmeVtoafew tenthsofaneV.Experimentsprobingthedensityof states confirm these predictions. Conductivity measurements on single nanotubes have shown rectification effects for some nanotubes and ohmic conductance for others. These properties suggest that nanotubes could lead to a new generation of electronic devices. Simulations to investigatetheinteractionofwatermoleculeswithananotubetiprevealedanatomistic understandingoftheinteraction,whichiscriticalindesigningcommercialqualityflat paneldisplaysaroundcarbonnanotubes.Theiruseasultrasensitiveelectromechanical sensorshasalsobeenexplored. 5.3 Dendrimers (Organic Nanoparticles) Inrecentyears,anewstructuralclassofmacromolecules,the dendriticpolymers ,has attractedtheattentionofthescientificcommunity.Thesenanometersized,polymeric systemsarehyperbranchedmaterialshavingcompacthydrodynamicvolumesinsolution andhigh,surface,functionalgroupcontent.Theymaybewatersolublebut,becauseof theircompactdimensions,theydonothavetheusualrheologicalthickeningproperties that many polymers have in solution. Dendrimers, the most regular members of the class, are synthesized by stepwise convergent or divergent methods to give distinct stagesorgenerations.Dendrimersaredefinedbytheirthreecomponents:acentralcore, aninteriordendriticstructure(thebranches),andanexteriorsurface(theendgroups). Over 50 compositionally different families of these nanoscale macromolecules, with over200endgroupmodifications,havebeenreported.Theyarecharacterizedbynearly spherical structures, nanometer sizes, large numbers of reactive end group functionalities, shielded interior voids, and low systemic toxicity. This unique combinationofpropertiesmakesthemidealcandidatesfornanotechnologyapplications inbothbiologicalandmaterialssciences.Thestateofreportsinthecurrentliteraturehas beendirectedtowardtheirapplicationsinabroadrangeoffields,includingmaterials engineering, industrial, pharmaceutical, and biomedical applications. Specifically, nanoscalecatalysts,novellithographicmaterials,rheologymodifiers,andtargeteddrug

9.18 NewerSyntheticStrategiesforNanomaterials deliverysystems,MRIcontrastagents,andbioadhesivesrepresentsomeofthepotential applications. 5.4 Polyhedral Silsesquioxanes (Inorganic-Organic Hybrid Nanoparticles) Hybrid inorganicorganic composites are an emerging class of new materials that holdsignificantpromise.Materialsarebeingdesignedwiththegoodphysicalproperties ofceramicsandtheexcellentchoiceoffunctionalgroupchemicalreactivityassociated with organic chemistry. New siliconcontaining organic polymers, in general, and polysilsesquioxanes, in particular, have generated a great deal of interest because of theirpotentialreplacementforandcompatibilitywithcurrentlyemployed,siliconbased inorganics in the electronics, photonics, and other materials technologies. Hydrolytic condensation of trifunctional silanes yields network polymers or polyhedral clusters. Hencetheyareknownbythe"notquiteonthetipofthetongue"namesilsesquioxanes. Eachsiliconatomisboundtoanaverageofoneandahalf(sesqui)oxygenatomsandto onehydrocarbongroup.Typicalfunctionalgroupsthatmaybehydrolyzedorcondensed includealkoxyorchlorosilanes,silanols,andsilanolates. Syntheticmethodologiesthat combine pH control of hydrolysis/condensation kinetics, surfactantmediated polymer growth,andmoleculartemplatingmechanismshavebeenemployedtocontrolmolecular scale regularity as well as external morphology in the resulting inorganic/organic hybrids (from transparent nanocomposites, to mesoporous networks, to highly porous and periodic organosilica crystallites) all of which have the silsesquioxane stoichiometry.Theseinorganicorganichybridsofferauniquesetofphysical,chemical, andsizedependentpropertiesthatcouldnotberealizedfromjustceramicsororganic polymersalone.Silsesquioxanesarethereforedepicted as bridging the property space betweenthesetwocomponentclassesofmaterials.Manyofthesesilsesquioxanehybrid materials also exhibit an enhancement in properties such as solubility, thermal and thermomechanical stability, mechanical toughness, optical transparency, gas permeability,dielectricconstant,andfireretardancy,tonamejustafew. 5.5 Nano-Intermediates Nanostructured films, dispersions, high surface area materials, and supramolecular assembliesarethehighutilityintermediatestomanyproductswithimprovedproperties suchassolarcellsandbatteries,sensors,catalysts,coatings,anddrugdeliverysystems.

SyntheticStrategiesinChemistry 9.19

Theyhavebeenfabricatedusingvarioustechniques.Nanoparticlesareobviousbuilding blocksofnanosystemsbut,requirespecialtechniquessuchasselfassemblytoproperly align the nanoparticles. Recent developments have lead to air resistant, room temperature systems for nanotemplates with features as small as 67 nm. More traditionally,electronbeamsystemsareusedtofabricatedevicesdownto40nm. 5.6 Nanocomposites Nanocompositesarematerialswithananoscalestructurethatimprovethemacroscopic properties of products. Typically, nanocomposites are clay, polymer or carbon, or a combination of these materials with nanoparticle building blocks. Nanocomposites, materialswithnanoscaleseparationofphasescangenerallybedividedintotwotypes: multilayer structures and inorganic/organic composites. Multilayer structures are typically formed by gas phase deposition or from the selfassembly of monolayers. Inorganic/organic composites can be formed by solgel techniques, bridging between clusters (as in silsequioxanes), or by coating nanoparticles, in polymer layers for example.Nanocompositescangreatlyenhancethepropertiesofmaterials.Forexample, ppm level impurities can result in the formation of nanoscale aluminide secondary phasesinaluminumalloys,increasingtheirstrengthandcorrosionresistance.Magnetic multilayeredmaterialsareoneofthemostimportantaspectsofnanocompositesasthey haveledtosignificantadvancesinstoragemedia. 5.6 a. Polymer-Clay Nanocomposites The large industrial demand for polymers has lead to an equally large interest in polymer composites to enhance their properties. Claypolymer nanocomposites are amongthemostsuccessfulnanotechnologicalmaterialstoday.Thisisbecausetheycan simultaneouslyimprovematerialpropertieswithoutsignificanttradeoffs.Recentefforts have focused upon polymerlayered silica nanocomposites and other polymer/clay composites. These materials have improved mechanical properties without the large loading require by traditional particulate fillers. Increased mechanical stability in polymerclay nanocomposites also contributes to an increased heat deflection temperature.Thesecompositeshavealargereductiongasandliquidpermeabilityand solventuptake.Traditionalpolymercompositesoftenhaveamarkedreductioninoptical clarity;however,nanoparticlescauselittlescatteringintheopticalspectrumandvery

9.20 NewerSyntheticStrategiesforNanomaterials little UV scattering.Although flame retardant additives to polymers typically reduce their mechanical properties, polymerclay nanocomposites have enhanced barrier and mechanical properties and are less flammable. Compressioninjection molding, melt intercalation, and coextrusion of the polymer with ceramic nanopowders can form nanocomposites. Often no solvent or mechanical shear is needed to promote intercalation. 6. Novel Materials at the Nanoscale ––Functional Nanomaterials Morphologycontrolled functional nanomaterials have unique chemical, mechanical, electrical,optical,magneticorbiologicalpropertiesthataredistinctlydifferentformtheir macroscopic analogs and provide new diverse opportunities for promising nanotechnologies.

Fig.9.12Quantumdots:Colourtuning[2] 6.1 Nanomaterials Size and Composition-Tunable Quantum DotsConducting Developmentofascalablesyntheticstrategyisnecessarytoaddressthequantumdots’ colourtuning and stability issues. Alloyed ZnCdS/Se nanocrystals have unique compositiontunableopticalpropertiesincluding:Highluminescence/stabilityIncreased narrowluminescencespectralwidth.Thesesizeandcompositiontunablequantumdots haveapplicationsintheoptoelectronicandbiomedicalsectors.(Fig9.12)

SyntheticStrategiesinChemistry 9.21

7. ZnO/TiO 2 Nanorods or Nanoarrays and Silica-Coated Metal Nanocrystals

Fig.9.13.NanoarraysofZnO/TiO 2 [16] Researchers have focused on largescale growth of wellaligned ZnO nanorods on selectedsubstrates.Theirapplicationsareshortwavelengthoptoelectronicdevices,solar energyconversion,transparentconductingcoatingmaterialsandsensors.Researcherstry tofindotherfunctionalnanomaterials withdiversemorphologiessuchastitaniananorods andsilicacoatednanocrystals.(Fig.9.13) 8. Polyhedral Oligomeric Silsesquioxanes (POSS)

Fig.9.14.POSSMaterial

9.22 NewerSyntheticStrategiesforNanomaterials

Uniquenanostructuredmaterial,hybridinorganicandorganiccompositionsatnanoscale, development and characterization of POSSmodified epoxy and POSS (Fig. 9.14) containing high performance polymers were synthesized to enhance its thermal and mechanicalproperties. Advantages : 1.Numerouspotentialapplicationssuchasmicroelectronics,photonics, aerospace,and coatings. 2. POSS materials are found to be compatible with mostly all thermoplastics and thermosets. 3.POSSmaterialspossesssizecontrollable,processableandtunableproperties. 8.a Conducting Polymer Nanofibers via Electrospinning : The major work in this area is to develop a novel methodology for synthesising conducting polymers with high molecular weight and excellent solubility. It is also necessary to fabricate conducting polymer nanofibers via electrospinning process. Investigating size/quantum confinement effect on the optical, electrochemical and conductingpropertiesofthenanofibersandfinallytoexplorethepossibleapplicationsof the conducting polymer nanofibers as OLED semissive layer, sensors and molecular electronics. CONCLUSION: The impact of understanding selforganizing behavior, and of finding ways to further directassemblytomakeexoticnanoscalepropertiesusefulatthemacroscale,clearlywill beenormous.Therearegeneralrulesofcontrolledsynthesisanddirectedassemblytobe discovered, andthesystematicapplicationofthesewillresultintheadditionofmany different nanostructured materials. Each and every success in the synthesis of nanomaterialswillmakeavailableanewsubsetofengineeringmaterials,anditiswell knownfromcenturiesofexperiencethatthediscoveryanddevelopmentofnewsynthetic stratergiesfornanomaterialsalwayshavebeenthesourceofnewtechnology. REFERENCES: 1. A.P. Alivisatos,Science,271(1996)993. 2. A.P.Alivisatos,J.Phys.Chem.,31(1996)13226. 3.A.Henglein.,Chem.Rev.,89(1989)1861.

SyntheticStrategiesinChemistry 9.23

4.M.B.Mohamed,C.Burda,andM.A.ElSayed,Nanolett,1(2001)589. 5.J.H.Fendler,Chem.Mater,8(1996)1616. 6.H.Gleiter,ActaMater.48(2000)1. 7. C.R.Henry,Surf.Sci.Rep.31(1998)231 8.K.J.Shea,D.A.Loy,Chem.Mater.13(2001)3306. 9.K.Strawhecker,E.Manias,Chem.Mater.12(2000)2943. 10.E.P.GiannelisAdv.Polym.Sci.138(1998)107. 11.B.C.Gates,Chem.Rev.,95(1995)511. 12. Thurstonetal.Inorg.Chem.42(6),(2003),2014. 13.www.ruf.rice.edu 14.QuldElyetalChimie8(2005)1906. 15.www.patentstorm.us 16.www.mrs.org 17.http://en.wikipedia.org/

CHAPTER - 10

THE ROLE OF SYNTHESIS IN MATERIALS TECHNOLOGY Andhesaiduntothem,itisnotforyoutoknowthe timesortheseasons,whichthe Fatherhathputinhisownpower. The Holy Bible (TheActs1:7) My times areinthyhand. The Holy Bible (Psalm1:15) Toeverythingthereisaseason,anda time toeverypurposeundertheheaven: Atimetobeborn,anda time todie;a time toplant,anda time topluckthatwhichis planted; A time tokill,anda time toheal;a time tobreakdown,anda time tobuildup; Atime toweep,anda time tolaugh;a time tomourn,anda time todance; A time tocastawaystones,andatime togatherstonestogether;a time toembrace,anda time torefrainfromembracing; A time toget,andatimetolost;a time tokeep,anda time tocastaway; A time torend,anda time tosew;a time tokeepsilence,anda time tospeak; Atime tolove,anda time tohate;a time ofwar,anda time ofpeace.

The Holy Bible (Ecclesiastes3:18)

Noah’s Ark – The Technological Marvel 13.AndGodsaiduntoNoah,Theendofallfleshiscomebeforeme;fortheearthis filledwithviolencethroughthem;and,beholdIwilldestroythemwiththeearth. 14.Maketheeanarkofgopherwood;roomsshaltthoumakeintheark,andshaltpitch itwithinandwithoutwithpitch. 15.Andthisisthefashionwhichthoushaltmakeitof:Thelengthofthearkshallbe threehundredcubits,thebreadthofitfiftycubits,andtheheightofitthirtycubits. 16.Awindowshaltthoumaketotheark,andinacubitshaltthoufinishitabove;and thedoorofthearkshaltthousetinthesidethereof;withlover,second,andthirdstories shaltthoumakeit. Genesis6:1316, The Holy Bible

10.2 RoleofSynthesisinMaterialsTechnology

What is new in Materials Technology? TheLivingCellistheAllTimeMarvelofAlmightyGod,TheCreator.TheLivingCell can apparently handle enormous number of unimaginable, uncomprehendable and difficult problems (functions) with ease and spontaneity. The Living Cell performs multiple functions (reproduction, growth, defense, protein synthesis, transport of nutrients,informationstorage,sitedirectedinformationtransfer,communication,energy conversion and energy storage, sensing) simultaneously. All vital functions for the sustenance of life takes place in the living cells. Thus the Living Cells are self replicating, selfcontaining and selfmaintaining. One of the goals of Materials Technology is to design and synthesize the material with artificial intelligence that replicateLivingCellinallaspects. Aninquisitivemindnowposesthefollowingquestions: WhatistheLivingCell?WhatdoesaCellmean?Wheredoestheterm‘Cell’originate from? How can the Living Cell be multifunctional and versatile? How cells form complexorganisms?WhatisthestructureoftheCell?Whatarethedimensionsofthe LivingCell?WhataretheconstituentsoftheCell?CantheLivingCellsbemimicked? Can such mimics of the Living Cells act as molecular machines and revolutionize MaterialsTechnology? Thequeriesarerecurring. TheCellisthebasisoflife.TheCellisthesmallest unit of allliving organisms whether it be unicellular (eg., bacteria) or multicellular (eg., human beings). Human beingshaveanestimateof100trillion(10 14 )cells.Atypicalcellisof10msizeand1 nanogrammass[1]. RobertHookeistheoriginatoroftheterm“Cell”andhasbeenderivedfromthelatin word ‘Cellula’ meaning ‘a small room’. It is worth knowing few good things about RobertHookewhicharedepictedinthenextfewlines. Robert Hooke (18 th July, 1635 - 3 rd March 1703) RobertHookewasaremarkablyindustriousscientistandphilosopher.Hewasanactive, restless,indefatigablegeniusevenalmosttothelastdaysofhislife.Oneoftheimportant contributionsofRobertHooketoBiologyishisbook“Micrographia”publishedin1665 [Fig.10.1(a)].RobertHookecoinedtheterm“Cells”.Thishasoriginatedfromhis microscopicalobservationofthinslicesofcork(tissueoflightsoftbarkofMediterranean

SyntheticStrategiesinChemistry 10.3 tree).Hecoinedtheword‘Cell’totheporesseparatedbywallsbecausetheobservation remindedhimofthecellofamonastery(smallroomswheremonkslivedin)[Fig.10.1 (b)]. He has recorded his study of the plant tissue in “Observation XVIII” of the Micrographiaasfollows[2]: “Icouldexceedingly,plainlyperceiveittobeallperforatedandporous,muchlikea honeycomb,butthatthepores,orcells,……..wereindeedthefirstmicroscopicalpores Ieversaw,andperhaps,thatwereeverseen,forIhadnotmetwithanyWriterorPerson, thathadmadeanymentionofthembeforethis….”. Truly,historyowestothisindustriousscientistandphilosopher.

(a)(b) Fig.10.1.(a)TitlepageofMicrographia(1665)[3],(b)RobertHooke’sdrawingsofthe cellularstructureofcork(planttissue)andaspringofsensitiveplantfromMicrographia [4].

Living cells are divided into two types, namely, the Procaryotic Cells and the EucaryoticCells.Procaryoticcellspossesssimplestructure(eg.Bacteriacell)[5].Cells

10.4 RoleofSynthesisinMaterialsTechnology

withoutanucleusweregroupedtogetheras“Prokaryotes’.Wehumanbeings,andmost of the animals and plants, belongs to the life form enjoying cell nucleus and are collectivelycalledEukaryotes[6].Eucaryoticcell(theycarrytheirDNAwrappedina cell nucleus) possesses complex structure as represented in Fig. 10.2. The cells are surroundedbyadoublelayermembrane.Inmanycasesthecellsarefurthershieldedby acellwall.Asa resultchemicalprocessestakingplaceinacellwouldnoteasilyget disturbedbythesurroundingenvironment. Golgi Apparatus

Cell Nucleus Membrane Rough Lysosome Endoplasmic Reticulum Cytoplasm Chloroplast Smooth Mitochondria Endoplasmic Reticulum Fig.10.2.Schematicrepresentationofaeukaryoticcellanditscompartments[6] The cell possesses secluded areas for specific functions. The major organelles (specializedsubunitwithinacellthathasaspecificfunctiontoperform)andcellular structureincludethenucleus,ribosomes,mitochondria,golgiapparatus,cytoplasm,rough endoplasmicreticulumandsmoothendoplasmicreticulum.Ashousesaredividedinto livingroom,diningroom,kitchenandbedroom,livingcellstooarecompartmentalized asshowninFig.10.2.Eachcompartmenthasaspecificsetoffunctionaltasks.For instance, the nucleus contains most of the DNA which is the carrier of genetic informationinallthecellularlifeforms.Themitochondriaarethepowerhousesofthe

SyntheticStrategiesinChemistry 10.5 cells.Thesearethesiteswherecellularrespirationandaconsequentreleaseofchemical energyfromfoodtakesplace.Ribosomesaretheproteinsynthesizingmachinesofthe cell.Ribosomesarethecellsproteinfactory. Can we imagine a data storage device of micrometer (10 -6 m) size but can squeez the data equivalent of five high-density floppy disks (5 x 1.44 MB = 7.2 MB) ? Yes,suchadatadeviceisexistinginNature.Thechromosome,averylongstretchof DNA wound up in a complicated way, which determinesthegeneticidentityofevery livingorganismonthisplantisanexampleofsuchdatastoringdevice. Can we imagine a motor that is running on and on and on but only of size measuring a few hundredths of a thousandth of a millimeter? Foryoursurprisethemotorisalreadyinexistence.Itisasystemmainlyconsistingofthe proteinsactinandmyosin.Thesystemservestopowerourmuscles.Actinisasoluble proteinfoundinmusclecells.Itisthemaincomponentofthethinfilaments.Myosinis the motor protein that generates the force and movement in contraction of muscles. Myosinistheoxygen–storageproteinofthemuscle.WhenMyosincarriesanoxygen molecule, the oxygen molecule is deeply burried within the structure of the protein. Becauseoftheweakinterationspresenttheproteiniscapableofrearrangingitsstructure tobeabletotakeupoxygenortosetitfreebywayofrearrangingitselfsothatatunnelis opened between the oxygen binding site and the rest of the world. For such rapid rearrangments to take place weak interactions should be present with in the protein structureaswellaswiththesubstrate.Thefollowingaresomeoftheweakinteractions foundinbiologicalsystems(TheCell). i. Hydrogen bonding: Hydrogenatomisnormallyboundtojustoneatomofoxygenor nitrogen.InHydrogenbondingtheHydrogenatomstartsinteractingwithasecondatom. Hydrogen bonding is responsible for the extremely high boiling point in spite of its modestmolecularweight. Ifhydrogenbonding didnot exist, water would be a gas at ambienttemperatures. ii. Salt bridges: Thisisbecauseoftheelectrostaticattractionbetweenpartsofmolecules havingoppositeelectricalcharges. iii. Van der Waals forces: Theseforcesexistbetweenthenegativelychargedcloudof electronsofoneatomandthepositivelychargednucleusoftheother.

10.6 RoleofSynthesisinMaterialsTechnology iv. The Hydrophobic interaction: The tendency of oily, wateravoiding molecular surfacestosticktogetherandshutoutanywatermolecules. Thusthefirstlessononeneedtolearnisthatthekeytogeneratematerialsthatreplicate living cells or biological systems lies in designing synthetic strategies based on weak interationsbetweenthereactingsystemsasrepresentedpictoriallyinFig.10.3.. Fig.10.3.Interactionsthatstabilizethelocalstructuresinproteins:(a)Hydrogenbonds (secondary structure), (b) disulfide bridges (tertiary or even Intermolecular, (c) salt bridges,(d)hydrophobicinteraction.Theovalshapesymbolizesthehydrophobicarea fromwhichwaterisexcluded[6].

SyntheticStrategiesinChemistry 10.7

Can we imagine a catalyst capable of converting the inert nitrogen gas from the air into nitrogen fertilizer at room temperature and atmospheric pressure? The enzyme nitrogenase present in nodule bacteria that live in symbiosis with certain plants and provide them with freshly made nitrogen fertilizer produced from air and water.Thereisnotechnicalcatalystproducingammoniafromelementalhydrogenand nitrogenatambienttemperatureandpressureasthenitrogenaseofnodulebacteria.Thus itisthedreamnotyetrealizedbythescientistsallovertheworld. Can we dream of the synthesis of natural products with 100 % enantiomeric excess (ee)? Inorganicchemistry,particularlyinthesynthesisofnaturalproducts,oneofthemajor issuesofconcerneventodayistosynthesizeexclusivelyoneofthetwomirrorimaged structuresofchiralcompounds.Literally,syntheticchemistsarebattlingforeverysingle digitenhancementinthevalueofeebeyondthestatistically50%.Interestinglyenzymes candistinguishthemirrorimageswith100%reliability. Can we synthesize catalysts (nonnatural)whichareonaparwithnaturalenzymesintheirperformance?Thisisyet anotherchallengefacingmaterialstechnologytoday. Other important challenges ahead in Materials Technology are achieving computer technology that matches with human brain, achieving telecommunications technology thatmatcheswithnervoussystemofhumanbody.The only means of realizing such breakthroughs in materials technology is through the evolving synthetic strategies in Chemistry. Some Benefits from Materials Technology Audiotapes,audiotapeplayers,audiotaperecorders,calculators,cameras,compactdisks (CD’s), CD players, Barcode, Colour Printers, Computers, Digital video disk (DVD), Electronic commerce, Electronic data interchange, Email, Internet, Fax machines, Laboratory equipment, Laser printers, Laser pointers, Liquid crystal display, LCD, Mailingservices,Measuringinstruments,Modem,Network/cabletelevision,Periodicals, Newspapers,Overheadprojectors,Photocopymachines,Playgroundequipment,Radio, Refrigerators, Slide Projectors, Scanner, Search Engines, Switching technology, Telephones, Transparencies, Type writers, Video cameras, Vedio conferencing are all someofthemarvelsofMaterialsTechnology[7].

10.8 RoleofSynthesisinMaterialsTechnology

Materials Technology Vs Material’s Technology Materials technology is a science and a knowledge area that describes properties, functionsandapplicationsofdifferentmaterials[8].Materialstechnologyisdependent onMaterial’stechnology,i.e.thewaymaterialsarearchitecturedorbuildorconstructed or fabricated or synthesized. To gain knowledge of Materials technology one should understandtheroleofsynthesisingeneratingthematerials. New materials, improved productiontechniques,andminiaturizationarethe three essential ingredients that have tremendouspotentialtotriggermajortechnologicalrevolutions. Role of Synthesis Materialssynthesisissovitalthattheearlyhistoryofmankind(fromthestoneagetothe iron age) is classified based on the materials that started new eras. So far the classificationoferasisassociatedwithorbasedonmaterialsthathaverevolutionizedthe respective eras. But the role of synthesis has now become so crucial in materials technology that the production methods are going to be used as milestones of development rather than the material itself. Initially materials that are either simply foundorgatheredfromnature(wood,bones,stones)servedmankind.Progresshasbeen madefromsuchnaturallyexistingmaterialstotheuseofmetalsretrievedfromoresusing fire.Metalswerefoundtobemoreversatileintheiruseratherthannaturallyoccurring materials. Currently,metals,glassandothermaterialsarebeingreplacedbypolymers.Thisage which we live can be truly regarded as polymer age because the survival of mankind becomesquestionablewithoutpolymers(substancesthatarechemicallyassembledfrom smaller molecules). Again polymers are more versatile than metals and naturally occurringmaterialslikewoodorstone.Milestonesintheuseofmaterialsduringthelast 10,000yearshavebeenillustratedinFig.10.4. Butstillwehavenotourdestinationintechnological advancement. Even though polymershavethepotentialtocatertomany ofour needs they can not fulfill all the demandswehaveforadvancedmaterials.Theproblemsassociatedwithpolymersare: valuableresourcesandenergyareusedupintheproductionofpolymers.Alsotheyare neitherbiodegradablelikewoodnorrecyclablelikeiron.

SyntheticStrategiesinChemistry 10.9

Fig.10.4.Timelineillustratingtheuseofmaterialsduringthepast10,000years[6].

10.10RoleofSynthesisinMaterialsTechnology

Eventhoughpolymershaverevolutionizedthewaywelive,incomparisontobiological materials they are far inferior as they in no way contain any significant amount of information,alsotheycannotstoreenergy,ortheycannotactinanintelligentway.Thus thefocusoftheevolvingsyntheticstrategieshasbeentoproducematerialsmimicking biologicalmaterials.Suchmaterialsshouldbefunctionalandadaptiveonamolecular scale. Like our fore fathers and ancestors we may be of the opinion that our present technology is modern, advanced, accomplished and ultimate. Also we may tend to believe that only incremental improvements can be brought about in the present day technology.Majortechnologicalbreakthroughsarebelievedtobehardlypossible.But thisisnottrue.Thereislargeroomforthechangetobettermentaschangehappenstobe thelawoflife.Theenormousscopeandpotentialforsuchamajoradvancementcanbe noticedifwecompareourpresentdaysyntheticstrategiesorproductionmethodologies withthesyntheticstrategies adoptedbymolecular machineryinlivingcells.Solong almost all the available synthetic strategies handled bulk amounts (countless atoms or molecules) which yielded materials whose propertiescanonly becontrolledwithina limit. But the properties of materials obtained by adopting such synthetic strategies dealingwithbulkamountsarenotadaptiveorintelligent.Eventhoughthetoolsand machinerythatonecurrentlyusestocarryoutsimpleprocessesautomaticallytheycanno waygodownbelowmacroscopicoratthemostmicroscopiclevels.Insharpcontrast,in the living cells, the structures of inorganic building materials are controlled at the molecular level guiding the precipitation of the mineral from solution for instance. Incidentally,themostcomplexmachinesofthecellarenotbiggerthan25–50nm.Thus thereisamplescopeandalsothewayislongtomeetthematerialstechnologymimicking theLivingCell.AtthemomentTheTimeofreachingthedestinationisnotclear. Informationtechnology,NanotechnologyandBiotechnologyareregardedasthethree eyesofthenewmillennium.Newsyntheticstrategiescanopenaccesstonewmaterials. Materialsaretransformedintousefulproductsbyworkingonthemwithhandsandtools. Productiontechnologyunderwentadrasticchange.Frommillenniausefulproductsare made manually. Now most of them are made by machines. Except those processes wheredecisionmakingisinvolvedallothersarebeingcarriedoutbymachines.Asa

SyntheticStrategiesinChemistry 10.11 consequencelengthscaleisnolongerlimitedtosuchpartsthathumanworkerscould graspwiththeirhands.Thusminiaturizationofproductionhaspavedthewayforthe miniaturizationoftheproducts. INFORMATION TECHNOLOGY: The Beginning of Information Technology – The Age of the Printed Book: Nowthisverysecondyouarereadingthisbook.Itmeansyouaregettingbenefitedby therevolutionofbookpublishingtechnologythathasstartedin15 th century.Johannes Gutenberg(1397–1468)conceivedtheideaofprintingbookswithmovabletype.He developed his idea into a working technology. It is no easy job to achieve uniform dimensionsofletters.Suchsufficientlyuniformdimensionsoflettertypefacilitatesaflat printingsurface.Hencesmoothlinesareobtained. Through years of persistent hard workJohannesGutenbergcouldovercomethisproblembymakingthetypefrommetal. Hecouldcastalloftheminthesamemold,whichcould be combined with different lettershapesandadjustedinitswidth.Thisdevelopmentisaresultofseveralyearsof hardwork.Toachievethisobjectivehehastoborrowconsiderableamountsofmoney. He could not have given the world the printing technology if he were not to be knowledgeableinmetallurgy.Hismajorsuccessinvolvesproducing160170copiesof the42lineBibleshowninFig.10.5.Thename42lineBiblereferstothenumberof linesofprintoneachpage.ThisworkofJohannesGutenbergisoficonicstatusasthis marked the beginning of ‘Gutenberg Revolution’ and ‘the Age of the Printed Book’. Theprintingof42lineBiblebyGutenbergisaremarkabledevelopmentinthehistoryof mankindasthishasbrokentheinformationmonopoly.Thenewtechnologyhasputan endtotheDarkAges.Ithasbroughtaboutthereformation,theenlightenmentandthe riseofmodernscience.Interestingly,withinaspanof50yearsaftertheproductionof Bible with printing technology developed by Johannes Gutenberg, more books were producedthaninthe1000yearsbefore.ThetechnologyJohannesGutenbergdeveloped lasted for more than 500 years. Later on this printing technology of Gutenberg was replaced by light reprography. This is followed by the new information revolution broughtaboutwiththeinventionofpersonalcomputers.

10.12RoleofSynthesisinMaterialsTechnology

Fig.10.5.(a)Acopyofthe42lineHolyBible,themajorworkofJohannesGutenberg [9].

Advent of Computers and Internet – Information Explosion: Advent of computers and widespread growth of availability and use in internet has broughtaboutrevolutioninthefieldofinformationtechnology.Sincethisdevelopment hasonlytakenplaceoverthepasttwodecadeswearefortunateenoughtowitnessthe progress. But how could this happen? What is the driving force for such a drastic explosionininformationtechnology?Literallywithcomputersandinternetthewhole worldisatourfingertips.Themarvelousprogresscanbeattributedtothenewsynthetic strategies that facilitated miniaturization of electronic elements and circuits. Miniaturization means not alone saving space or materials but miniaturization of electroniccircuitsmadethemcosteffective,efficientandfaster.1000foldenhancement ismemorycapabilitiesandcalculationspeedshavebeenachievedwithinashortspanof 10yearsi.e.,fromearly1980’sto1990’s.Thenumberoftransistorsintypicalmicrochip kept on increasing exponentially from 1971. For every 18 months the number of transistors on a microchip doubled. In 1971 the microchip contained only 2300 transistors when Intel launched the world’s first microchip. Nearly a hundred fold increaseinthenumberoftransistorshasbeenachievedby1985withthecreationofIntel 386processorwith2,75,000transistors.AndtodaytheTukwilacontained2billions transistors.Veryrecently,i.e.,on14 th April2008,Intel,themicrochipleader,unveiled

SyntheticStrategiesinChemistry 10.13 two drastically different processors. They succeeded in creating the worlds smallest microchip(computerprocessor)namelytheatomchipaswellastheworldsbiggestchip namelytheTukwila.Thesizeoftheatomchipisofababy’sfingernail.Thechippacks 47milliontransistors.Butconsumesonlyoverahalfofawattofpower.Theatomchip canpowerhandheldinternetdevicesnamelythemobileinternetdevices,smallhandheld computersthatwillhelpinemailing,letterwritingandcalculations.Onthecontrarythe Tukwila,thenewIntelchippossessingthehighestnumberoftransistorseverputonthe slabofsiliconwith2billiontransistorscandotheworkoffourcomputers.Thisworld’s biggestmicrochipconsumes130170wattsofpower.Thiswillhelpscientiststobuild gaint ‘number crunchers’ far more powerful than the TataCRL ‘Eka’ super computer built in India (the TataCRL ‘Eka’, computer is the world’s forthfastest computing machine).Withthehelpofsuchsupercomputersfuelledbymicrochipswith2billion transistorscompletegeneticsimulationofahumancellcanbeachievedwithinaspanof a decade or even lesser time period. This facilitates doctors to precisely simulate an unhealthycellinahuman.Thusexactmedicationcanbepossible.Withtheadventof nanotechnologyunimaginableadvancementsinthefieldofinformationtechnologyare anticipated. NANOTECHNOLOGY: ThewordNanotechnologywasusedforthefirsttimebyTaniguchiattheUniversityof Tokyo,Japan,in1974.Hewasthenreferringtoneedofelectronicsindustrytoengineer materialsatnanometerscale[10]. On29 th of December, 1959, Richard P. Feynman took the shiny example of the livingcelltodrivehomehispointthatindividualatomscanbearrangedinthewaywe want them to be. With this ultimate degree of miniaturization all the information containedinallthebooksintheworldcanbestoredinthegrainofasand.Livingcellis notonlycapableofstoringenormousamountofinformationinaverysmallvolumebut alsoequippedwiththehardwaretoreadouttheinformationandretrievethesamewhen neededandputthesameintoaction.InananalogouswayRichardP.Feynmanprofessed thatitshouldbepossibletowritetheentireEncyclopaediaBritannicaontothepointofa needle.Inthosedayswhencomputerswerehugemachinesthewiringofwhichfilledthe

10.14RoleofSynthesisinMaterialsTechnology wholeroomcompletely,heisgeniusenoughandforesightedtoprofessthatcomputers ofthefutureshouldbemadeofwiresthatwouldonlybe10or100atomsindiameter. Role of Synthesis in Nanotechnology: CarbonmaterialsareveryimportantforNanotechnologytoflourish.Carbonmaterials are vital, versatile, amazing and unique. Carbon exists in different allotropic forms, namely,graphite,diamond,fullerene,andnanotube.Graphitewithitstwodimentional hexagonalarrayofcarbonatomsanddiamondwithits three dimensional structure are wellknown.Fullereneandnanotubesarenewlydiscoveredallotropicformsofcarbon. Synthetic Strategy that lead to the formation of Fullerenes: AdreamteamoffivescientistsnamelyKroto,Heath,O’Brien,CurlandSmallyinRice Quantum Institute, Texas were trying to understand how longchain carbon molecules (cyclopolyynes) are formed in interstellar space. To find an answer, they started vapourizinggraphite,thegrandoldallotropeofcarbon,byirradiatingwithlaser.This experiment has lead to the serendipitous discovery of C 60 molecule, named as

Buckminsterfullerene.ThestructureofC 60 moleculeconsistedof32faces,12ofwhich arepentagonalandtheremaining20arehexagonal.ThestructureofC60isanalogousto common foot ball shown in Fig. 10. 6. (a) The experimental set up containing the vapourization chamber is shown in Fig. 10. 6. (b). Later on many studies have been carriedoutonthepropertiesandapplicationsoffullerensandfullerenederivatives.Not oniy that, fullerene research paved path for the discovery of multiwalled carbon nanotubesbyIjima.Simplythesyntheticmethodologyadoptedhaschangedthecourse anddestinationofcarbonscienceandtechnology. Carbonspeciesfromthesurfaceofasolidgraphitediskarevapourizedusingapulsed lasersourceinthepresenceofHeenvironment.Nd:YAGlaserproducingpulseenergies of~30mJisused.Inatypicalexperimentthepulsedvalveisopened.Vapourization laserisfiredontotherotatinggraphitedisk.Carbon species start vaporizing into the heliumstream,cooledandpartiallyequilibratedintheexpansion.Asaresultmolecular beamisformedwhichtravelsintotheionizationregion.Theclusterswereionizedbya laserpulseandtheproductswereanalysedbymassspectrometer. Duringtheprocess graphite disk is rotated slowly to produce a smooth vaporization surface. The vaporizationlaserbeamisfocusedthroughthenozzletostrikethegraphite.Thespecies

SyntheticStrategiesinChemistry 10.15 inthevaporizedgraphiteplasmaarecooledandclusteredbythethermalizingcollisions of He carrier gas. Also the carrier gas provides necessary wind to carry the cluster throughtheremainingofthenozzle.Theclusterfilledgasexpandsfreelyatthenozzle andformasupersonicbeamwhichisprobedbymassspectrometer[11]. Kroto,Smalley and Curl were awarded nobel prize for Chemistry in 1996 for their discovery of Fullerenesin1985. Fig.10.6.(a)Afootball(theC60moleculeissupposedtohavethestructureformedwhen eachvertexontheseamsofsuchaballisreplacedbycarbonatm,(b)Schematicdiagram ofthepulsedsupersonicnozzleusedtogeneratecarbonclusterbeams EventhoughFullerenswerediscoveredin1985,itisnotuntil1991whenKratschmerand Huffmanevolvedasyntheticstrategybasedonarcdischargeforthemassproductionof fullerenesthatfullereneresearchgrewrapidlyandinonewaythisisafoundationstone forthefuturediscoveryofnanotubesbyIjimain1991.Hencesyntheticstrategiesplaya pivotalroleinthebirthaswellasdestinationofanynewtechnology. WiththewaysbeingavailableforthemassproductionofFullerenes,derivativesof fullerenes could be synthesized. Endohedral compounds, exohedral compounds and heterofullerenesarethethreeclassesoffullerenederivative[12]. Fullerenederivativeswithatleastoneatomorion located inside the 7 Å cage of fullerene are called endohedral fullerenes. Endohedral compounds are more polar comparedtotheparentfullerenemakingtheseparationoffullereneseasier.Electronic properties of fullerenes can be tuned by forming endohedral derivatives. Endohedral compoundsfindutilityinthefabricationofsolarcells,linearopticalunitsandinphoto conductors.Someofthewaysofsynthesizingendohedralderivativesarebyheatingthe fullerenedirectlywiththeguestgasunderpressure;evaporatingfullereneinpresenceof

10.16RoleofSynthesisinMaterialsTechnology metals or metal oxide to be hosted by using laser source or by generating an arc. Endohedralcompoundswithnoblegases,alkalimetalsandlanthanidesarewellknown.

RecentlyN@C 60 hasalsobeensynthesized.La@C 82 couldbesynthesizedbyevaporating graphiteimpregnatedwithLa 2O3usingalaser.Rg@C 60 (Rgraregas),Li@C 60 havealso beensynthesized[13].

Exohedral compounds: ByreactingC 60 withCrO 2(NO 3)2acompoundC 60 Cr 2N6O21 was obtained.Inthiscompound50%CrspeciesareinCr 3+ andtheother50%werefoundto be in Cr 6+ state. Such exohedral compounds are useful for catalytic and medicinal applications. Intercalation Compounds: Alkalimetalintercalatedfullerenecompoundswerefoundto be super conductors. For instance, K 3C60 has a transition temperature of 19.8 K and

Cs 3C60 exhibitedatransitiontemperaturesupto40K.Compounds like (NH 3)6Na 3C60

(NH 3)8Na 2C60 showedimprovementsintransitiontemperatures.Ba 6C60 andSm 3C60 have alsobeenfoundtobesuperconductors. Carbon Nanotubes Synthesisofcarbonnanotubeswithdesiredpropertiesinlargequantitiesisachallenge ahead. The second problem in the successful utilization of CNT’s for a variety of Technologiesistoremovetheimpuritiespresentsuchasthecatalystnanoparticles(Fe, Co, NiY and few others), amorphous carbon and fullerenes. The third hurdle is to develop synthetic strategies that exclusively give either SWNT’s or MWNT’s as currentlythereisnopathavailabletoseparatethem.Makingthemsolubleisalsoanote worthyproblem. Carbon nanotubes are a key component of nanotechnology. Carbon nanotubes (multiwalled tubes) were discovered by the Japanesescientist Iijimain1991.Carbon nanotubes were obtained as a byproduct during fullerene synthesis. Discoovery of carbon tubes has supplemented the fullerene research to a major extent. Multiwalled carbon nanotubes should be distinguished from single walled carbon nanotubes. The differencebetweenthemispictoricallyrepresentedinFig.10.7.Singlewalledcarbons nanotubes contain one single cylinder whose wall is made up of hexagonal carbon structure. Multiwalled carbon nanotube contains concentric cylinders (one inside the other)witheachcylindricalwallbeingmadeofagraphenesheet.

SyntheticStrategiesinChemistry 10.17

Fig.10.7.Schematicrepresentationofsinglewalledandmultiwalledcarbonnanotubes [12] Thediscoveryofsinglewalledcarbonnanotubes(SWCNT’s)isincidental.Aseriesof failed attempts to synthesize MWCNT’s (multiwalled carbon nanotube’s) filled with transitionmetalsresultedintheformationofsinglewalledcarbonnanotubes.Thecredit of the discovery of single walled carbon nanotubes should be given to two research groupswhoworkedindependently,namelyIijimaandIchihashi,NEC(NipponElectric Company),JapanaswellasBethuneetal.,fromIBM,California[14]. Nanotubes of carbon are not soluble. As a result separation and purification are a problem. This has hindered the large scale production of carbon naotubes partially. Fortunatelythesetubes arelesssusceptibletocombustion.Soheating inoxygencan bruntheimpurities(otherformsofcarbonnamelyamorphouscarbon)andcanyieldpure CNT’s[13]. Singlewalledcarbonnanotubescanbeenvisagedorconceptualizedorunderstoodas seamless(continuousanduniformthroughout)cylindersrolledupfromgraphenesheet as represented in Fig. 10. 8. A graphene sheet is a monolayer of sp 2 bonded carbon atoms.

10.18RoleofSynthesisinMaterialsTechnology

Singlewallednanotube GrapheneSheet

Fig.10.8.Hexagonalnetworkofcarbonatomsrolleduptomakeaseamlesscylinder [15]. Dependingonthewayinwhichthegraphenesheetisrolleduptherecanbethreetypes ofcarbonnanotubes,namely,zigzag,armchairandchiral.InzigzagtubessomeoftheC Cbondslieparalleltothetubeaxis.Thenamezigzagcomesfromthefactthattheedge ofthetubepossesseszigzagstructure().InarmchairtubesomeoftheCCbonds lieperpendiculartothetubeaxis.Thenamearmchaircomesfromtheveryshapeofthe edgeofthetubewhichlookslikearmchair().Intermediateorientationsof the graphene sheet result in ‘chiral’ carbon nanotubes. Thus the classification of nanotubesintozigzagorarmchairisbasedonthe appearance of the rim of the tube formed.Theformationofthethreetypesofnanotubesbychangingthewayofrollingthe graphenesheetisrepresentedschematicallyinFig.10.9 Cabonnanotubeshaveseveralpotentialapplications.Thisisbecauseoftheirunique properties.Duetothehighmechanicalstabilitycarbonnanotubesarenowbeingusedin carboncarboncomposites.Ascarbonnanotubesarecapableofemittingelectronsfrom thetubeendstheyareusedforflatscreenapplications.Carbonnanotuesserveashost materialsforLiorH 2 andcanbeexploitedinenergystorageapplications.

SyntheticStrategiesinChemistry 10.19

Fig.10.9.Schematicrepresentationoftherelationbetweennanotubeandgraphene[16]. Evolution in Synthetic Strategies Someofthewaysofsynthesizingnanotubesinclude: 1.Archdischargeorvaporizationprocess(inthepresenceoftransitionmetalcatalyst) 2.Laserevaporationofgraphite(Laserfurnaceprocess) 3.ChemicalVapourDeposition,CVD(CatalyticPyrolysisofhydrocarbons)orCatalytic ChemicalVapourDeposition(CCVD) 4.TemplateCarbonizationMethod Thefinedetailsofthesyntheticstrategiesaregiveninthenextfewpages. 1. The Arc-Discharge Process: Carbonnanotubeswerefirstintroducedtotheworldbyusingthissyntheticmethodology. As stated earlier carbon nanotubes occurred as a byproduct during the synthesis of fullerenes.Kratschmerandcoworkershaveusedthesamearcdischargemethodin1990 for the mass production of fullerene. The method they have employed comprises of

10.20RoleofSynthesisinMaterialsTechnology evaporating the graphitic anode. The two electrodes were made to contact with each otherbyapplyinganacvoltageinaninertgasatmosphere.Thisresultsinthegeneration ofanarcthatevaporatestheanodicgraphite.Thus fullerenes were generated in bulk amounts. Fig.10.10.SchematicrepresentationoftheapparatususedforthesynthesisofCNT’s [17] ThearcdischargeapparatususedfortheproductionofcarbonnanotubesisshowninFig. 10.10.Thechamberisfirstmadefreefromatmosphericairbyevacuatingthereaction chamberwithavacuumpump.Ambientgas(HeorArorCH 4)isintroducedintothe chamber.Adc(directcurrent)arcvoltageisappliedbetweenthetwographiteelectrodes (rodsofcarbon).Theanodicgraphiteevaporates.Fullerenesintheformofsoothare depositedinthechamber.Inadditiontofullerenes,thegreattreasure,CNT’swerealso foundtobedepositedontothecathodefromtheevaporatinganode.TheseCNT’swere foundtobemadeofcoaxial(concentric)graphenesheetsandaretermedasmultiwalled carbon nanotubes. These results are obtained when pure graphite rods free from any

SyntheticStrategiesinChemistry 10.21 metalimpuritiesareusedasanodeandcathode.Byusingthisarcdischargemethodand byemployingHeenvironmentlargescalesynthesisofMWCNT’scouldbeachieved. If the synthetic condition is slightly changed by using metal catalyst (Fe or Co) containing graphite rod as anode instead of pure graphite rod, single walled carbon nanotubescouldbeobtainedinsteadofmultiwalledcarbonnanotubes.Ineithercasesthe cathode is only pure cathode. Experiments have been carried out by changing the atmospherewithinthereactionchamber.AmongHe,ArandCH 4,CH 4gaswasfoundto be the best as it resulted in the formation of highly crystalline nanotubes with few coexistingnanoparticleswhicharenotwanted.Themajorandessentialdistinguishing featureinthesynthesisoffullerensandCNTsisthat,fullerenecannotbeproducedwhen reaction chamber contains hydrogen containing gases (CH 4 or even H 2). At the same timepresenceofenvironmentofH 2facilitatestheformationofCNT.Duringtheprocess ofarcevaporationinCH 4,thermaldecompositionofmethanetakesplaceleadingtothe insitugenerationofH 2asindicatedinthefollowingequation.

CH 4C 2H2+3H 2

In presence of CH 4, the evaporation of graphitic anode thus takes place in pure hydrogengasenvironment.Thushydrogenarcdischargeiseffectiveinthegenerationof highly crystalline carbon nanotubes (multiwalled). As hydrogen arc results in high temperature and resultant high activity, production of MWCNT’s in hydrogen atmosphere(CH 4environment)ismoreeffectivethaneitherHeorAratmosphere.Not onlythat,theunwantedcarbonnanoparticles(amorphouscarbon)whichareubiquitous andunavoidablecanbeminimizedbyusinghydrogenarcprocess SWNT’sweresynthesizedintheyear1993,justtwo years after the discovery of MWNT’sintheyear1991.SWNT’sweresynthesizedbythearcdischargeprocessusing agraphiteanodebutwithmetalcatalyst(eitherFeorCoormetalalloysinsomecases likeNiY).InsharpcontrasttothesynthesisofMWCNT’s,SWNT’swerenotfoundon thecarbondepositoncathodebutwereobtainedfromthesootinthegasphase. Xinluo Zhao and coworkers has succeeded in synthesiszing highly crystalline SWCNT’swithacleansurfaceinlargequantitiesbyemployingarcdischargeprocess. TheSWNT’swereobtainedwith70at.%purity.

10.22RoleofSynthesisinMaterialsTechnology

Fig.10.11.Arc–dischargechamberwithawebofSWNT’s[18](ArrowAshowsthe starting point of the SWNT web; arrow B shows the black thick region of SWNT’s; arrow C shows the halftransparent thin region of SWNT’s growth; arrow D points towardstheroofofthearcdischargechamberwherethethinfilmofSWNT’sislocated. Thisfilmcanbepeeledofintolargeslices) The carbon electrode employed comprises of 1 at% Fe catalyst. Inert atmosphere is maintained in the reaction chamber by H 2Ar mixture. The catalyst Fe nanoparticles presentintheproductCNT’scouldbecompletelyeliminatedbyheatingtheproductin airat693KandalsobythesubsequentacidtreatmentwithHCl. Thetwographiteelectrodeswereheldverticallyoneovertheotherwiththeanodeat the lower end and the cathode at the upper end separated by 2 mm distance. The electrode assembly was held in the centre of the vacuum chamber. The anode is the carbon containing Fe catalyst and the cathode is pure carbon. Inert atmosphere is maintainedinthereactionchamberbypassingH 2Armixture.Thesynthesistimewas roughly3mm.ThishasresultedintheformationofmacroscopicwebofSWNT’sas

SyntheticStrategiesinChemistry 10.23

showninFig.10.11.ThelengthoftheSWNT’sisnearly 30 cm. H 2Ar gas mixture playedacrucialroleinthesynthesisofSWNT’s. Fig.10.12.TEMimagesof(a)asgrownCNT’s(lowmagnification,ironparticlesare seenalongwithtubes),(b)asgrownCNT’s(highmagnification,ironparticlesareseen alongwiththegraphenelayers)(c)purifiedCNT’s(lowmagnification,onlyCTN’sand noFeparticles)(d)purifiedCNT’s(highmagnification,onlygraphenefoldedsheetsand noFeparticles)[18]. TEMimagesofasgrownandpurifiedSWNT’sareshowninFig.10.12.Boththe lowmagnificationandhighmagnificationimagesareshown.ItisclearfromtheTEM images that after heat treatment and the subsequent acid treatment with HCl the particulateimpurities(catalystaswellasamorphouscarbon)werecompletelyremoved yieldingcleanandpureSWNT’s. 2. Laser Furnace Process Theenergydensityoflasersishighercomparedtoanyothervaporizingdevice.Solasers are appropriate means to vaporize materials like carbon with high boiling temperature [17]. Typical laser furnace experimental set up is shown in Fig. 10.13. The essential componentsareafurnace,aquartztubeprovidedwithawindowatoneendandatrapfor

10.24RoleofSynthesisinMaterialsTechnology

CNT’sprovidedwithawatercirculationattheotherend,flowsystemforinertgas(Ar), lasersourceandthecarbontarget.

Fig.10.13.Laserfurnace(vaporization)apparatus[19] ThecarbontargetusedisacompositeofcarbondopedwithcatalyticCoNialloy.The carbon composite (CoNi/graphite) is placed at the centre of the quartz tube having windowatoneendthroughwhichthelaserbeampenetratesintothequartztube.The laser source used is Nd:YAG (Neodymium : Yttriumaluminiumgarnet) and this can produceatemperatureof1200ºCinthefurnace.CO2canalsobeused.Alaserbeam fromtheaforementionedsourceisintroducedintothequartztubethroughthewindow andisfocusedontothecarboncomposite.Argasiscirculatedthroughoutthefurnace. The carbon composite is vapourized when the laser beam hit the target and forms SWNT’s.TheSWNT’sproducedarecarriedtotheotherendofthequartztubeprovided withatrap(withwatercirculationfacility)wheretheSWNT’sarecollected. Itisimportanttonotethatthesurfaceofthecarboncompositeshouldbekeptasfresh aspossiblefortheprocessofvaporizationtobehomogeneous.Sincethereisnowaythe targetcanbemoved,thefocusofthelaserontothetargetshouldbechangedfromtimeto time.Laserfurnacesynthesisisanefficientroutetosynthesizebundlesofsinglewalled carbonnanotubeswithanarrowdiameterdistribution.

SyntheticStrategiesinChemistry 10.25

3. Chemical Vapour Deposition Inthismethodofsynthesisahydrocarbon(acetylene,ethylene,benzeneormethane)is thermallydecomposedinthepresenceofatransitionmetalcatalyst(Fe,CoorNi)or catalystsupport(alumina,silicaorzeoliteserveasusefulsupportsfortransitionmetals) Theprocessofsynthesiscanbecarriedoutevenatrelativelylowertemperatures(600 1200ºC)thanthosewhicharenormally encounteredineitherarcdischargeprocessor laser – vaporization processes. But we have to forgo the quality of the nanotubes obtainedintermsofcrystallinitywhenthesynthesistemperaturesarelower.Sincethis synthetic methodology can be employed at relatively lower temperatures and ambient pressure,CVDissimpleandalsoformsaviablemeansofproducinglargeamountsof nanotubes. Hydrocarbons in either liquid (benzene, alcohols) or solid (camphor, napthelene)orgaseous(CH 4orC 2H2)canbevirtuallyemployedascarbonprecursors.

Fig.10.14.ExperimentalsetupforChemicalVapourDepositionSynthesis,(b)probable modelsofCNTgrowth[17] TheexperimentalsetupusedforaCVDorCCVD(catalyticchemicalvapourdeposition) synthesisisshowninFig.10.14(a)andthepossiblewaysinwhichthenanostructures growoncatalystparticlesisrepresentedpictoriallyinFig.10.14(b). InatypicalprocessofCVD,thecatalystisplacedinthemiddleofaquartztube. The quartz tube with the catalyst is placed in a furnace capable of generating and sustainingtemperaturesbetween(6001200ºC).Thehydrocarbonvapourisallowedto pass through the quartz tube containing catalyst material present at sufficiently high

10.26RoleofSynthesisinMaterialsTechnology temperature.Thehydrocarbongetsdecomposed.CNT’sstartgrowingonthecatalyst particles. The temperature of the furnace is then cooled to room temperature. The CNT’sarecollected.Ifthecarbonprecursorsweretobeinliquidformasinthecaseof either benzene or alcohols then an inert gas like Ar is bubbled through the flask containing the liquid hydrocarbon. The liquid hydrocarbon in the flask is heated simultaneouslysothatvapoursofthehydrocarbonaregenerated.Thevapoursofthe hydrocarbonarethuscarriedbytheinertgasthroughthecatalystparticleslocatedinthe hotzoneofthefurnace.Ifthecarbonprecursorsweretobeinsolidstatelikethoseof naphthaleneorcamphor,thehydrocarbonvapoursinsuchcasesaregeneratedbyplacing theminanotherfurnacekeptatalowertemperaturepriortothemainfurnacewherethe deposition of carbon takes place over the catalyst particles. Analogous to the carbon precursorsthecatalystmaterialscanalsobeineithersolidorliquidorgaseousstate.In thehightemperaturezonethehydrocarbonvapourgetsdecomposed.Avarietyofcarbon species are formed. Such carbon species are capable of dissolving in the metal nanoparticlesofthecatalyst.Onceacatalystparticleissupersaturated, carbonspecies startsprecipitatingout. Initially,fullerenedomelikestructurewillpetrudeoutofthe catalyst particle which extends into carbon cylinders. The position and direction of growth of carbon from the metal nanoparticle depends on the interaction between the metalparticleandthesupport.Iftheinteractionbeweenthemetalparticleandthesupport isstrong,theparticleisliterallyimmovable.The decomposed carbon species will be adsorbed into the particle initially from all the direction where the catalyst particle surfaceisexposedtothecarbonspecies.Oncealevelofsupersaturationoftheparticle with carbon species is reached no more carbon species are adsorbed in to the metal particle.Nowcarbonhastoprecipitateoutfromthemetalparticle.Sincetheparticleis stronglyheldbythesupport,theonlydirectioninwhichthecarboncanprecipitateoutis fromthetip.Theprocessofdissolutionnowtakesplace from the sides of the particle near the base and the excess carbon is precipitatedatthemouthortipoftheparticle. Since the grown process starts at the sides of the base this is known as base grown carbon. Onthecontrary,ifthereweretoexistweak interaction between the catalyst particle and the support, the particle can now be easily lifted vertically up above the supportsurfacethemovementwhentheparticlegetssupersaturatedwithcarbonthatis

SyntheticStrategiesinChemistry 10.27 beingabsorbedfromthetip.Sincethegrowthofthetubeisnoworiginatingatthetipof themetalparticlethemechanismisregardedattipgrownCNTprocess.Dependingon thesizeofcatalystparticle,wegeteitherSWNT’sorMWNT’s. 4. Template Carbonization Method: Thestructuredirectingmaterial(template)usedforpreparingnanotubesinthismethodis theporousaluminiumplate.Channelsarecreatedinthealuminiumplatebytheprocess ofanodicoxidationinthepresenceofsulphuricacid.Carbonisdepositedpyrolytically ontothechannelsinaluminiumoxidefilmat800ºCininertatmosphere.

Fig.10.15.Synthesisofcarbonnanotubesbytemplatecarbonizationmethod[12] Aftertheprocessofcarbondepositionandsubsequentcarbonizationthealuminiumoxide templateisremovedbytreatmentwithHF(hydrofluoricacid)solution.(Note:HFshould behandledwithutmostcare.ProperhandglovesshouldbeusedwhilehandlingHF.Itis sohazardousthatthroughskinitcandamageeventhebones).Thisresultedinuniform monodisperse carbon nanotubes with uniform length, diameter and thickness. As the temperatureofthecarbonizationisnothigh(800ºConly),thetubescontainedstructural imperfections. Typical synthetic steps involved in a template assisted synthesis are pictoriallyrepresentedinFig.10.15.

10.28RoleofSynthesisinMaterialsTechnology

Can a carbon source as common as kerosene be used for the synthesis of nanoforms of carbon materials? Keroseneisusedasafuelforcookingandlightening.Itisaresidueofthepetroleum refinery. Kerosene contains a mixture of various short and long chain aromatic and aliphatic hydrocarbons. Sharon and coworkers have used kerosene as precursor for preparingvariousnanoformsofcarbon.Themethodofsynthesiscomprisesofpyrolysis ofkeroseneat1000ºC.Theexperimentalarrangementcomprisesofaquartztubeheld horizontally in a furnace. Kerosene is placed in a round bottom flask and heated thermostaticallyat90ºC.ThevapoursofkerosenealongwithArgaswereallowedto passthroughthequartztubeheldinafurnace.Stainlessstealplatesinrectangularshape wereplacedinthecenterofthequartztube.Thestainlessstealplateservesasasubstrate forthedepositionofvariousformsofcarbonmaterials.Straight,stiffandlongfibers, flexible, thin hair like threads, soft wool like clusters of carbon, uniform nanotubes, bittergourdlikeroughfibers,earthwormlikenanofibersandcarbonthinfilmdeposits were found around and on the stainless steal substrate. SEM (scanning electron microscope)imagesofthevariousnanoformsofcarbonsynthesizedareshowninFig. 10.16.Itistobenotedthatthesubstratestainlessstealisacomplexcatalystofdifferent compositionswhichgovernthegrowthandorientationofthenanostructures.Important questions one should now consider are: How a single carbon precursor under given conditionsofsynthesisyieldsdifferentnanostructuressimultaneously?Whatfactorsare responsibleforcontrolofsizeandshapeofthenanostructures?Caneachofthedifferent carbonnanostructuresbeseparated?Howdoesthesyntheticstrategyadopteddictates theultimatematerialstechnology?Orinotherwords,whatimpactdoesthissynthetic strategyhaveontheultimatematerialstechnology?Theultimatematerialstechnologyis afunctionofmaterialstructureaswellasmaterialpropertywhicharecontrolledbythe specificsyntheticstrategyemployed.Forinstance, as in the present case, the fibers obtained were found to be conducting and such conducting fibers are useful in biosensors,ionactivatedmolecularswitchesandfor the fabrication of microelectrodes formedicinalapplications.Compositefiberscanbeobtainedfromhairlikeandwool likefibers.Thebittergourdlikefiberswithextremelyirregularoutersurfaceyieldslarge specificsurfaceareavaluesandwillbeusefulforcatalyticapplications.Theelectrodes

SyntheticStrategiesinChemistry 10.29 fabricatedfromthethinfilmcarbonsynthesized wasfoundtobeasubstitutestoan expensiveRuO 2coatedtitaniumsheetwhichisusedasanodeagainstamercuriccathode fortheelectrolysisofbrinesolution.Theelectrochemicalpotentialrangeoftheelectrode fabricated from carbon thin film material was found to be 1.24 to 1.67 V Vs SCE (standardcalomelelectrode)indicatingthatwaterwillnotbeelectrolysedbyusingthe electrodes(bothanodeandcathode)fabricatedbythiscarbonmaterial.Theelectrodes fabricatedfromthinfilmcarboncouldelectrolyze30%NaClsolutionat300mAcm 2for more than 110 h continuously with out any deterioration [21]. Thus the electrodes fabricated from the carbon obtained by kerosene pyrolysisareuseful forChloroalkali industryofferingasolutionfortoxicityproblembyeliminatingthehazardousmercury contamination[20].

(a)(b)(c) Fig.10.16.SEM(scanningelectronmicroscope)imagesof(a)hairlikefibers,(b)bitter –gourdlikeroughfibersand(c)carbonthinfilmgrownonastainlesssteelsubstrate Carbon Nanotubes as STM and AFM Tips: Gerd Binnig and Heinrich Rohrer at the IBM (International Business Machines Corporation)researchlaboratoriesin Zurichinthelate1970’sdevelopedamethodof structuralanalysisnamelytheSTM,scanningtunnelingmicroscopy.Fewdetailsonthe principleofoperationofSTMareworthknowing.Electronisnotonlyaparticlebutalso a wave. In our daily life walking through a brickwallisnotpossible[7].Imaginea nanometerscaleequivalentofabrickwall.Letthisbeanenergybarrierthatanelectron followingprequantumphysicswouldnotbeabletoovercome.Butquantummechanics tellsusthatthereisstillacertainprobabilitythattheelectronisfoundontheothersideof thewallsinceelectronisnotonlyaparticlebutalsoawave.Inthiscasetheelectron

10.30RoleofSynthesisinMaterialsTechnology behavesmorelikeawaveandthiseffectiscalled tunnelingwhich formsthebasisof STM analysis. The empty space between the surface to be studied and the probe is regardedasthebarrierorthewallthroughwhichanelectronnormallybutoccasionally willnotpasses.Thetipoftheprobeshouldbeone atom wide. The next layer can containmoreatoms.Theprobabilityoftunnelingdecreasesbyafactoroftenforeach 0.1nmofadditionaldistance.Sothesecondlayerhasnovirtualsignificance.Theone atomtipissuspendedabovetheobjectjustafewatomicradii.Thisdistanceisreadjusted usingthemeasuredtunnelingcurrent.FromthedistancebetweentheSTMtipandthe surfacetobeprobedandthemeasuredtunnelingcurrentonecanfeelthestructureofthe surface.Thepeaksandtroughsofthesurfacecanbefeltatatomicscaleswithoutever touchingit.Ithasnowbecomepossibletocontroltheplacementofthetipwithsufficient accuracyestablishingSTMasastandardmethodofanalysisinmaterialsscience. Avariationofthethemeofelectrontunneling found as in the case of STM was presented in 1986 by Binnig, C. F. Quate and C. Gerber which was latter called as AtomicForceMicroscopy(AFM).Inthistechniquetheprobewillactuallytouchthe surface.Theverticalmovementofthetipoftheprobewillbecontrolledbytherepulsive forcemeasuredwhenthesurfaceistouched.By1990’sindividualbiomoleculessuchas thedoublestrandedDNAcouldbepicturedwhichmade the researchers elated. Thus bothSTMandAFMtechniquesareinprinciplesuitablefor“feeling”thefinestructureof asurfaceatombyatom. TheperformanceofSTMandAFMinstrumentshasbeenlimitedbythequalityofthe tipwhichactsasaprobethateitherexchangeselectronwiththesurfaceasinthecaseof STMortouchesthesurfaceasinthecaseof AFM. The virtual characteristics a tip shouldpossesstobeusedasprobeare:suchtipsshouldendinasingleatom;possessa welldefinedgeometry,shouldbeconductiveandchemicallyinert.Sofarthetoolthat workedasagoodprobetipisobtainedbysimplycuttingametalwirewithanordinary pair of scissors. Is it not surprising? Even though the afore mentioned material possessed no such virtual attributes as mentioned previously for an ideal probe, the materialderivedbycuttingthroughametalwirewithapairofscissorsworkedverywell. Thediameterofsuchatipisofafewhundrednanometersindicatingthattheoutermost

SyntheticStrategiesinChemistry 10.31 layer contains thousands of atoms. Interestingly, alternative methods like etching the wiretipsorsomeotherimprovementsinshapingthemetalwireonlyfailedtofunction. Tremendous improvements have been brought about in the advancement of utility of STM and AFM techniques for probing the surface structure when carbon nanotubes (CNT’s)wereusedforthefirsttimeasAFMtipsbytheresearchersfromthelaboratory ofNobelLaureateRichardSmalley.Asinglecarbonnanotubeof10nmwideand100 nmto1mlongcappedwithfullerene–likehemisphereisgluedasmolecularantenna ontoaconventionalprobe.Theconventionalprobewascoatedinitiallywithasuitable glueandthendippedintoabundleofnanotubes.Inmostcases,thiseffortculminated intothegluingofjustonetubeontothetip. Thenewprobeontowhichcarbonnanotubeisgluedfulfilledtheidealconditionsthe STMorAFMtipshouldsatisfy.TheprobewithCNTtipshasawelldefinedandwell knownmolecularstructure,isconductive,chemicallyinertandalsoverythin.Useof carbon nanotubes as AFM tips provided additional advantages by being stable to withstandtheforcesappliedtothetipandalsobeingelasticenoughtoavoidunwanted collisionswiththesurface.Moreover,nowadays,reliableandreproduciblesynthetic strategiesareatdisposalformakingsuchnewprobes(carbonnanotubes). Role of Synthesis in Leather Technology: Transformationofanimalhidesandskinsintoattractive,aestheticandusefulartifactshas been one of the oldest technologies of mankind. When wet the animal skins are susceptible to bacterial attack and putrefy. On the contrary drying makes the skin inflexible and useless for clothing and other applications. Such problems can be over comebytheuseofbactericideduringsoaking.Thisformsagoodsolutionfortheshort termpreservationofskinsandhides.Additionofbactericidepreventthebacterialattack onhideandskin.Additionofbactericideconverttheputresciblebiologicalmaterialsinto astablematerialresistanttomicrobialactivitieswithenhancedresistancetowetanddry heat.Thebactericidekillthegrowthofmicroorganismtherebypreventingthedamage toskinandhide. Mercurycompoundsaswellasamixtureofsulfiteandaceticacidwereextensively usedasbactericides.Eventhoughtheaforementionedmaterialsareeffectivetheyare damageableandharmfultotheenvironment.Sotheyarenolongerusedforprotecting

10.32RoleofSynthesisinMaterialsTechnology the hides and skins from bacterial attack. Instead, bronopol which is a highly active antimicrobialchemicalcompoundhasbeentheubiquitouschoicetoleatherindustryto preventbacteriaattackingtheskinsandhides.Bronopolis2bromo2nitropropane–1, 3–diol.BronopolwasinventedbyTheBootsCompanyPLC,Nottingham,Englandin theearly1960’s.Itwasusedasapreservativeforpharmaceuticalsforthefirsttime[21]. Earlierreportsonthesyntheticpathwaysofbronopolincludereactionofbromonitro methylcyclohexanolwithaliphaticaldehyde.Themaindrawbacksassociatedwiththis methodofsynthesisaretheuseofcyclohexanolwhichisexpensiveandalsotheuseof hazardoussodiumethoxide.LakshimiMuthusubramanian and Rajat B Mitra [22] have succeeded in deloping a simple but cleaner synthetic strategy that helped Leather manufacturingtechnology.

Scheme10.1.SyntheticstrategyforBronopol Thismethodofsynthesisofbronopolincludesreactionofformaldehydewithnitroalkane inthepresenceofsodiumhydroxide(inMeOH)whichonbrominationyieldedbronopol as represented in Scheme 10.1. The advantages of this method include complete replacementofhazardousandexpensivehydroxideandmethanolwhichareinexpensive andreadilyavailable.Currentlyscaleupstudiesarebeingcarriedoutfortheproduction ofbronopolusingthisenvironmentallybenignsyntheticstrategy. CONCLUSION: The living cells are versatile in its design and function. They provide all necessary inspiration to design and synthesize new materials with specific functions that bring

SyntheticStrategiesinChemistry 10.33 about revolutions in Technology and also give birth to Advanced Technologies. Understandingandimitationofnaturalmachineryofthelivingcellsholdsgreatrewards. Butsuchendeavoursarenotfreefrombarriersandobstacles. Forinstance,molecular leveldetailsoftheworkingofribosomes(theirfunctionofproteinsynthesis)isunclear even today and remains one of the hardest problems in biology. Therefore our knowledge and understanding of the processes going on in a living cell and also the mechanochemicalfunctionsofvariouscellcomponentsislimited.Anyimprovementsin such an understanding facilitate imitation and mimicking of the synthetic strategies involvedinlifeprocess(auniquenetworkofchemicalreactions).Thisknowledgewill inturnbringaboutdrasticchangesandadvancementsinMaterialsTechnology. REFERENCES: 1.http://en.wikipedia.org/wiki/cell_(biology) 2.http://www.ucmp.berkeley.edu/history/hooke.html 3.http://www.roberthooke.org.uk/ 4.http://images.google.co.in/imgres?imgurl=http://cache.eb.com/eb/image%3Fid%3D997 68%26rendTypeId%3D4&imgrefurl=http://www.britannica.com/ebc/art99713/Robert Hookesdrawingsofthecellularstructureofcork and&h=450&w=339&sz=44&hl=en&start=1&um=1&tbnid=8NZeLWw7jw9MzM:&tbn h=127&tbnw=96&prev=/images%3Fq%3DRobert%2BHooke%2Bcell%26um%3D1%26 hl%3Den%26sa%3DN 5.http://www.cellsalive.com/cells/3dcell.htm 6.TravelstothenanoworldMiniatureMachineryinNatureandTechnology,Michael Gross,PerseusPublishing,Cambridge,Massachusetts,1999. 7.NorthCentralRegionalEductionalLaboratory,18003562735,199799. 8.http://www.sp.se/en/areas/material/Sidor/default.aspx 9.http://en.wikipedia.org/wiki/Gutenberg_Bible 10.WarrenH.Hunt,Jr.JOM,October2004,13. 11.H.W.Kroto,J.R.Heath,S.C.O’Brien,R.F.CurlandR.E.Smalley,Nature318 (1985)162 12.P.Scharff,Carbon36(1998)481 13.T.Pradeep,CurrentScience,72(1997)124

10.34RoleofSynthesisinMaterialsTechnology

14.MarcMonthionx,VladimirL.Kuznetsov,Carbon44(2006)1621 15. Purushottam Chakraborty, Everyman’s Science, Vol. XXXIX, No. 4, October November’04 16.NanotubeandNanofibers,YuryGogotsi(Ed.,)Taylor&Francis,2006 17.YoshinoriAndo,XinluoZhao,ToshikiSugai,andMukulKumar,MaterialsToday, October2004,2229 18. Xinluo Zhao, Sakae Inone, Makoto Jinno, Tomoko Suzuki, Yoshinori Ando, ChemicalPhysicsLetters,373(2003)266 19.R.Saito,G.DresselhausandM.S.Dresselhaus,PhysicalPropertiesofCarbon Nanotubes,ImperialCollegePress,1998 20.MukulKumar,P.D.Kichambare,MaheshwarSharon,YoshinoriAndoandXinluo Zhao,MaterialsResearchBulletin34(1999)791 21.http://en.wikipedia.org/wiki/Bronopol 22.LakshmiMuthusubramanian,RajatBMitra,Journalofcleanerproduction,14(2006) 536

Chapter - 11 ELECTROCHEMICAL SYNTHESIS MHelen INTRODUCTION Electrochemistry isabranchofsciencewhichdealswithelectricalenergyandchemical change. Spontaneous chemical reactions liberate electrons and they are taken place in GalvanicorVoltaicCells .Thesecellsareexploitedinbatteriesandfuelcellstoproduce electricpower.Ontheotherhand ElectrolyticCells arenonspontaneouswhereelectrical energyisrequiredtocarryoutchemicaltransformations.Foranexampleintheprocess calledwaterelectrolysiselectricalenergyissuppliedtosplitwaterinproducinghydrogen and oxygen. Electrolysis is exploited in electroplating, chlorine gas production and in refining metals. Electrochemical synthesis is the production of chemical products or materialsusingelectricityasthedrivingorcontrollingfactor.Electrochemicalsynthesis is achieved by passing an electric current between two electrodes separated by an electrolyte.Thatis,thesynthesistakesplaceattheelectrodeelectrolyteinterface.This methodhassignificantpotentialforimprovingavailabilityofspecialtyproductsandfor improving environmental compatibility in several industrial sectors by destroying or convertingunwantedbyproductsintousefulproducts.Muchprogresshasbeenmadein thelastfewdecadesinadvancingthebasicunderstandingandindustrialapplicationsof electrochemical processes, but many aspects of electrochemical synthesis are still inadequatelyunderstoodorexplored. FEATURES OF ELECTROCHEMICAL SYNTHESIS Thefeaturesthatdistinguishelectrosynthesisfromothersyntheticmethodsare:  The experiments are simple to perform and the instruments required are inexpensiveandreadilyavailable.  Electrochemicalsynthesistakesplaceattheelectrodeelectrolyteinterfacewhich hasaveryhighpotentialgradientof10 5Vcm 1.Undertheseconditions,the reactions often lead to products which cannot be obtained in a conventional chemicalsynthesis.  Anelectrochemicalsynthesisisaredoxreaction.Byfinetuningtheappliedcell potential, the oxidizing or reducing power can be continuously varied. This

11.2 ElectrochemicalSynthesis

possibility of continuous variation is not available in conventional chemical synthesis.  Theproductisnormallydepositedontheelectrodeintheformofathinfilmora coating.  Thefilmcompositioncanbecontrolledbyvaryingthebathcomposition.  Electrochemical synthesis is a lowtemperature process limited by the boiling pointoftheelectrolyte.  Reaction taking place can be controlled kinetically by controlling the current passedthroughthecell,whileitcanbethermodynamicallycontrolledbychoosing theappliedpotential. Insummary,electrochemicalsynthesisisa‘green’ routetoproducetofabricatehigh puritymaterialswithoutanyadditives. ELECTROCHEMICAL SYNTHESIS: DESIGNING Any electrochemical reaction depends on the proper choice of a number of reaction parameterssuchas: (1)Choiceofanelectrode (2)Choiceofanelectrolyte (3)Choiceoftemperature,pH,concentration,andcompositionoftheelectrolyte solution (4)Choiceofthecelldesigndividedorundivided (5) Mode of electrolysis potentiostatic or galvanostatic (constant potential or constantcurrent) In a typical electrosynthesis, the reactant, which is dissolved in the electrolyte is depositedasasolidproduct.Whenametallicsaltisdissolvedinwateritdissociatesto formpositivelychargedions.Thesolutionthatcontainsthesechargedionsisreferredto asanelectrolyteoraplatingsolution.Bypassingelectriccurrentthroughthiselectrolyte, one can reduce the metal ions to form solid metal. This process is referred to electroplating or electrochemicaldeposition .InFig.11.1electroplatingisexplainedby takingsilverastheanode,forkasthecathode(forktobeplatedwithsilver)andaqueous solutionofsilvernitrateasanelectrolyte.Bothanodeandcathodeareconnectedtothe externalbattery.Whentheexternalpowerisonsilvermetalattheanodeisoxidizedto

SyntheticStrategiesinChemistry 11.3 silverionsandmovestowardscathode.Atthecathodesilverionsisreducedtosilverand depositsonthefork.Thisresultsinthincoveringofsilveronthecathode(fork).

Battery

e

Ag

Ag +

Anode Fork Cathode

aq.AgNO 3 Fig.11.1Representationofsilverelectroplating Twoparametersdeterminethecourseofthereactioni)thedepositioncurrentand(ii)the cellpotential.Ofthetwo,anyoneofthemcanbecontrolledasafunctionoftimeduring thereaction. In a galvanostatic synthesis (Fig. 11.2), a constant current is applied through the electrolytic cell leading to deposits with good adhesion and a controlled morphology. However the cell potential drifts as the activity (concentration) of the reactant is decreased.Thedriftinthecellpotentialmayleadtoamultiplicityofproducts. Apotentiostaticsynthesisiscarriedoutwithathreeelectrodeelectrolyticcell(Fig. 11.3).Thesynthesisiscarriedoutbypolarizingtheelectrodetoadesiredpotentialwith respecttoareferenceelectrode.Thecellcurrentusuallydecaysrapidlyasthereaction proceeds,bothduetolowratesofdiffusionofthereactantmoleculesfromthebulktothe electrodesurfaceaswellasduetodecreaseintheactivityofthereactant.Thereaction yieldsapuresinglephaseproductselectedforbytheappliedpotential.

11.4 ElectrochemicalSynthesis

V G - e 1 2 3 W.E C.E Fig.11.2.GalvanostaticSynthesis, G,galvanostat;V,voltmeter;WE,workingelectrode;CE,counterelectrode;1, electrochemicalcell;2,electrolyte;3,Lugincapillary

P

W.E C.E

R.E Fig.11.3.P,potentiostat;R,recorder;RE,referenceelectrode;WE,workingelectrode; CE,counterelectrode

SyntheticStrategiesinChemistry 11.5

VARIOUS TECHNIQUES FOR ELECTROCHEMICAL SYNTHESIS Varioustechniquesareemployedinelectrosynthesis.Alistafewofthemandthenature ofproductsobtainedfromeacharegiveninTable11.1. Table11.1.SummaryoftheElectrosyntheticTechniques Technique Product Application Anodicoxidation Coatings/films/powders/ Synthesis of compounds with conductingpolymers high oxidation state, corrosion control,electrochromism Cathodicreduction Coatings/films/powders Synthesis of electrode materials for energy systems, fabrication of hydroxide films/coatings Electrolysis Single crystals, pure metals Crystal growth at moderate offusedsalts,water andgases temperature, pure gas production Electromigration Polycrystallinepowders/ Electrode materials for ofreactantspecies singlecrystals/carbon batteriesandelectrochromism Alternate Layerbylayer Synthesisofcomposites/solid voltage/current films/coatings solutions synthesis Electrospraying Microornanoparticles Biomedicalapplications Electrospinning Microornanofibres Drugdelivery,tissue engineering,sensors Duringanodicoxidation ametalioninaloweroxidationstateoramonomerlikepyrrole oranilineisoxidizedtoahigheroxidationstateorpolymeranodically.Theanodic oxidationtechniqueisespeciallysuitedforthesynthesisofcompoundswithmetalionsin unusualhighoxidationstates.InTable11.2,importantanodicsynthesesarelisted. Table11.2.OxidesandPolymersSynthesizedbyAnodicOxidation Compound Application

Al 2O3 Template,molecularfilters

CoO 2. nH 2O Electrochromicdevices,lithiumion batteries,supercapacitors,andthe protectionfilmofcathodesinmolten carbonatefuelcells FeOandMnO 2 Electrodematerial Ta 2O5 Protectivecoatingforchemicalequipment,

11.6 ElectrochemicalSynthesis

electronicandsensordevices NiO(OH),Co 2O3 Alkalinewaterelectrolysis

PbO 2 Battery,Organicdegradation

Fe 3xLi xO4 Magneticdevices,electrodematerial

WO 3withCo,Cr,Fe,Mo,Ni,Ru,andZn Electrochromicdevices RuO 2 Supercapacitor TiO 2 Photocatalyst,humiditysensor

V2O5nanofibers Batterymaterial,activematerialsin electrochromicandchemochromicdevices Polypyrrole,polyaniline,polythiophenes, Sensors,Electroluminescence,Protective polyacetylenes,polyindole coating,FET,Organicsemiconductors, batterries Incathodicreductionelectriccurrentispassedthroughametalsaltsolutionandhencethe metalisdepositedatthecathode.Thisprincipleiswidelyusedtoobtainmetalcoatings. But depending upon the deposition potential, choice of the anion and the pH of the solutionvariousotherreactionstakeplaceatthecathode.Switzerin1987introducedthis techniqueforthefirsttimeasasyntheticroutetoobtainorientedceramicfilmsaswellas polycrystallinepowders.PolycrystallineCeO 2 powderwassynthesizedfromacerous nitrate solution. Various oxide materials are synthesised by using electrogeneration of basebycathodicreductionandtheirtypicalapplicationsarelistedinTable11.3. Duringelectrolysisoffusedsalts,alowmeltingsaltcontainingthetransitionmetal oxide,ismeltedandelectrolyzedatelevatedtemperaturesusinganinertPtelectrodeora reactivemetalelectrodesuchasFe,Co,orNidependingonthedesiredproduct.During water electrolysis pure oxygen and hydrogen is collected at the anode and cathode respectively. Synthesis by electromigration, is based on kinetic control over the reaction by electrochemistry.Thistechniqueinvolvesintercalationordeintercalationofaguestion inahostlatticebyapplyinganelectricpotentialbetweentheelectrodes. Duringpulsedelectrolysis,theworkingelectrodeisalternatelypolarizedanodicallyfora lengthoftime, t1 (calledtheontime)andthencathodicallyfortime, t2 (calledtheoff time).VoltageaswellascurrentpulseswereusedtoobtainoxidefilmsinthePbTl

SyntheticStrategiesinChemistry 11.7 systemusingastainlesssteelelectrodeandamixedPb(II)Tl(I)bath.Atlowcurrent densities,thefilmswereTlrichwhileathighcurrentdensitiesthefilmswerePbrich. CathodicdepositionoftheprecursorfilmfortheYBaCuOsystemhasbeencarriedout fromamixedmetalnitratebathcontainingKCNaswellasacomplexingagent.The superconductivefilmsobtainedfromacyanidebathshowa Tc~92Kwhichisgreater than all other reported values for this class of superconductors, obtained by electrochemical techniques. High intensity pulsed electric fields is an interesting alternativetotraditionaltechniqueslikethermalpasteurizationinpreservationofliquid foods such as fruit juices or milk. Conventional preservation methods such as heat treatmentoftenfailtoproducemicrobiologicallystablefoodatthedesiredqualitylevel. Highintensitypulsedelectricfieldsprocessingcandeliversafeandshelfstableproducts withhighnutritionalvalue.Pulsedelectrodepositionisaneffectivemethodtoprepare nanomaterialsofdifferentmorphologies.

Table3.OxidesSynthesizedbyElectrogenerationofBasebyCathodicReduction (reproducedfrom[1]) Compound Application

CeO 2 Gassensor,fuelcells

La 1xMxCrO 3 (M=Ca,SrorBa) Electronicconductor ZrO 2 Ionicconductor

BaTiO 3 Dielectriccomponents

LaFeO 3 Oxidecoating

Al 2O3, Cr 2O3, Ln 2Cr 3O12 .7H 2O Corrosionprotectivecoating

PbO 2 Electrodematerial

Mo 1xMxO3(M=Co,Cr,Ni,WorZn) Opticallightmodulators

TiO 2 Photocatalyst

Nd 2CuO 4 Superconductor ZnO Opticalandelectronicdevices

LaMnO 3 Giantmagnetoresistance(GMR)

WO 3 Electrochromicdevices

11.8 ElectrochemicalSynthesis

MICRO- AND NANOPARTICLE PRODUCTION BY ELECTROSPRAYING Electrospraying isaprocessofliquidatomisationbyelectricalforces.Itisadynamic processofdropletgenerationandsimultaneouslychargingbymeansofanelectricfield. Droplets spraying out of a capillary nozzle, produced by electrospraying are highly charged.Chargeddropletsareselfdispersinginspace,resultingintheabsenceofdroplet coagulation.Thedepositionefficiencyofachargedsprayonanobjectishigherthanfor an uncharged spray. This feature can be advantageous in thinfilm formation or in surfacecoating.Generationofthedropletanditssizecanbecontrolledbycontrolling the flow rate of the liquid and the voltage at the nozzle. Nearly monodisperse size distribution is achievable. The size of the electrospray droplets can range from micrometerstonanometers.

Fig.11.4.Schematicforproductionofparticlesofuniformsizebyelectrospraying (reproducedfrom[2]) Electrosprayingtechniqueisasinglestep,lowenergy,andlowcostmaterialprocessing technology, which can deliver products of unique properties. Electrospraying is exploited in many industrial processes such as painting, fine powder production, or microandnanothinfilmdeposition.Itisalsoemployed in microfluidic devices and nanotechnology. Spraying solutions or suspensions allows production of particles,

SyntheticStrategiesinChemistry 11.9 rangingfrommicrotonanometersize.Productionofparticlesofuniformsizecanbe accomplished by using pulsed or ac superimposed on dc bias voltage for liquid jet excitation.Bytuningthefrequencyofacvoltage,uniformsizeofdropletsareformed bydisintegratingtheliquidjet.Variousparameterscancontrolthedropletsizetheyare thebiasandacvoltage magnitudes,acvoltagefrequency,andvolumeflowrateofthe liquid.Uniformdropletscanbeachievedwhenacanddcvoltagesareadjustedsuch thatthedropletsareformedanddetachedatthevoltagepeaks.Aschematicdiagramof asystemforharmonicsprayingisshowninFig.11.4. Electrosprayingisexploitedforthegenerationofmicro/nanospheresforbiomedical applications since the process has an advantage of not making use of any external dispersion/emulsion phase which often involves ingredients that are undesirable for biomedical applications. Chitosan micro/nanospheres were synthesized by electrosprayingwithaceticacidsolutionisexploitedfordrugdeliveryapplications.This technique is also used to prepare polycaprolactone (PCL) polymer particles with a different microstructure by the evaporation of solvents during the electrospraying process. MICRO- AND NANOFIBRES PRODUCTION BY ELECTROSPINNING Electrospinning isaprocessinwhichahighvoltageelectricfieldisappliedtoameltor polymersolutioninordertoattainchargerepulsionontheliquidsurface.Thisovercomes surfacetensiontherebyathinliquidjetisejected.Narrowjetdiameterisattaineddueto the electrostatic repulsion caused between the charges on the liquid surface and the collectorthathasadifferentelectricfield.Solidorpolymerfibresrangingfrom10mto 10nmisattainable.AsshowninFig.9.5,atypicalelectrospinningsetupconsistsofa syringewithacapillarynozzlethroughwhichtheliquidtobeelectrospunisforced;a high voltage source with positive or negative polarity to charge the liquid jet and a groundcollector. Electrospuntextilesareexploitedinpreparingfilters,semipermeablemembranes,as scaffoldingfortissueengineeringandindrugdelivery.Electrospinninghasflexibilityin selecting materials for drug delivery applications. Either biodegradable or non degradable materials can be used to control whether drug release occurs via diffusion aloneordiffusionandscaffolddegradation.Duetotheflexibilityinmaterialselectiona

11.10 ElectrochemicalSynthesis numberofdrugscanbedeliveredincluding:antibiotics,anticancerdrugs,proteins,and DNA.

Syringe

Sample

( Voltagegenerator

Nonwovenfiber

Collector

Ground Fig.11.5.Schematicofanelectrospinningsystem CONCLUSION Electrochemical techniques such as anodic oxidation, cathodic reduction, alternating current pulsing, electrospraying and electrospinning provide simple, cost effective alternative routes for the production of micro or nanomaterials, thin films, coating, composites having unique properties and applications. It is hoped that the ease and versatilityofthistechniquewillfindwillfinditapermanentplaceinsyntheticchemistry. REFERENCES 1. G.H.A.ThereseandP.V.Kamath, Chem.Mater., 12(2000)1195. 2. A.Jaworek, PowderTechnol., 176(2007)18. 3. A.Jaworek,A.T.Sobczyk J.Electrosta., 66 (2008)197. 4. N. Arya, S. Chakraborty, N. Dube, D.S Katti, J Biomed Mater Res B Appl Biomater., 2008(InPress) 5. S.Tan,X.Feng,B.Zhao,Y.Zou,X.Huang, Mater.Lett., 62(2008)2419. 6. T.J.Sill,H.A.vonRecum, Biomaterials, 29(2008)1989.

Chapter – 12 NEWER REACTIONS AND PROCEDURES: CATALYTIC AND NONCATALYTIC M.Banu

Thefollowingchapterfocusesmainlyonsomedevelopments of industrially important reactionswithandwithoutcatalyst.Brieflythecontentsofthischapterinclude: • Introduction • Biodiesel production with new source by transesterification reaction with and withoutcatalyst • Conversion of glycerol to valuable chemicals by heterogeneously catalysed liquidphaseoxidation • Catalytichydrodesulfurization • Catalyticandnoncatalyticstudyofoxidativedehydrogenationreactionforethane conversiontoethyleneastheoneoftheindustriallyimportantproduct • Noncatalytic supercritical fluid method for the preparation of various polyorganosiloxanes 1.0. INTRODUCTION Thegeneraldefinitionforcatalystis“chemicalmarriagebrokers”.

Fig.1(a)Fig.1(b) Fig.12.1(a)Generaldiagramforcatalyticreaction;(b)Energyprofilediagramfor catalyticreaction[1]

12.2N ewerReactionsandProcedures:CatalyticandNoncatalytic

The presence of a catalyst facilitates reactions that would be kinetically impossible or veryslowwithoutacatalyst.Thecatalystdoesnotaltertheoverallthermodynamicsof thereaction. 1.1 IMPORTANCE OF CATALYST • Morethan70%ofallexistingprocessesonanindustrialscalerelyoncatalysis. • Morethan99%oftheworldgasolineproductionoccursviacatalyticcrackingof oilfractionsandothercatalyticprocesses. • Morethan90%ofallnew industrialprocessesarecatalytic. • Enzymesarecatalyststhatfacilitatecomplexreactionswith100%selectivityat extremelymildreactionconditions, i.e. inourbodies • Thechemicalprecisiondisplayedinenzymaticreactionsisasourceofinspiration forallcatalysischemists. In our life mainly one is depending on oil mostly for transport, food, pharmaceutical,industryandentirebasisofmodernlife.Sothedemandforenergy sources more in India compared to other countries. Biodiesel is the one of the important energy source. Biodiesel is a domestic, renewable fuel for diesel engine derivedfromnaturaloils.Biodieselcanbeusedinanyconcentrationwithpetroleum baseddieselfuelinexistingdieselengineswithlittleornomodification.Biodiesel production can be carried out by catalytic route and also by noncatalytic route. Biodieselisnotthesamethingasrawvegetableoil. It is produced by a chemical process which removes the glycerin from the oil. The glycerol is the one of the byproductsinthebiodieselproduction.Itcanbeconvertedintovaluableproductsby environmentallyfriendlycatalyticroute.Theindustrialenergysourceslikepetroland diesel contain more amounts of sulphur and it leads to the formation of more pollutionwhichisveryharmfulforhumanbeings.Forremovalofsulphurcontentin oil, hydrodesulfurisation is the important process which can be carried out by catalyticroutewithMo,Ni,andColoadedonvarioussupports.Althoughcatalytic routeisfeasibleinindustrially,someofthenoncatalyticroutesarealsopossiblefor producing industrially important products like ethane and polyorganosiloxanes. Ethene is the second major component of natural gas and it is also abundant in

SyntheticStrategiesinChemistry 12.3

refinerygases.Polyorganosiloxanesaretheimportantmaterialinchemicalindustries whichcanbepreparedbycatalystfreesupercriticalfluidmethod. 2.0 BIODIESEL PRODUCTION BY TRANSESTERIFICATION REACTION Biodieselisafuelcomposedofmonoalkylestersoflongchainfattyacidsderivedfrom varietyofvegetableoilsoranimalfats.Biodieselhasbecomemoreattractiverecently because of its environmental benefits and the fact that it is made from renewable resources.Althoughtherearemanywaysandprocedurestoconvertvegetableoilintoa diesellikefuel,thetransesterificationprocess[1]wasfoundtobethemostviableoil modificationprocess. 2.1 Transesterification Reaction Transesterificationistheprocessofusinganalcohol(e.g.methanol,ethanolorbutanol), inthepresenceofacatalyst,suchassodiumhydroxideorpotassiumhydroxide,tobreak themoleculeoftherawrenewableoilchemicallyintomethylorethylestersofthe renewableoil,withglycerolasabyproduct.

Scheme.12.1.TransesterificationReaction TheFatsandoilsarebigmoleculeswithaspinalofglycerolonwhichareboundthree fatty acid rests as shown in Fig. 12.2 (a). The fatty acid rest are removed from the glycerolanditwillformbondwithmethanolbytransesterification method. Further it leadstotheformationofonemoleofglycerolandthreemolesoffattyacidmethylester whichisshowninFigs.12.2(b)and12.2(c).

12.4N ewerReactionsandProcedures:CatalyticandNoncatalytic

(a)(b)(c) Fig.12.2(a)Molecularstructureoffattyacidrestswithglycerol;(b)Formationofbond withmethanol;(c)formationofglycerolandmethylester[1]. Thefollowingmethodsareemployedforthepreparationofbiodiesel • BatchBaseCatalyzed • ContinuousBaseCatalyzed • AcidCatalyzedProcesses • NonCatalyticProcesses Feedstocksusedinbiodieselproductionmainlytriacylglycerolorfatsandoils(e.g. 100 kg soybean oil), primary alcohol (e.g. 10 kg methanol) and catalyst (e.g. 0.3 kg sodiumhydroxide)andalsotheneutralizer(e.g.0.25kg)sulfuricacid.Thetriglycerids sources are rendered from animal fats like beef tallow, lard, and vegetable oils like soybean,canola,palm,etcandchickenfatandalsorenderedgreaseslikeyellowgrease (multiplesources).Therecoveredmaterialsisbrowngrease,andsoapstock,etc. 2.2 BASE CATALYSED TRANSESTERIFICATION REACTION Generally in the base catalysed transesterification method the catalyst is dissolved in methanolbyvigorousstirringinasmallreactor.Theoilistransferredintothebiodiesel reactor,andthen,thecatalyst/alcoholmixtureispumpedintotheoil.Thefinalmixtureis stirred vigorously at particular temperature and ambient pressure. A successful transesterificationreactionproducestwoliquidphases that is ester and crude glycerin. Crude glycerin, the heavier liquid, will collect at the bottom after several hours of settling.Phaseseparationcanbeobservedwithin10minandcanbecompletewithin2h ofsettling.Completesettlingcantakeaslongas20h.Aftersettlingiscomplete,wateris addedattherateof5.5%byvolumeofthemethylesterofoilandthenstirredfor5min, andtheglycerinisallowedtosettleagain.Washingtheesterisatwostepprocess,which isperformedwithextremecare.Awaterwashsolutionattherateof28%byvolumeof oiland1goftannicacidperliterofwaterisaddedtotheesterandgentlyagitated.Airis

SyntheticStrategiesinChemistry 12.5 carefully introduced into the aqueous layer while simultaneously stirring gently. This process is continued until the ester layer becomes clear. After settling, the aqueous solution is drained, and water alone is added at 28 % by volume of oil for the final washing. The basic batch reactor diagram is given in Fig. 12.3. The transesterification canbecarriedoutbywithoutcatalystalso. Alcohol Water Water

TG Ester

Biodiesel Alcohol catalyst Dryer Alcohol W at Wash er Water Acid Batch Reactor Crude Glycerol

Neutralized Glycerol Fig.12.3.BaseCatalysedReactorSystem[1] 2.3 ACID CATALYSED PROCESSES Acid catalyzed processes are used for direct esterification of free fatty acids in a high FFAfeedstock,ortomakeestersfromsoapstock.Thesensitivitiesoftheacidcatalysed reactionishighFFAcontentrequireswaterremovalduringreaction.InAcidcatalyzed reactiontheratioofalcohol:FFAis40:1andalsothisreactionrequireslargeamount(5 to25%)ofcatalyst.

12.6N ewerReactionsandProcedures:CatalyticandNoncatalytic

Fig.4.AcidcatalysedFFApretreatsystem[1] 2.4 NON-CATALYTIC SUPERCRITICAL METHANOL TRANSESTERIFICAION REACTION In this method, the reaction is performed by cylindrical autoclave maintained at particular temperature and pressure [2]. The autoclave will be charged with a given amountofvegetableoilandliquidmethanolwithdifferentmolarratios.Aftereachrun, thegasisvented,andtheautoclaveispouredintoacollectingvessel.Alltherestofthe contents are removed from the autoclave by washing with methanol. The variables affectingthemethylesteryieldduringthetransesterificationreaction,suchasmolarratio of alcohol to vegetable oil and reaction temperature were already investigated. The viscositiesofthemethylestersfromthevegetableoilswereslightlyhigherthanthatof diesel fuel. This method shows that increase in temperature especially critical temperature has a favorable influence of ester conversion. A typical supercritical methanoltransesterificationsystemisshowninFig.12.5. The transesterification reaction of rapeseed oil in supercritical methanol was investigated without using any catalyst. In addition, it was found that this new supercriticalmethanolprocessrequiresashorterreactiontimeandasimplerpurification procedurebecauseofthatthereisnocatalyst. Biodieselpreparationisalsocarriedoutbybatchvscontinuousflowmethod.Batchis bettersuitedtosmallerplants(<1milliongallons/yr).Thisbatchdoesnotrequire24/7 operation.Thisprovidesgreaterflexibilitytotuneprocesstofeedstockvariations.The

SyntheticStrategiesinChemistry 12.7 continuousoperationallowsuseofhighvolumeseparationsystems(centrifuges)which greatlyincreasesthethroughputandalsohybridsystemsarepossible.

Fig.12.5.Supercriticalmethanoltransesterificationsystem.(1)Autoclave,(2)Electrical furnace,(3)Temperaturecontrolmonitor,(4)Pressurecontrolmonitor,(5)Productexit valve,(6)Condenser,(7)Productcollectingvessel[2]. Fig.12.6.HybridBatch/ContinuousBaseCatalyzedProcess[1]

12.8N ewerReactionsandProcedures:CatalyticandNoncatalytic

2.5 PROCESS ISSUES Themainissueofthebasecatalyzedtransesterificationprocessisfreefattyacidintheoil which when reacted with alkaline catalyst will form soaps and it will lead to loss of catalystandreductionintheoil.

ROH+KOHKOR+H 2O Acid+KOHSoap+water Andwaterformationisanotherissueinbasecatalysedtransesterificationreaction.Water willdeactivatethecatalystsandalsoitrequiresthedryingofoil.Waterhydrolysesfatsto formfreefattyacidsandthefreefattyacidsreactwithalkalicatalystsformingsoaps. Soapssemisolidmixtureglycerolseparation Triglycerids+water Diglycerids+fattyacid Anotherissueofthetransesterificationreaction is the use of alcohol. Methanol is commerciallyused.Inmethanolysis,emulsionformsandseparatedintolower glycerol portion and upper ester portion. Reaction time is small. In ethanolysis, emulsions are stableandrequiremorecomplicatedseparationandpurificationprocessandalsoreaction timeislarge. Thistransesterificationreactioncarriedoutgenerallybytheuseofhomogeneousand heterogeneous catalysts. For homogeneous catalysed reaction, the basic catalyst used were NaOH, KOH, NaMeO and acid catalysts like H 2SO 4, PTSA, MSA, H 3PO 4, and

CaCO 3. For heterogeneous reaction the catalysts employed are sulfated zeolites and clays,hetropolyacidsmetalOxides,Sulfatescompositematerials. BasecatalyzedreactionisnotsuitableforhighFFAfeedsbecauseofsoapformation. Most of the nonedible oils available in India contain high FFA (212%). In order to decreasethecostofbiodiesel,itisimperativetoutilizehighFFAoilorfattyacids.Sothe preferred method for high FFA content feedstock is acid catalysis followed by base catalysis. The main barriers of homogeneous catalyst are thefocusonthesensitivitytoFFA andwatercontentofthefeedstocks,removalofcatalyst, formation of soap with FFA

SyntheticStrategiesinChemistry 12.9 feedstock.Alargequantityofeffluentwateristhemainissueasaresultofremovalof catalyst,whichnecessitiespretreatmentofoilincaseofhighFFAcontentandalsono scopeforregenerationorreutilization.Inheterogeneouscatalystthemainutilizationis catalystregeneration,decreaseofcatalystcost,utilizationoflowerqualityfeedstocksfor biodiesel production, simplification of separation process, decrease of production cost anddecreaseofwastewaterandalsodevelopmentofenvironmentalfriendlyprocess. 2.6 BIODIESEL FROM JATROPA PLANT Thejatropaplantisagoodsourceforproducingbiodiesel.Itishavingthefollowingmain advantages • Itthrivesonanytypeofsoilanditneedstheminimalinputsormanagement. • Ithasnoinsectpestsanditisnotbrowsedbycattleorsheep • Itcansurvivelongperiodsofdroughtandthepropagationbyseed/cuttingis easy • Itishavingtherapidgrowthandthenitleadstogivetheyieldfromthe2 nd yearonwards. • Theyieldfromestablishedplantationsis5tonneperhactare. • 30% oil from seeds by expelling and the seed meal is excellent organic manure.

Fig.12.7.JatropaPlant The Estimated biodiesel production per hectare = 3,000 litres/700Gal and the potentialyieldsof12tonnesperhectareand55%oilextractionarealsoattainable.The literaturesurveysshowthatthe2500treesperhectareproducestheseed(6.9tonnes), seedcake(4.2tonnes),vegetableoil(2.7tonnes),glycerol(0.27tonnes).Ithassomeanti erosive property like it reduces wind and water erosionofsoilandleadstoimproved

12.10NewerReactionsandProcedures:CatalyticandNoncatalytic absorption of water by soil. Mainly the seedling preparation by 10×20 cm bag and allowedtogetgerminationby3days. 2.7 CONCLUSION Finallyitisconcludedthatthebiodieselisarenewablefuelfordieselenginesthatcanbe madevirtuallyfromanyoilorfatfeedstockanditcanprovidehugeruralemployment potentialof40to50millionfamiliesandtransformtheruraleconomy.Itisusedinthe remote village electrification and power for agriculture application. The technology choiceisafunctionofdesiredcapacity,feedstocktypeandquality,alcoholrecovery,and catalystrecovery.Thedominantfactorinbiodieselproductionisthefeedstockcostwhich is around 70%, with capital cost contributing only about 7 % of the product cost. ThereforehighFFA,lowerqualityfeedstockshouldbepromotedforbiodieselproduction inIndia. 3.0 CONVERSION OF GLYCEROL TO VALUABLE CHEMICALS BY ENVIRONMENTALLY FRIENDLY PROCESS Asarenewablefeedstockandduetoitshighfunctionalityglycerol,isanattractive reactantfortheproductionofalargenumberofvaluablecompounds.Oxidationreactions are of industrial importance for the synthesis of fine chemicals, even though stoichiometric oxidizing agents (e.g. permanganate) or biotechnological processes are usedandalargenumberofbyproductsareoftenformedwhichdecreasetheselectivity tothedesiredoxidationproduct.Anenvironmentallyfriendlyalternativeistheoxidation inthepresenceofaheterogeneouscatalystandoxygen[3]. Heterogeneously catalyzed liquidphase oxidation of glycerol performed under atmosphericpressureandatconstantpHusingcarbonsupportedongoldascatalyst.The aimofthisworkistoproducehighlyinterestingchemicalslike glycericacidfromthe environmentallyfriendlyoxidationofabiosustainablesourcewithhighyields.Oxidation reactionoftheglycerolcanbeexplainedinthefollowingscheme.

SyntheticStrategiesinChemistry 12.11

Scheme.12.2.Reactionnetworkoftheglyceroloxidation 3.1 Monometallic and Bimetallic Gold Catalysts Preparation

Monometallic gold catalyst is prepared by goldsol method. 86 mg of HAuCl 4 is dissolvedin5mlof1.2MHClbyaddingdropwisewithin30minwithstirredcarbon suspension(5gcarbonin133mldistilledH 2O).After5himpregnationtime10mlofa 20%formaldehydesolutionandsubsequently11mlofa40%KOHsolutionwereadded under stirring. After 30 min the catalyst was filtered, washed with distilled water and dried in air [4]. Bimetallic gold catalyst prepared by the precursor metal solution of

HAuCl 4andH 2PtCl 6addedtotheTHPCsolutionandthenaddedtothestirredcarbon suspension.TheICPOESanalysisshowsthatthegoldcatalystspreparedbythegoldsol methodtypicallyprovideparticlesizessmallerthan10nm. 3.2 Oxidation Experiments Theglyceroloxidationiscarriedoutunderatmosphericpressureina300mlsemibatch glassreactorequippedwithanautomatedtitratorprovidingcontinuouspHcontrolduring thereactiontimeandwithafourbafflepropeller.Thereactionswereperformedwith150 mlofanaqueousglycerolsolutionatconstantpH,atanoxygenflowrateof300mlmin 1 checkedbyamassflowcontroller,at60 0Candastirringrateof500rpm.Thereactions werestartedbyswitchingthegastooxygenandsubsequentlyincreasingthestirringto 500rpm. 3.3 Results and Conclusion The gold catalysts prepared by the goldsol methods show a higher activity than the catalystpreparedbytheprecipitationmethod.Accordingtothisstudy,thehighestAu/C

12.12NewerReactionsandProcedures:CatalyticandNoncatalytic catalystactivityisachievedbyusing(i)thegoldsolmethodand(ii)THPCasreducing agent. To improve the catalyst properties in theglycerol oxidation by modifying gold withasecondmetal(Pt).Catalystswith1wt.%goldand0.5wt.%platinumonactivated carbons were prepared by the goldsol method with THPC as reducing agent. The presenceofplatinuminAu/BPcatalystssignificantlyincreasestheglycerolconversion rate. The increase in activity can be maximized by promoting the monometallic gold catalystswithplatinumbytheformationofAu/Ptalloyswithaplatinummolefractionin the range from 0.2 to 0.4, which corresponds to a Au 0.8 Pt 0.2 composition. Also the selectivityisaffectedbyintroducingasecondmetalintheAu/Ccatalysts.Infact,by promotingthegoldcatalystwithplatinum,itwaspossibletoincreasetheselectivityto dihydroxyacetonefrom26%to36%at50%conversion. 4.0 HYDRODESULFURIZATION REACTION The basic operation of a refinery is the conversion of crude oil into products such as LPG, gasoline (boiling point <150°C), kerosene (boiling point150250°C), diesel oil (boiling point 250370°C), fuel oil (>370°C), base oils for lubricants, bitumen, and feedstocks for petrochemical industries. After separation of the crude oil into different fractionsbyatmosphericdistillation,thesestreamsaretransformedintoproductswitha highadditionalvaluethroughawidevarietyofcatalyticallypromotedchemicalreactions such as hydrogenation, isomerization, aromatization, alkylation, cracking and hydrotreating. Hydrotreatingreferstoavarietyofcatalytichydrogenationprocessesinwhichsulfur, nitrogen, oxygen and metal atoms are removed and unsaturated hydrocarbons are saturated. Characteristic for hydrotreatment operations is that there is essentially no change in molecular size distribution, this in contrast to, for instance, hydrocracking. Whilehydrodesulfurization(HDS)isassuminganincreasinglyimportantroleinviewof thetighteningsulfurspecifications,hydrodenitrogenation(HDN)isnecessarytoassure theviabilityofsubsequentupgradingprocesses[5]. 4.1 Importance of Sulphur Removal from Oil HydrodesulfurizationfirstcameintopracticeduringWorldWarIIintheproductionof petroleum.Sulfurreductioningasolineispromptedbyseveralfactors.

SyntheticStrategiesinChemistry 12.13

• Manycatalystsinreformerunitsaresensitivetotheamountofsulfurinthefeed. In fact, some bimetallic reforming catalysts require the sulfur content to be limitedtothevicinityof1ppmorless. • Airpollutioncontrolstandardsrequireremovalofsometimesupto80%ormore ofthesulfurthatwouldbepresentinvariousfueloils. • Someofthesulfuringasoilfedtoacatalyticcracker is in the form of coke, which is then hydrogenated and released as sulfur dioxide in the combustion gases. This is not desired as this proposes environmental harms. The organosulfur contentof thefeedtothehydrocrackermustbereducedtoavoid poisoningofthehydrocrackingcatalyst. • Thereductionofsulfurreducestheamountofcorrosionintherefiningprocess, improves the odour of the product, and reduces the amount of sulfur that can poisonthecatalyticconvertertoanautomobile. Oneofthebiggestmovementsinrecentlegislationforreductionofsulfuringasoline productswasstartedbyaspeechbyBillClintononMay1,1999.Heannouncedanew Environmental Protection Agency regulation calling for a 90% reduction of sulfur contentinautomobilegasolineintheUnitedStatesbytheyear2004.Similareffortsare underwayaroundtheworld. The hydrodesulfurization process involves catalytic treatment with hydrogen to convertthevarioussulfurcompoundspresenttohydrogensulfide.Thehydrogensulfide isthenseparatedandconvertedtoelementalsulfurbytheClausprocess.Fromthispoint, someofthehydrogensulfideisoxidizedtosulfurdioxidebyairandsulfurisformedby theoverallreaction: 2H 2S+SO 23S(s)+2H 2O Originally the interest in hydrodesulfurization was initially stimulated to the availabilityofhydrogenfromcatalyticreformers.Howeverthedemandforhydrogenfor hydrodesulfurizationandhydrotreatingismorethanthatcanbegeneratedbyarefinery. Becauseofthis,mostrefineriesrecyclethehydrogenformedfromsidedehydrogenation reactionsbacktotheinlet.Sincehydrogenisso expensivetomanufacture,itisvery

12.14NewerReactionsandProcedures:CatalyticandNoncatalytic importanttorunallhydrodesulfurizationandhydrotreatingprocessesattheiroptimumto reducecosts. Thesupportedmolybdenumsulfidecatalystcontainingcobaltisoperatedunderpressures of150160psihydrogenat300400°C.Thesulfurcontentinoilof15%isreducedto 0.1%ingasolineandfuturesulfurlimitsmaybereducedtoaslittleas0.0030.04%.For lowpointandmiddleboilingpointdistillates,typicalHDSreactionconditionsareabout 300to400°Cand0.7to5MPahydrogenpressure.Thehighertheboilingpointofthe feedstockis,thehigherthesulfurcontent.Moresevereoperatingconditionsareneeded forhigherfractionboilingpoints.Thenhighpressureandlowtemperaturecombinations areusedtoreducethehydrogenconsumptionandcorrespondingcosts. HDSreactionsareexothermic.Mostreactorsareadiabaticfixedbedsandmaybe multistage. Adding additional hydrogen between the stages usually does cooling; the term“coldshot cooling”isusedtodescribethisprocess. If the feed for the reaction conditionsisamixedvapourandliquid,theliquidisnormallycausedtoflowcounter currentlydownwardthroughafixedbedcatalyst,or“tricklebedreactor”. Thesulfurispresentlargelyintheformofthiols,sulfides,andvariousthiophenesand thiophene derivatives. Mercaptans and sulfides react to form hydrogen sulfide and hydrocarbons. RSSR’+H 2RH+R’H+H 2S RSH+H 2RH+H 2S RSR’+2H 2RH+R’H+H 2S RandR’arevarioushydrocarbongroups.

+2HS 2H 2S+C 4H8(mixedomers) Scheme.12.3.Reactionpathwayofthiophene

SyntheticStrategiesinChemistry 12.15

Studies have indicated that the hydrodesulfurization and subsequent hydrogenation reactionsoccuronseparatesites.Thethiopheneringisnothydrogenatedbeforesulfuris removed,althoughthefirststepmayinvolveanessentiallysimultaneousremovalofa sulfuratomanddonationoftwohydrogenatomstothemolecule. Forbenzothiophene,substitutedorunsubstituted,thethoipheneringishydrogenated tothethiophanederivativebeforethesulfuratomisremoved,incontrasttothebehavior ofthiophene.Thereactionpathwaysfordibenzothiopheneareasfollows:

Scheme.4.Reactionpathwayofdibenzothiophene 4.2 Preparation of Catalyst CatalystsusedinindustryarederivedfromoxidesofsuchelementlikeMo,W,Co,Ni supportedondifferentcompounds,althoughthemostcommonlyusedisalumina.The catalystusedinHDSisalmostalwaysCoMo/Al 2O3,andsometimesNiMo/Al 2O3.The ratioofmolybdenumtocobaltisalwaysconsiderablygreaterthan1.

Themolybdenumsulfidecatalystispreparedbyimpregnationof γAl 2O3withan aqueous solution of ammonium molybdate and cobalt nitrate or nickel nitrate. This precursorisdriedandcalcined,whichconvertsthemolybdenumtoMoO 3.Thisisthen treatedwithamixtureofH 2SandH 2orafeedcontainingsulfurcompoundsandH 2.The resulting molybdenum catalyst is almost completely sulfided. If the catalyst is not completely sulfided, then there is the possibility,thatitwillnotbeactingasaactive catalyst. 4.3 Conclusion Since the mechanism for the hydrodesulfurization of thiophenes is not completely understood,therehasbeenextensiveworktotryanddevelopthemechanismandkinetics forthereactionsinordertodevelopbettercatalysts.Nickeltreatedcompoundshavehad

12.16NewerReactionsandProcedures:CatalyticandNoncatalytic some success, and while the nickel containing catalysts appear to be better at sulfur removal,theCocontainingcatalystsgiveslightlymoreoilyield.Intheend,itmaybea simplematterofeconomicsthatdetermineswhichcatalystisused. 5.0 CATALYTIC AND NONCATALYTIC STUDY OF OXIDATIVE DEHYDROGENATION REACTION Etheneisoneofthemostbasicfeedstocksinchemicalindustryanditsdemandissteadily increasing. The main commercial routes for production of ethene are steam thermal crackingandFCC(fluidcatalyticcracking)processes.Thedrawbackofthesemethodsis highenergyinputrequiredbythehighlyendothermicreactions,highoperationcostsdue to coke deposition on catalyst and reactor, and generation of low molecular weight alkanes. The reserves of raw materials for these processes are becoming increasingly limited.Consequently,alternativeprocesseswithhigherefficiency,whichutilizesmore abundant and economic sources for ethene production, are becoming increasingly necessary.Ethaneisthesecondmajorcomponentofnaturalgasandisalsoabundantin refinerygas. 5.1 Oxidative Dehydrogenation (ODH) Production of ethene via oxidative dehydrogenation (ODH) of ethane [6] has received increasingattention,owingtoitspotentialadvantages,suchasexothermicreactionheat andlesscokedeposition.Thismethodcanbecarriedoutatrelativelylowtemperaturesin thepresenceofproperlyselectedcatalysts.Untilnow,numerouscatalystswereemployed fortheODHofethane,suchascompositeoxidesbetweenalkalineearthsandrareearths, halogen (particularly F and Cl) and/or alkali ionpromoted, as well as some transition metal(Mo,V,Bi,etc.)oxidebasedcatalysts. Asanalternativetotheheterogeneousroute,aheterohomogeneousprocessforthe ODHofethaneattemperatureshigherthan900 ◦C,thesocalledautothermaloxidative dehydrogenation,wasalsoemployed.Thefollowingreaction carried out by vanadium magnesiumcatalystandwithoutcatalyst. 5.2 Catalyst Preparation

MesoVMg catalysts were prepared by using vanadium source like V 2O5 and the magnesium source like magnesium chloride (MgCl 26H 2O). Surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecylbenzene sulfonate (SDBS),

SyntheticStrategiesinChemistry 12.17 benzyltrimethylammoniumbromide(BTAB),andthetemplateishexadecylamine(HDA) wereusedforthesynthesisofthemesoporousmaterials.

MgCl 26H 2O and the template were dissolved into an aqueous solution of hydrochloric acid. The vanadium source was dispersed homogeneously into distilled waterwithvigorousstirring.Thenthesolutioncontainingvanadiumwasaddedslowly intothatcontainingmagnesium,withvigorousstirringatroomtemperature.ThepHof themixturewasadjustedto4.0or10.0.Afterstirringatroomtemperaturefor24h,the mixturewasallowedtoagestaticallyatroomtemperaturefor2days.Thesolidformed wasrecoveredbyfiltration,washedwithdistilledwater,anddriedat100 ◦Cfor12h.To removethesurfactant,thepreparedspecimenswereheatedinaflowofargonfromroom temperature up to 750 ◦C at a rate of 10 ◦Cmin −1 and kept at that temperature for 4 h. MesoV was prepared using the same procedure, except that no magnesium was introducedandthepHofthemixturewasadjustedto7.0. The MixVMg catalysts were prepared via a solidstate reaction. Powders of vanadium and magnesium source were mixed together and ground thoroughly in a mortar,andthemixtureobtainedwascalcinedat750 ◦Cfor2hafteritwasheatedfrom roomtemperatureatarateof4 ◦Cmin −1. 5.3 Reaction Set Up Thecatalyticperformancewasdeterminedatatmosphericpressureinatubularfixedbed quartzmicroreactor(internaldiameter=5mm,operationlength=30cm).Thereactor was packed as the middle of the reactor was plugged with quartz wool and a catalyst (about0.25g)waslocatedoverit.Thespaceofthereactorabovethecatalystbedwas filledwithquartzgranules.Thereactorwasplacedintoatubularfurnacewiththecatalyst bedlocatedintheconstanttemperaturezone. Inaddition,fourotherreactorconfigurationsfree of catalyst were employed in the studyofthenoncatalyticconversionofethane, namely, ET (the empty tube), FQ (the reactorfilledwithquartzgranulesuptoaheightof1cm),HFQ(theupperhalfofthe reactorwasfilledwithquartzgranulesandtherestofthereactorwasempty),andFFQ (theentirereactorwasfilledwithquartzgranules).Twothermocoupleswereemployedto monitorandcontrolthetemperature.Oneofthemwasembeddedinthefurnace,andthe otheronewaslocatedinthecenterofthecatalystbedandtightlycontactedtheexternal

12.18NewerReactionsandProcedures:CatalyticandNoncatalytic surface of the quartz reactor. The temperatures measured by these two thermocouples werealmostthesame. Thecompositionsofreactant mixtures (N 2, O 2, and C 2H6) and gaseouseffluentsfromreactorweredeterminedbyonlinegaschromatographywithFID andTCDdetector 5.4 Results TheresultsareshowninFig12.8.Atlowtemperatures, the differences between the yields of ethene for the noncatalytic and catalyticthermolysisandODHofethaneare small when the conversions of ethane are low. The highest yield of ethene for the noncatalyticODHoccursforaconversionofabout7%;however,theyieldtoethenefor thecatalyticODHcanbehigher,beinglargerfortheMesoVMgcatalyststhantheMix VMgonesandthehighestfortheMesoVMg3catalyst.

Fig. 8(a). Yield of ethene as a function of conversion of ethane for noncatalytic and catalyticthermolysisandODHofethaneat550 ◦C(Thethermolysiscasesaremarkedby T.)[6]

SyntheticStrategiesinChemistry 12.19

Fig.8(b)Yieldofetheneasafunctionofconversion of ethane for noncatalytic and catalyticthermolysisandODHofethaneat700 ◦C.(Thethermolysiscasesaremarkedby T.)[6] ThehigherperformanceofthemesoVMgforODHofethanethantheMixVMgones may be due to the V 2O3 phase containing highly dispersed magnesium species and possessing large specific surface area in the former case. The Mg species probably moderate the redox capability of V 2O3, thus controlling the activations of ethane and oxygen. The activation mechanism of ethane over these catalysts is dependent on temperature and the heterogeneous processes occur at low temperatures, whereas heterogeneous–homogeneous ones account for the behavior of the catalysts at high temperatures. 6.0 PREPARATION OF POLYORGANOSILOXANES BY SUPERCRITICAL FLUID METHOD Polyorganosiloxanesorsiliconesarethemostpopularsiliconbasedpolymericmaterials in which the backbone is composed of repeating Si–O linkages. It has good thermal stability,lowtemperaturestability,weatherability,transparency,andelectricinsulation. The materials are used in almost all industries including automobile, construction, electronics, personal and household care, and chemical industries. Silicones manufacturedbysequentialhydrolysisandpolycondensationreactionsofchlorosilanes withorwithoutusingorganicsolvents.

12.20NewerReactionsandProcedures:CatalyticandNoncatalytic

Thehydrolysisandpolycondensationprocessesinanorganicsolventareconducted inthepresenceofanacidcatalystbecausethehydrolysisrateofalkoxysilanesissmaller than that of chlorosilanes. The neutralization process is also necessary to obtain final products even though alkoxysilanes were used as the substrates. In addition, a large amountoforganicsolventwasteisgeneratedinbothcases.Accordingtothesefacts,the current silicone manufacturing processes cannot be regarded as an environmentally benign process. Typical examples for syntheses of poly(phenylsilsesquioxane) starting fromphenyltrialkoxysilanesandfromphenyltrichlorosilane,bothofwhichareperformed inasignificantamountofanorganicsolventformorethan10h,havebeenreported 6.1 Supercritical Fluid Method This technology has attracted significant interest for the last two decades from an environmentalviewpoint.Themostattractiveaspectofthistechnologyistoreduceor eliminatetheuseoforganicsolventsinprocess.Supercriticalcarbondioxide(scCO 2)or supercriticalwater(scH 2O),isusedasasolvent.Manyindustrialapplicationsareunder developmentofseveralwhichhavebeengloballybeencommercializedbasicallydueto reduction of hazardous material waste. Several SCFbased technologies have been proposedtosynthesizesiliconesandotherSicontainingmaterialswithoutusingorganic solventslikePolysiloxanesynthesisbyacatalyticpolymerizationofsiloxanecyclicsand silanolterminated siloxane oligomers, a hydrosilylation process yielding functional polysiloxanes,andsilicaaerogelproductionbyscCO 2dryingwerereportedasscCO 2 assistedtechnologies.Ontheotherhand,scH 2Oassistedtechnologiesincludei)recycling ofwastesiliconeelastomersviatreatmentwithMeOH/H 2Omixtureathightemperature, ii)siliconeparticleformationbydegradationofelectrophotographicdevelopercarriers, iii) silicon nanotubes formation, and iv) silicon oxide nanowires formation. These technologiesarenottailormadesiliconesyntheticprocesses.Thefollowingmethodis the first example of a catalystfree silicone synthesis via sequential hydrolysis and polycondensation reactions of alkoxysilanes [7]. Organic solventsoluble nonlinear silicones such as silsesquioxanes were selected as target materials due to their high potentialfordevelopmentofvalueaddedproducts.

SyntheticStrategiesinChemistry 12.21

6.2 Equipment Thismethodneedthefollowingequipments • AreactorandnarrowtubingforthehighT&Pprocessweremadeof½”and1/8” stainlesssteeltubing. • Theinternalvolumesofthereactorare10andthetubingwere2mL. • ApressuregaugewasaKH15pressuretransmitterandasandbathwasusedasa heat source in which sand is circulated by a compressor to maintain the temperaturedeviationwithin±3 ◦C. • ProductsareanalyzedbyGCMS 6.3 Synthesis of Poly(phenylsilsesquioxane) (PPSQ) PTMSanddeionizedwaterwereloadedinaoneendcappedreactorandsealed withaconnectorattachedtonarrowtubing(stainlesssteelcoilwiththediameterof1/8”; length:90cm;internalvolume:2mL)asatrapandapressuregauge.Thereactorwas placedinapreheatedsandbathat300 ◦Ctostartthereaction.Duringheating,pressure insidethereactorwasmonitoredperiodically.Afterthepressurevaluebecameconstant, thereactorwaspulledoutofthesandbathandpouredintoawaterbathtoterminatethe reaction.Aftertheworkupsimilartothatforthetrapfreesystem,asolidproductwas obtained.

Scheme.12.5.Reactionsyieldingphenylsilsesquioxanebyhydrolysisandcondensation ofPTMS Thisprocessshowsthattheorganicsolventsarenotneededtoincreasethediffusability ofsubstratesinthehighTandPwaterassistedprocessindicatingthatanorganicsolvent freeprocessbeessentiallypossible.Inaddition,acatalystfreeprocesscanbeexpectedif theionproductofhighT&Pwatersuchassubcriticalwaterishighenoughforhydrolysis

12.22NewerReactionsandProcedures:CatalyticandNoncatalytic ofalkoxysilanes.Inthisstudy,alkoxysilaneswereusedassubstratessincechlorosilanes arenotappropriateduetoevolvingcorrosivegasofHCl. SynthesisofPPSQviahydrolysisandsubsequentpolycondensationreactionsofPTMS wasselectedasthefirstprocessbecausehydrolysisofphenylbasedalkoxysilanesathigh temperaturesprovedtobemorecontrollablethanthatofmethylbasedalkoxysilanes.The overallreactionintendedisasfollows:

C6H5Si(OCH 3)3 +3 /2H 2OC 6H5SiO 3/2 +3CH 3OH Theinitialstudywasmadebyabatchprocessusingastainlesstubereactorwiththe diameter of ½” and the volume of 10 ml. The reactor containing PTMS and excess amountofwaterwasheatedat300 ◦C.Duringthisreactionmethanolformedabove200 ◦C asabyproduct.

Fig. 12.9. Reaction equipment for a trapattached system in which the narrow tubing betweenthereactorandthepressuregaugeactsasthetrap.Thesizesofthereactorand thetrapare10and2ml,respectively[7] Astheamountofsubstratesincreases,themolecularweightofPPSQincreasesto thetopvalueandthendecreasesindicatingthatthereactionisstronglydependingonthe pressure. This isbecause the reaction is an equilibrium reaction in which three molar methanolasabyproductgeneratesfromonemolarphenyltrimethoxysilane.Thepressure range at which the PPSQ’s molecular weight becomes the maximum is between

SyntheticStrategiesinChemistry 12.23 approximately 3 and 7. It is notable that the molecular weight of PPSQ increased by introducingnarrowtubingasatrapinthereactionsystem.Thisisbecausetrappingof methanol shifts the equilibration to the productside. The process yields polyorganosiloxanes with relatively high content of Si–OH and Si–OMe groups. Synthesisofpolysiloxaneswithotherstructuralunitsbyusingvariousalkoxysilanesis alsopreparedbythesamemethod.Itcanexplaininthefollowingmanner. 6.4 STRUCTURAL UNITS OF POLYORGANOSILOXANES

Fig. 12.10. Structural units of polyorganosiloxanes. R: methyl, phenyl, vinyl, H, and otherreactivegroupssuchasaminopropylandglycidoxypropyl. Q unit: SiO 4/2

Tetramethoxysilane(TMOS) yielded aninsolublesolidproductbasedonQunitswith significant amount of OH/OR groups. This can be as a part of the alkoxysilanes was trappedintothetubingbyevaporationaswaterisandthenundergoeshydrolysistoyield insolublesolidinthetubing.

Tunit:MeSiO 3/2 Methyltrimethoxysilane(MTMS)wasusedasaTsource.Thisreactioncarriedoutby withouttrapbecauseofthehighvolatilityofthiscompoundanditleadstotheformation ofhighlyreactivehydrolysatewithhighcontentofsilanol/methoxythatturnedintoan insolublematerial.

Dunit:Me 2SiO 2/2 PolycondensationbetweenasilanolterminatedoligodimethylsiloxaneasaDsource and other alkoxysilanes such as PTMS, MTMS, and methylphenyldimethoxysilane (MPDMS)provedtobepossible.Thisreactionshowsthatthisoligomerisstableat300 ◦C inthepresenceofexcesswaterwithoutbothofdehydrativeselfcondensationasshown in the following Scheme and siloxane bond cleavage. These results indicate that the polycondensationobservedhereproceedsbyademethanolreaction.

12.24NewerReactionsandProcedures:CatalyticandNoncatalytic

Scheme.12.6.Dehydrativecondensationofsilanolterminatedpolydimethylsiloxanes 6.5 CONCLUSION It can be concluded that polysiloxanes composed of any D, T, and Q units are easily synthesized. The volatility of the substrates is a critical factor to select the reaction systemwithnarrowtubingasamethanoltrap.Sincesiloxanebondcleavagehardlytakes place,thepresentsystemisadvantageousforthematerialdesignbywhichthestructure ofsiloxanecontainingstaringmaterialremainsinthepolysiloxaneproduct.Thelargest advantageofthisnewprocessisnocontaminationofvolatileorganiccompoundsinthe productbecausetheproductisobtainedasasolventfreeform.Inaddition,asthepresent syntheticmethodissimplifiedduetoskipoftheneutralizationprocessbeingnecessary foraconventionalsolutionbasedprocess,thisisalsoadvantageousfromtheeconomical standpoint. 7.0 REFERENCES 1.http://biofuels.coop/pdfs/4_commercial.pdf 2.AyhanDemirbas,EnergyConversionandManagement,44(2003)2093. 3.S.Demirel,K.Lehnert,M.Lucas,P.Claus,Appl.Catal.B:Environmental70(2007) 637. 4.R.Garcia,M.Besson,P.Gallezot,Appl.Catal.A:Gen.127(1995)165. 5.http://www.che.lsu.edu/COURSES/4205/2000/Mattson/HDS.htm 6.ZiShengChaoandEliRuckenstein,JournalofCatalysis222(2004)17. 7.TakuyaOgawa,JunWatanabe,YoshitoOshima,JournalofSupercriticalFluids45 (2008)80.

Chapter - 13 SYNTHETIC STRATEGIES - FROM LABORATORY TO INDUSTRY S.Chandravathanam INTRODUCTION Scaleupisanactoftransferringalaboratoryprocesstothelargerequipmenttypicalofa commercialplant,ordesigningapieceofcommercialequipmentbasedonresearchscale models.Thisisoftenacomplexmatterinwhich,forsomeprocesses,trailanderrorstill hasasignificantfoothold.Evenwithcarefulplanningandstrictmethodology,scaleup can be fraught with difficulty and unexpected problems. The reasons for this are numerous; many common laboratory methods cannot be applied at the large scale, equipmentmayexhibitunexpectedbehaviouratsizesneverusedbefore,orcriticalheat ormasstransferphenomenamaynotbediscernibleatlaboratoryscale. Thedesignofanewplantorcommercializationofanewchemicalprocessrepresents atremendousinvestmentoftimeandmoney.Theriskisconsiderableandtheeconomic penalty,iftheplantorprocessfailstoproduceasexpectedissevere.Tominimizesuch risks, industries undertake lengthy and expensive process research and development programs. Aninventionmightsometimeslayunusedforageswithoutpayingbackintermsof industrialrealizationandofprofitablebusinessforitslackoftechnicalknowhow.For example,thepigmentindigowasextractedfromtheseaanimalsduringthe13 th century. Onegramofthepigmentcouldbeobtainedfromalmost10,000numbersoftheanimal. Sothatitwastheveryexpensivedyematerial,andthecolourwasrestrictedtotheRoyal familyonly,andsonamedastheRoyalpurple.ItwasthesituationtillBayerinthe19 th centurystudiedthedyematerial,andstartedsynthesizingitfromtheeasilyavailableraw materials. Thistextdealswiththetechnicalknowledgeandonthetoolsthatarenecessaryto changeaninventionintoatrueinnovation.Thetermscaleuphasusuallybeenexplained as how to design an industrial reactor able to replicate the results obtained in the laboratory.Thesearedonewithmostofthetimeinnovativeideas,andsometimeswith

13.2 SyntheticStrategies–FromLaboratorytoIndustry manymistakes.Thescaleupusuallymeansthatthescalingfacorof1000timesfromthe laboratoryprocesstothepilotplantscale,andfurther1000timesfromthepilottothe industrialscale.Typical laboratoryscalepreparationlieswithintheproductionof100 g/day.Apilotplantwilltypicallyproduce150Kg/day,andtheindustrialplantwillbe aimedwiththeproductionoftons/day. Fewoftheindispensablestepswhichareundertakenwhilegoingfromthelaboratory scaletothecommercialorindustrialscaleare,costestimation,processdesign,pilotplant studiesmayormaynotbeaccompanyingwiththedimensionalanalysis. COST ESTIMATION Thecostestimationforthescaleupofproductionisaverycrucialstep.Forthisprocess, chemicalengineerslongbeenrelyingasaruleathumbontheuseofpowerlaw.Asper thelawtheinvestmentisproportionaltothescaleofaproductionfacilityraisedtosome constantpower,characteristicoftheparticularprocess. Thesocalledpowerlawofinvestmenttakestheform,

1/n I 2/I 1=k(V t,2 /V t,1 )

WhereIisinvestment,V tisproductionrateandkandnareconstants. Thepowerlawisbasedonthe‘minimizationlaw’,whichstatesthatpeopleminimize theireffortsperunitofdimensionwheneverachangeofscaleorvolumeisrequired.The constantnisthenumberofdimensionsoftheproductionratelimitingactivity,typically iseitherone,twoorthreecorrespondingtothelinear,areaandvolumetricdimensions. PROCESS DESIGN Process Design is the heart of the process of scaleup. When the research department discovers a new reaction to make an existing product or a new material, the process departmentwillhavetotranslatethesediscoveriesintoanewprocesswhichcouldbe commerciallyandtechnicallyfeasible. Thefewofthefactorsofimportanceduringthisprocessdesignstageare, Productionrate Temperatureandpressureconditionsofthereaction Everyavailableinformationaboutoperatingvariables,thermodynamicsandkineticsof thechemicalreaction Stoichiometryofthemainreaction

SyntheticStrategiesinChemistry 13.3

Desiredyieldoftheproduct Desiredproductpurityandthepossibilityofthefeedrecycling Theimportanceoftheprocessdesignstepcanbeexplainedwithoneofthecasestudies withtheprocessdesignforthemanufactureofGrignardReagent.Grignardreagentisone of the important reagents in organic synthesis for the introduction of alkyl groups on carbonylcarbonatoms.It’srequirementisvastbutitsmanufacturinginlargescalewas not undertaken for a long time for some reasons like the high exothermicity of the reaction,thehighinflammabilityofthesolventdiethylether,andthehighsensitivityof thereagentforwater,whichneedstobeusedforcoolingpurpose. Grignard reagent is prepared in situ when alkyl halide reacts with Mg metal scraps in diethylether. Throughcarefulprocessdesignstudies,thesedrawbackshadbeenovercome;withthe useofalkylchloridesorbromidesinsteadofiodides,theexothermicity ofthereaction canbereducedasaresultofthereductioninthereactionrateofchloridesandbromides comparedtotheiriodidecounterpart.Alongwiththat,itcouldalsoreducethecostasthe chloride is cheaper than the iodide. The risky diethyl ether was replaced with tetrahydrofuran(THF),whichhasatleast30 °Chigherboilingpointthantheformer.Inert gases which can play both as the cooling system and blanket replaced water, as the productGrignardreagentismoresensitivetowater,ifthereisanyleakage. PILOT PLANT Apilotplantisgenerallyacollectionofequipmentdesignedtoallowoperationofanovel processatascalesmallenoughtobesafelymanageablebutlargeenoughtoprovidea realistic demonstration of operations and physical principles as they might apply in a commercial facility and to allow the collection of meaningful engineering data for a furtherscaleup. Pilot plants provide important information on the best ways to handle reactants, intermediates, products and waste streams, on energy transfer, on the best choice of

13.4 SyntheticStrategies–FromLaboratorytoIndustry separationtechnologiesandanoperatingprocedures.Someofthemanyareasofprocess developmentinwhichapilotplantcanplayanimportantroleare, Confirmingtheoperationalfeasibilityofanewprocess Identifyingscaleupeffectsonyieldandselectivity Collectingkineticsanddesigndata Testingmaterialsofconstruction Testingtheoperabilityofcontrolschemes Assessingprocesshazardsandsafetyissues Commercialviabilityoftherawmaterial Processtroubleshootingandoptimization Testingprocessrecyclestreams Pilotplantscanbeclassifiedaccordingtonumerouscriteria.Foremostisthefundamental distinction between a pilot plant for a continuous process versus one for a batch or semibatch process. In the continuous process, pilot plants tend to be singlepurpose, productdedicatedfacilitiesthataregenerallysmaller.Batchpilotplants,typicalofthe finechemicalindustry,tendtobemultipurpose.Therequisiteflexibilitytohandleawide varietyofproductsandprocessescanaddconsiderablytothecomplexityandcostofa plant. DIMENSIONAL ANALYSIS Dimensional analysis is a process by which the dimensions of equations and physical phenomena are examined to give new insight into their solutions. It is a powerful technique for spotting errors in equations. It shows how physical dimensions of the variablesthatgovernaproblemcanbeusedtofindphysicallaws.Themainrequirement for dimensional analysis is dimensional homogeneity – it states that all parts of an equation must have the same dimension. In other words, we can add or compare quantitiesthathavesimilardimensionsonly. Dimensionalanalysisresultsinmanyadvantageslike thereductionofnumberofvariablesinavariableset;ifthereare‘n’variablesor parametersofconcern,thenthetotalnumberofdimensionlessnumbersisreduced tonr,where‘r’isthenumberofbasicdimensionsofalltheparametersof concern(Buckinghampitheorem).

SyntheticStrategiesinChemistry 13.5

Givestheguidelinesforscalingtheresultsfrommodeltesttothefullscale. Otherwisedimensionalanalysissetstheruleunderwhichfullsimilarityinmodel testcanbeachieved. Nondimensionalparametersaremoreconvenientthandimensionalparameters sincetheyareindependentofthesystemofunits. Inthefollowingpassagetwocasestudiesofapplicationofdimensionalanalysisare given. CASE STUDY 1: Correlation between Meat Size and Roasting Time Whatistheroastingtimefortwotimesthemassofthemeat? Themainparametersofconcernforthisproblemarelistedbelowalongwiththeir dimensions.

Physical quantity Symbol dimension

roastingtime θ T

meatsurface A L2

thermaldiffusivity a L2T1 surfacetemperature T0 Θ

temperaturedistribution T Θ

Thehighertheheatconductivityk,ofthemeat,thefasteristhecooking.

Thehighertheheatcapacity ρCp,thesloweristheheattransfer.

∴Thermaldiffusivitya=k/ ρCp Thetotalnumberofdimensionlessnumbersare53=2. ∏ =T/T or(T T)/T ) 1 0 0 0 ∏ =aθ/A F 2 0

WhenthetemperaturedistributionT/T 0isachievedthroughoutthemeatthenitcanbe saidthatthemeatiscooked. T/T =f(F ) 0 0

13.6 SyntheticStrategies–FromLaboratorytoIndustry

T/T =idem F aθ/A=idem θαA idem=identical 0 0 Thisequationrelatestheroastingtimewiththeareaofthemeat.Butinrealitythe roastingtimeneedstobecalculatedwithrespectedtothemassofthemeat. massisrelatedtoareawiththefollowingequation. 3 3/2 m=ρVαρL αρA thedensity ρremainsthesameirrespectiveofthemeatsize.Therefore,

ρ=idem and A ∝m 2/3 θ /θ 2/3 2 1 α(m2/m1) Thatis, 2/3 0.67 θαm =m ∴doublingthemassofmeat,theroastingtimeincreasesby,2 2/3 =1.58times.Thefinal equationcontainsonlytwoparameters,insteadoftheinitialfiveparameters,therefore, thenumberofsupplementaryexperimentalrunsisgettingdrasticallyreduced. CASE STUDY 2: Homogeneous Irreversible 1 st Order Reaction in a Tubular Reactor Howmuchisthevolumeandresidencetimeofthereactortobeincreasedforthe increaseofvolumethroughputbyafactorofn(q T=nq M)? Theimportantparametersofconcernare,vflowrate,d,L–diameterandlengthofthe tubularreactor, ρ,fluiddensityandviscosityrespectively.T 0–inlettemp. cin ,c out –inletandoutletconc.k eff. –effectivereactionrateconstant keff. =k 0 exp(E/RT) Theparametersofmassandheattransferare,DDiffusioncoefficient,Cp–heat capacity,kthermalconductivity,c in HR heatofreactionperunittimeandvolume,

T0inlettemp., Ttemp.differencebetweenfluidandtubewall. Thecompleterelevantlistofparametersis,

v,d,L,ρ,,c ,c ,k E/R,D,Cp,k,c Η ,T ,Τ out in 0, in R 0

SyntheticStrategiesinChemistry 13.7

Fromthepitheorem,thenumberofnondimensionalparametersshouldbe,156=9. Theobviousfivenondimensionalnumbersare, L/d,C /C ,E/RT andΤ/Τ out in 0 0 Theremainingfournondimensionalnumbersarederivedusingthefollowing combinationoftheparameters. 1 Π1=v/Lk0=(k0τ) (meanresidencetimeτ L/vatpipeflow)

2 1 Π2=/ρd k0=(k0τReL/d)

2 1 Π3=D/L k0=(k0τReScL/d)

1 Π4=(ρCpΤ0)/cinΗR=Da (Da=Damkohlernumber)

2 1 Π5=(kΤ0)/cinΗRd k0=(k0τRePrDaL/d) ∴Theninedimensionlessnumbersare,

L/d,Cout/Cin,E/RT0,Τ/Τ ,κ τ,Re,Sc,Pr,Da 0 0 Duringscaleup

•Nochangeofreactiontemp. ∴T0 and k 0 are constant •Nochangeinthephysicalandchemicalpartnersofthereaction ∴thekineticand materialnumbers,remainunchanged. • L/d = identical

•Toattainspecifieddegreeofconversion, c out /c in = identical Therefore,thefollowingtwoaretheonlytwoparametersneedtobeadjusted. Re=(vdρ)/andk τ (k L)/v 0 0 Butitisimpossibletohaveboth, vdvL=idemandL/v=idemandL/d=idem

Forthegivenconditionof qT=nq M,, dT=nd M, 2 2 2 q ∝vd andv TdT =nv MdM

13.8 SyntheticStrategies–FromLaboratorytoIndustry as, Reαvd=idem V =(V )/n T M 3 2 VT=n VMandhenceτΤ=n τΜ(τ=V/q) Where, Mmodelscale Ttechnologicalorindustrialscale q–volumethroughput n–factor V–volumeofthereactor Theresultsstatethatfortheincreaseofvolumethroughputntimesthevolumeofthe reactorneedstobeincreasedbyn 3timesandtheresidencetimeneedstobeincreasedby n2times. REFERENCES 1.KirkOthmer,ConciseEncyclopediaofChemicalTechnology,FifthEdition,Vol.2, JohnWiley&Sons,2007. 2.JohnJ.Mcketta(Ed.,),EncyclopediaofChemicalProcessingandDesign,Vol.49, 1994. 3.MarkoZlokarnik,ScaleupinChemicalEngineering,,WileyVCH,2002. 4.KeldJohansen,‘Aspectsofscaleupofcatalystproduction’,StudiesinSurface ScienceandCatalysis,143(2002)1. 5.GianniDonati,RenatoPaludetto,CatalysisToday,34(1997)483.

Chapter - 14 SYNTHESIS OF CHEMICALS FROM CARBON DIOXIDE T.M.Sankaranarayanan INTRODUCTION Carbondioxideisanatmosphericgascontainsonecarbonandtwooxygenatoms.Itis wellknownchemicalcompoundanditschemicalformulaCO 2. Dryice isnothingbut solidstatecarbondioxide.Itwasoneofthefirstgasestobedescribedasasubstance distinctfromair. Inthe17thcentury,theFlemishchemistJanBaptistvanHelmontobservedthatwhen heburntcharcoalinaclosedvessel,themassoftheresultingashwasmuchlessthanthat of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" ( spiritus sylvestre ). The Scottish physician Joseph Black studied some of the properties more thoroughlyinthe1750’s.Hefoundthatlimestone(calciumcarbonate)couldbeheatedor treatedwithacidstoyieldagasashetermed"fixedair."Heobservedthatthefixedair wasdenserthanairanddidnotsupporteitherflameoranimallife.Healsofoundthatit would, when bubbled through an aqueous solution of lime (calcium hydroxide), precipitate calcium carbonate, and used this phenomenon to demonstrate that carbon dioxideisproducedbyanimalrespirationandmicrobialfermentation.In1772,Joseph Priestleyusedcarbondioxideproducedfromtheactionofsulfuricacidonlimestoneto preparesodawater,thefirstknowninstanceofanartificiallycarbonatedbeverage.CO 2 was first liquefied (at elevated pressures) in 1823 by Humphrey Davy and Michael Faraday.TheearliestdescriptionofsolidcarbondioxidewasgivenbyCharlesThilorier, whoin1834openedapressurizedcontainerofliquidcarbondioxide,onlytofindthatthe coolingproducedbytherapidevaporationoftheliquidyieldeda"snow"ofsolidCO 2. Most of the combustion of organic matter, volcanic out gassing and respiration processesoflivingaerobicorganismsarethemainsourcesofthecarbondioxide.Various microorganisms from fermentation and cellular respiration also produce it. During the photosynthesisplantstakeinthecarbondioxideandusingboththecarbonandoxygento formcarbohydrates.Inaddition,plantsalsoreleaseoxygentotheatmosphere,whichis subsequently used for respiration by heterotrophic organisms, forming a cycle. It is

14.2 SynthesisofChemicalsfromCarbondioxide presentintheearth’satmosphereatalowconcentrationandactsasagreenhousegas.It isamajorcomponentofthecycle. Greenhouseeffectisnothingbutanincreaseintheabsorptionofradiationenergyfrom suncausedbytheexistenceofgasesintheearthatmosphere.Becauseofthisabsorption theearthatmospherictemperatureisraisingwhichiscalled“globalwarming”.Someof the gases like CO 2, CH 4, O 3, CFCs and H 2O vapors are called “green house gases”.

Mainlythesegasesareresponsibleforthisabsorption.CO 2beinganimportantmember ofthesegasesisresponsibleformanyclimaticchangesdemonstratingtheimportanceof

CO 2 contentinatmosphere.BecauseofmountingofIndustrialRevolution,thePercentage ofcarbondioxideintheearthatmosphereisincreasing virtually at the rate of 1% per annum;from250ppmofthepreindustrialperiodtoapresentlevelof400ppm(315ppm in1958,340ppmin1984).Becauseofthisglobalwarmingthesnowcoverinthenorthern hemisphere and floating ice in the artic ocean havedecreasedconsiderably.Moreover, globallyseawaterlevelincreasedupto8inchesinthelastdecade.Thereisanincreasein worldwideprecipitationbyonepercent.Thereisabnormalrainfallthroughouttheworld. Unfortunately,greenhousegasesarelikelytoincreasetherateofclimatechanges. CARBON DIOXIDE - CHEMICAL AND PHYSICAL PROPERTIES 1) Carbondioxideisacolorlessgas. 2) When inhaled at high concentrations (a dangerous activity because of the associatedasphyxiationrisk),itproducesasourtasteinthemouthandastinging sensationinthenoseandthroat. 3) Thesekindofeffectsresultfromthegasdissolvinginthemucousmembranes andsaliva,formingaweaksolutionofcarbonicacid. 4) Itsdensityat25°Cis1.98kgm −3 ,about1.5timesthatofair. 5) Ithasnoelectricaldipole.Asitisfullyoxidized,itisnotveryreactiveand,in particular,notflammable. 6) Attemperaturesbelow−78°C,carbondioxidecondensesintoawhitesolidcalled dry ice. Liquid carbon dioxide forms only at pressures above 5.1 atm; at atmosphericpressure,itpassesdirectlybetweenthegaseousandsolidphasesina processcalledsublimation.

SyntheticStrategiesinChemistry 14.3

7) Waterwillabsorbitsownvolumeofcarbondioxide,andmorethanthisunder pressure.About1%ofthedissolvedcarbondioxideturnsintocarbonicacid.The carbonicacidinturndissociatespartlytoformbicarbonateandcarbonateions. Test for Carbon dioxide Whenalightedsplintisinsertedintoatesttubecontainingcarbondioxide,theflameis immediatelyextinguished,ascarbondioxidedoesnotsupportcombustion.(Certainfire extinguisherscontaincarbondioxidetoextinguishtheflame).Tofurtherconfirmthatthe gas is carbon dioxide, the gas may be bubbled into calcium hydroxide solution. The calciumhydroxideturnsmilkybecauseoftheformationofcalciumcarbonate. Applications 1) Liquidandsolidcarbondioxideareimportantrefrigerants,especiallyinthefood industry,wheretheyareemployedduringthetransportation and storage of ice creamandotherfrozenfoods.Solidcarbondioxideiscalled"dryice"andisused forsmallshipmentswhererefrigerationequipmentisnotpractical. 2) Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation in beer and sparkling wine come about through natural fermentation, but some manufacturers carbonate these beverages artificially. 3) Theleaveningagentsusedinbakingproducecarbondioxidetocausedoughto rise. Baker's yeast produces carbon dioxide by fermentation within the dough, whilechemicalleavenerssuchasbakingpowderandbakingsodareleasecarbon dioxidewhenheatedorexposedtoacids. 4) Carbondioxideisoftenusedasaninexpensive,nonflammable pressurized gas. Life jackets often contain canisters of pressured carbon dioxide for quick inflation.Steelcapsulesarealsosoldassuppliesofcompressedgasforairguns, paintball markers, for inflating bicycle tires, and for making seltzer. Rapid

vaporizationofliquidCO 2isusedforblastingincoalmines. 5) Carbondioxideextinguishesflames,andsomefireextinguishers,especiallythose designedforelectricalfires;containliquidcarbondioxideunderpressure.Carbon dioxidealsofindsuseasanatmosphereforwelding,althoughintheweldingarc, itreactstooxidizemostmetals.Useintheautomotiveindustryiscommondespite

14.4 SynthesisofChemicalsfromCarbondioxide

significant evidence that welds made in carbon dioxide are quite delicate than thosemadeinmoreinertatmospheres,andthatsuchweldjointsdepreciateover time because of the formation of carbonic acid. It is used as a welding gas primarily because it is less expensive than more inert gases such as argon or helium. 6) Liquidcarbondioxideisagoodsolventformanyorganiccompoundsandithas begun to attract attention in the pharmaceutical and other chemical processing industriesasalesstoxicalternativetomoretraditionalsolventssuchasorganic chlorides.It'susedbysomedrycleanersforthisreason. 7) Plants require carbon dioxide to conduct photosynthesis, and greenhouses may

enrichtheiratmosphereswithadditionalCO 2toboostplantgrowth.Ithasbeen proposed that carbon dioxide from power generation be bubbled into ponds to growalgaethatcouldthenbeconvertedintobiodieselfuel.Highlevelsofcarbon dioxideintheatmosphereeffectivelyexterminatemanypests.Greenhouseswill

raise the level of CO 2to10,000ppm(1%)forseveralhourstoeliminatepests suchaswhitefly,spidermites,andothers. 8) Inmedicine,upto5%carbondioxideisaddedtopureoxygenforstimulationof

breathingafterapneaandtostabilizetheO 2/CO 2balanceinblood. 9) A common type of industrial gas laser, the carbon dioxide laser, uses carbon dioxideasamedium. 10) Carbondioxideiscommonlyinjectedintooradjacenttoproducingoilwells.It willactasbothapressurizingagentand,whendissolved intothe underground crude oil, will significantly reduce its viscosity, enabling the oil to flow more rapidlythroughtheearthtotheremovalwell.Inmatureoilfields,extensivepipe networksareusedtocarrythecarbondioxidetotheinjectionpoints. Carbon dioxide - Dry Ice 1) Dryiceisageneralizedtrademarkforsolid("frozen")carbondioxide.PrestAir Devices,acompanyformedinLongIslandCity,NewYorkin1923,coinedthe termin1925.

SyntheticStrategiesinChemistry 14.5

2) Dryiceatnormalpressuresdoesnotmeltintoliquidcarbondioxidebutrather sublimatesdirectlyintocarbondioxidegasat−78.5°C(−109.3°F).Henceitis called"dryice"asopposedtonormal"wet"ice(frozenwater). 3) Compressingcarbondioxidegastoaliquidform,removingtheheatproducedby the compression, and then letting the liquid carbon dioxide expand quickly producedryice.Thisexpansioncausesadropintemperaturesothatsomeofthe

CO 2freezesinto"snow",whichisthencompressedintopelletsorblocks. Supercritical Carbon dioxide Carbon dioxide also could be used more widely as a solvent and for example 0 supercriticalCO 2(thestateexistingat31. Cand72.8atm).NowadaysCarbondioxide could be used more widely as a solvent and for example supercritical carbon dioxide offersadvantagesintermsofstereochemicalcontrol,productpurificationsynthesizing finechemicalsandpharmaceuticals.Peopleareextractingcaffeinefromcoffeebyusing supercriticalcarbondioxide.MoreovertheadvantageofusingCO 2isoilgasrecovery andpondsofgeneticallymodifiedalgaethatcanconvertpowerplantCO 2intobiodiesel.

Asnotedabove,CO 2isgenerallyconsideredtobeagreenorenvironmentallybenign solventandisnaturallyabundant.CO 2hasbeensuggestedasasustainablereplacement fororganicsolventsinanumberofchemicalprocessesandiscurrentlyusedinthedry cleaning, and in parts degreasing. While CO 2 is certainly not a panacea, there are a number of characteristics, which suggest that CO 2 could provide environmental and economic benefits. The nontoxic nature of CO 2 has a number of advantages. For example,infoodandpharmaceuticalapplications,usageofCO 2greatly reduces future liabilitycostsandcanalsofacilitateregulatoryapprovalofcertainprocesses.Anexample is the conversion of pharmaceuticals into nanometersize particles for injectable use. Another instance in which supercritical carbon dioxide could be advantageous is in situationsinvolvingcontactbetweenhydrophilicandhydrophobicsolvents.Inthiscase, themutualsolubilityofthetwophasesisdesignedtobesmall.However,somecross contaminationisinevitable,typicallyleadingtoacostlyremediation.TheuseofCO 2 as thehydrophobicphaseproducescontaminationthatisbothbenignandreadilyreversible. Examplesincludeliquidliquidextractionbetweenorganicandaqueousphasesaswellas emulsionpolymerizationofwatersolublemonomers.Inapplicationswhereemissionsare

14.6 SynthesisofChemicalsfromCarbondioxide

unavoidable,CO2isrelativelybenigntotheenvironment.Examplesrangefromuseof

CO 2inenhancedoilrecoverytouseasafoamingagentorasthesolventindrycleaning.

UsingsupercriticalCO 2asasolventalsohasadvantagesthatarisefromchemicaland/or physicalproperties.Inreactionsinvolvinggaseousreactantsinliquidphases,theuseof supercritical CO 2withitsabilitytodissolvelargeamountsofmost gases could allow kineticcontrolofreactionsasopposedtolimitingofreactionratesbythetransportofthe gaseousreactantacrossthegasliquidinterface.InreactionswhereCO 2isareagent,its useasasolventwouldalsofavorthereaction.Carbondioxidemayalsoofferadvantages inreactionssuchasfreeradicalpolymerizationsandoxidationswhereachemicallyinert solventisrequired. CHEMICALS SYNTHESIS FROM CARBON DIOXIDE

ItiswellknownthatCO 2isverystablegasandhighlyunreactivebutplantsareutilizing it for example in the photosynthesis of carbohydrate from CO 2. Can we also find the ways to make chemicals from CO 2 artificially? Surely it will be an environmentally benignroute,andthisprocesswillleadtogreenchemistry.Themainprocesswhichuse

CO 2are 1) Synthesisofurea. 2) Synthesisofsalicylicacid 3) Synthesisofcycliccarbonateandpolycarbonate 4) Synthesisofmethanol.

SynthesisofureabyusingCO 2 currentlyisawellestablishedprocess.Thecapacityis approximately90millionmetrictonsperannumasper1997statistics.Otherreactions areinpilotplantscale.Besidestothesereactionstherearemanyreactions,whichutilize

CO 2.Itwillbeagreatfeedstockformakingcommoditychemicals,fuelsandmaterials.It alreadyplaysamajorroleforavarietyofapplications.Buttherearefewcatches.Oneis thatCO 2isverystable,whichmeansittakesextraefforttoachievethemoleculessothat itwillreact.ProfessorChristopher.M.RayneroftheUniversityofLeeds,inEngland, has been working on CO 2 conversion. He published a review article recently on the potentialofCO 2insyntheticorganicchemistry.Approximately115millionmetrictons of CO 2 is used annually by the global chemical industries but really that does not compare to the approximate 24 billion metric tons. Bulk chemicals already produced

SyntheticStrategiesinChemistry 14.7

routinely from CO 2 include urea to make nitrogen fertilizers, salicylic acid as a pharmaceuticalingredient,andpolycarbonatebasedandplastics.Thesimplestreactions of CO 2 are those in which it is simply inserted into an XH bond. Examples are the insertionofCO 2intoorganicaminestoaffordcarbamicacids,whichmaybeconverted intoorganiccarbamates.MorerecentexamplesincludetheinsertionofCO 2inPNbonds of P (NR 2)3 compounds to form P (NR 2)(OCONR 2) 2 compounds and the reaction of ammoniumcarbamates(derivedfromCO 2)withalkylhalidesinthepresenceofcrown etherstoformusefulurethaneintermediates.ThisisanexampleofusingCO 2toreplace phosgene, a highly toxic intermediate in chemical synthesis. Reactions are known in which CO 2 undergoes insertion into SnC bonds of allyl tin compounds to form carboxylated allyl derivatives and which are catalyzed by Pd complexes. Another interestingreactionistheinsertionofCO 2intoalkanessuchasmethanetoformacetic acid.

The activation of a CH bond and CO 2 insertion are much fascinating .The thermodynamicsofthisreactionaremarginal;however,adjustingthereactionconditions andcouplingthisreactionwithenergeticallyfavorableproductprocessescouldimprove conversionefficiencies.Carbonates,(RO) 2CO,canalsobepreparedbyinsertingCO 2into OH bonds followed by dehydration or by oxidative carboxylation of olefins. This syntheticapproachhasthepossibilityofprovidinganewroutetocompoundsthathave verylargepotentialmarkets.RelatedreactionsinwhichCO 2isincorporatedintoproduct moleculeswithoutreductionhavebeenusedinthesynthesisofpolymers. ThegroupsofInoueandKuranperformedinitialworkinthisarea.Inrecentyears,a numberofnewcatalystshavebeendevelopedforcopolymerizationofCO 2andoxiranes toformpolycarbonate.Thesestudieshaveincreasedtheproductivityofthisreactionby 100 times and have also expanded the range of applicable monomers (oxiranes). Polypyrones are another potentially interesting new class of polymer. It has been preparedfromdiacetylenesandCO 2inthepresenceofNicatalysts;arelatedreactionis thetelomerizationofbutadieneandCO 2toproducelactones.Urethaneshavealsobeen preparedbythereactionsofdicarbamateionsformedbyinsertionofCO 2intodiamines, followedbyPdcatalyzedcouplingto1,4dichloro2butene.Reductivecarboxylationsin whichtheCO 2unitisincorporatedintotheproductarealsoknown.

14.8 SynthesisofChemicalsfromCarbondioxide

In the case of alkynes and olefins, electrochemical reductive carboxylations result in effective addition of the formic acid CH bond to CC double or triple bonds. For example,buildingontheearlierstoichiometricresultsofHoberg,Dunachandcoworkers usedNibipyridinecomplexesandsacrificialMganodestoreductivelycoupleacetylene andCO 2toformpropenicacid.Similarly,Sylvestrireportedthatthereductivecoupling of CO 2withstyreneiscatalyzedbybenzonitrile.Bromoarenes can also be reductively carboxylatedtoformthecorrespondingcarboxylicacidusingNidiphosphinecatalysts.

Morerecently,thesequentialreductivecouplingoftwomoleculesofCO 2tobutadieneto form3hexen1,6dioicacidhasbeenreported.Thisapproachprovidesanewroutetoa Nylon precursor. Another important monomer, ethylene, can be prepared by electrochemicalreductionofCO 2inaqueoussolutionswithcurrentefficienciesashighas 48%.Theproductionofthismonomerbythisremarkable12electronsreductionoffersa potential route to polyethylene from CO 2. The preceding results clearly indicate that it may be possible to produce a large variety of polymers in the future using materials derivedfromCO 2.

Underoxidativeconditions,CO 2mayreactwitholefinstogivecycliccarbonatesthat find wide industrial applications. In these transformations heterogeneous catalysts are currentlymorepromisingandviablethanhomogeneousones.Anotherpotentiallyuseful reaction of CO 2 is the dehydrogenation of hydrocarbons. Examples are the dehydrogenation of ethyl benzene and propane over metal oxides to form styrene and propene,respectively.Inthesereactions,nopartoftheCO 2moleculeisincorporatedinto the organic product, rather the oxygen of CO 2 serves to remove two H atoms of the hydrocarbon.CO 2iscurrentlyusedasanadditiveinthesynthesisofmethanolfromCO and H 2, and it is believed that reduced forms of CO 2 are kinetically important intermediates in this process. Recently, efficient heterogeneous catalysts have been developed for CO 2 hydrogenation to methanol. However, the thermodynamics for methanol production from H 2 and CO 2 are not as favorable as that for production of methanol from H 2andCO.Forexample,at200°Ctheequilibriumyield of methanol from CO 2isslightlylessthan40%whiletheyieldfromCOisgreaterthan80%.The reduction of CO 2 can be rendered more favorable by the use of hybrid catalysts that dehydratemethanoltoformDimethylether.Othercopperzincbasedcatalystshavealso

SyntheticStrategiesinChemistry 14.9

beenusedformethanolsynthesis.FisherandBell etal. studiedCu/ZrO 2/SiO 2catalysts by insitu infrared spectroscopy and suggested some mechanism for the route to methanol. Ethanol has also been produced by the hydrogenation of CO 2. This fuel is attractivebecauseithasasomewhathigherenergydensitythanmethanolanditisnotas toxic.However,theselectivityforethanolproductioniscomparativelylow(<40%).

ThehydrogenationofCO 2tomethaneandhigherhydrocarbonsisalsoknown.ForC 2 andhigherhydrocarbons,hybridcatalystssuch asCuZnOCr 2O3 and HY zeolite are generallyused.Noyori etal .havecarriedoutpioneeringworkonthecatalyticsynthesis of formic acid derivatives by CO 2 hydrogenation, together with other substrates, in supercritical CO 2. In part because of the high solubility of H 2 in CO 2, an economical synthesisofdimethylformamideisachieved.

HomogeneouscatalystsarealsoknowntomediatetherapidhydrogenationofCO 2to formate, because this reaction is not thermodynamically preferential, amines and supercriticalCO 2havebeenusedtodrivethisreaction.Undertheappropriateconditions, very high turnover numbers and rates can be achieved. For example, Leitner et al. examinedcomplexesofthegeneraltype[R 2P(X)PR 2]Rh(hfacac)(X=bridginggroup; hfacac) 1,3bis(trifluoromethyl)acetonylacetonate). All the compounds are active catalysts for formic acid production from H 2 and CO 2, but the most effective has X=

(CH 2)4andR=cyclohexylshowinggoodresultsat25°Cand40atmof1:1H 2:CO 2.The selectivitytoformicacidisnearly100%. PossiblepathwaysfortheopposinginteractionofLowValentCatalystswithProtons orCO 2 oftheCO 2reductionproductobserved.Ifthereducedformofthecatalystsreacts withCO 2toformaMCO 2 complex,protonationyieldsametallocarboxylicacid;further reactioncanthenproduceCObyCObondcleavagetoformhydroxideorwater.Thus, reactionofareducedformofthecatalystwithCO 2,asopposedtoprotons,leadstoCO formation. If the reduced form of the catalyst reacts with protons to form a hydride complex,subsequentreactionofthehydridewithCO2leadstoformateproduction.An interesting example of such selectivity is CO 2 electrochemical reduction catalyzed by polymeric films based on [Ru(NN)(CO) 2]n (NN=poly pyridine ligand) in aqueous media. Deronzier and Ziessel et al. found that bipyridyl ligands with electron withdrawinggroupsinthe4,4positionsgavecatalystswhicharehighly selectivefor

14.10SynthesisofChemicalsfromCarbondioxide formateatpH>5whilethosederivedfromtheunsubstantiated2,2bipyridineorthe4, 4dimethylanalogueprimarilygiveCOatpH>7.165Formatewasthoughttoarise from an intermediate metal hydride, whereas CO was thought to arise from a metallo carboxylicacid generated by carbonation of an intermediate anion followed by protonation. It is unusual for homogeneous catalysts to form reduction products that requiremorethantwoelectrons. However, Tanaka and coworkers, reported that the formation of glycolate

(HOCH 2COO), glyoxylate (OCHCOO), formic acid, formaldehyde, and methanol as 2+ CO 2reductionproductsusing[Ru(tpy)(bpy)(CO)] complexesaselectrocatalysts(bpy= 2,2bipyridine,andtpy=2,2:6,2terpyridine).Althoughturnovernumberswere notgivenforthesehighlyreducedspecies,theirformationraisestheexcitingpossibility that a singlesite catalyst can result in multi electron reductions of CO 2 and even CC bondformation.

TanakaandGibson etal .recentlysucceededinisolatingkeyRuC 1compoundswith polypyridine ligands that are models for catalytic intermediates. Gibson et al . also isolatedReC 1complexeswithpolypyridineligands.Theimportanceofphotochemistryin reactionsofsomehavetheReandRucomplexeshasbeendemonstrated.Theformation offormaldehydehasalsobeenreportedinelectrochemicalCO 2reductionusingtransition metal terpyridine complexes polymerized on glassy carbon electrodes. The relatively mildconditionsandlowoverpotentialsrequiredforsomeofthehomogeneouscatalysts makethemattractiveforfuturestudies;however,anumberofbarriersmustbedefeated beforeusefulcatalystsareavailableforfuelproduction.

Photochemical reduction of CO 2 is one of the significant reactions many of the reactions described above rely on energy input either in the form of reactive bonds (alkenes, alkynes, etc.), hydrogen, or electricity. Photochemical systems have been studied in an effort to develop systems capable of directly reducing CO 2 to fuels or chemicals using solar energy. Transitionmetal complexes have been used as both catalystsandsolarenergyconverters,sincetheycanabsorbasignificantportionofthe solarspectrum,havelonglivedexcitedstatesareabletopromotetheactivationofsmall molecules, and are forceful. Carbon dioxide utilization by artificial photo conversion presentsachallengingalternativetothermalhydrogenationreactions,whichrequireH 2.

SyntheticStrategiesinChemistry 14.11

ThesystemsstudiedforphotochemicalCO 2reductionstudiescanbedividedintoseveral 2+ 2+ groups: Ru(bpy) 3 bothasaphotosensitizerandasacatalyst.Ru(bpy) 3 as a photo sensitizerandanothermetalcomplexasacatalyst.ReX(CO) 3 (bpy)orasimilarcomplex 2+ 2+ asaphotosensitizers.Ru(bpy) 3 andRu(bpy) 3 typecomplexesasphotosensitizersin microheterogeneous systems. Metalloporphyrins also act as a photosensitizers and catalyst.PhotochemicalCO 2reductionisnormallycarriedoutlessthan1.0atmCO 2at roomtemperature.Therefore,theconcentrationofdissolvedCO 2inthesolutionislow

(e.g., 0.28 M in CH 3CN,0.03Minwater).Thesesystemsproduceformate and CO as products.Inthemostefficientsystems,thetotalquantumyieldforallreducedproducts reaches40%.InsomecaseswithRuorOscolloid,CH4isproducedwithalowquantum yield.Underphotochemicalconditions,theturnovernumberandtheturnoverfrequency are dependent on irradiation wavelength, light intensity, irradiation time, and catalyst concentration, and they have not been optimized in most of the photochemical experimentsdescribed.TypicalturnoverfrequenciesforCOorHCOO arebetween1and 10h 1, andturnovernumbersaregenerally100orless.The abovementioned molecular sensitizers can be replaced with semiconductor electrodes or particles to achieve light harvesting. These systems may use enzymes or catalysts to promote electron transfer fromthesemiconductorsolutioninterfacetoCO 2orreduceCO 2directly.Typicallythese reductions require a potential bias in addition to solar energy input to achieve CO 2 reductionandelectrodecorrosionsamajorconcern.Thiscorrosioncansometimesbe defeated using high CO 2 pressures. Fascinating examples of stoichiometric photochemicalreactionsofCO 2promotedbymetalcomplexeshavealsobeenreported.

Thus, Aresta et al. found that CO 2 can be incorporated into cyclopropane to afford butyrolactone.Kubiak etal. demonstratedthereductionofCO 2totheradicalanionand subsequentcouplingtocyclohexenebytheuseofaNicomplex.

Ranyar etal. haveworkedonthecatalyticprocessesforreducingCO 2toformicacid. It has potential of power fuel cell for electricity generation and automobiles and as a precursorforotherfuels,syntheticchemicalsandfibers,includingpolymers.

ConversionofCO 2toCOwillbemorechallengingprocessbecauseCOcanbeused inahostoforganicsynthesisandoneoftheimportantfeedstockinthechemicalindustry formakinghigherhydrocarbonthroughFisherTropschSynthesis.

14.12SynthesisofChemicalsfromCarbondioxide

Nicolas Eghbali and his group working on conversion of CO 2 and olefins into cyclic carbonatesinwater.GeoffreyW.Coates etal. havedevelopedthecatalysttoincorporate

CO 2 into polymers by using βdiminate zinc acetate and salen cobalt carboxylate complexes. These catalysts promote alternating co polymerization of various epoxides with CO 2tomakebiodegradablealiphaticpolycarbonate.The polymers which contain

3050% CO 2 by weight have gas barrier and degradation properties that make them attractive for feed packaging foamcasting to make automotive parts and electronic processingapplications.Thesekindsofpolymersalsocanbeusedtoreplacepropylene oxidesegmentsinpolyurethanefoams,whichwouldhelpcutcosts.Thefoamsareused forinsulationandseatcushions,amongotherapplications About150milliontonsofplasticsproducedgloballyinayear,andmostofitisnon biodegradableandfromenergyintensiveprocessesthatusepetroleumbasedfeedstocks. Coastes et al. working on limeonene oxide derived from citrus fruit waste as a potential epoxide monomer for copolymerization with CO 2.other research in progress involvesdevelopingacatalyticsystemthatcanuseuntreatedCO 2 directlyfromindustrial wastestreamstomakepolymer. Artificialbioinspiredsystemsisfarlesscomplicated and therefore easier to study thannaturalphotosynthesis,inwhichsunlightwaterandCO 2areconvertedintoO 2and carbohydrates. Byusinginorganicmaterial,peopleareutilizingthecarbondioxide.Thereactionof

CaCO 3 and CO 2 in water to form Ca (HCO 3) 2 isresponsibleforthefixationoflarge quantitiesofCO 2intheoceans.However,itiskineticallyslow.Similarly,CO 2canalso be fixed by naturally occurring minerals. Even though the reactions are thermodynamicallyfavorable,theyareslowandwouldneedtobeenhancedkinetically beforetheycouldcontributesignificantlytoadjusting the carbon balance. In addition, thiswouldgenerallyrequireminingandprocessing enormous amountsofmaterialsto store relatively little CO 2.Currently, large quantities of CaCO 3 are converted into CaO andCO 2(whichisreleasedintotheatmosphere)incementmanufacture.Ifanaturalore couldbesubstitutedforCaO,asignificantreleaseofCO 2 intotheatmospherecould,in principle,beavoided.

SyntheticStrategiesinChemistry 14.13

Fujita has found success by using rhenium tri carbonyl complexes to mediate CO 2 reduction. The researchers homed in on one set of catalysts bearing bipyridine (bpy)

ligandswhichincludes(bpy)Re(CO) 3anditsdirheniumanalog.Butthereactionrates

forcoproductionareslowduetothestabilityofCO 2

PeopleareworkingonCO 2→CObutultimatetargetisCO 2toMethanolthatcould beusedasafuel.

All Possible Reactions

Fig.14.1.Overallchemicaltransformations

(reproducedfromHironiriArakawa, chemicalReviews,101 (2001)976) CONCLUSION Synthesizingchemicalsfromcarbondioxideisoneofthechallengingprocessaswellas ecofriendlyandenvironmentallybenignroute.Usingcarbondioxideasarawmaterial

14.14SynthesisofChemicalsfromCarbondioxide

nevergoingtoreduceatmosphereCO 2leveloritwillbeverylittletheeffectonclimate changebutwecanreducetheproductionofCO 2andreducetheusageoffossilfueland shallwemakeit? REFERENCES 1.J.M.Desimone, Science,265 (1994)359 2.HironiriArakawa, chemicalReviews , 101 (2001)976 3.Chemical&EngineeringNews, April30 ,2007,11 4.Chemistry&Industry, July13 ,2007,13 5.X.Xiaoding,J.A.MoulijinEnergyandFuels, 10 (1996)305 6. Greenhouseissues 1992Feb 7.M.A.ScibiohandB.ViswanathanProc. IndianNatnSciAcad .,70A(3)(2004)407

Chapter -15 CARBOHYDRATES TO CHEMICALS VamsiKrishnaNunna, INTRODUCTION Carbohydratesarethemostabundantclassoforganiccompoundsfoundinlivingorganisms. They originate as products of photosynthesis , an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll. Carbohydrates can be

writtenascarbonhydrates,C n(H 2O) n,hencetheirname.

nCO 2+nH 2O+energy C nH2n On+nO 2 Thecarbohydratesareamajorsourceenergyrequiredformetabolism.Asidefromthesugars and starches that meet this vital nutritional role, carbohydrates also serve as a structural material(cellulose),acomponentoftheenergytransportcompoundATP,recognitionsites oncellsurfaces,andoneofthreeessentialcomponentsofDNAandRNA. MONOSACCHARIDES Monosaccharidescanbeinturncanbeclassifiedasketosesandaldosesbasingonthefunctional grouppresent,i.e.ketoneoraldehyde. Glucoseisamonosaccharide,analdohexoseandareducingsugar.Thegeneralstructureof glucose and many other aldohexoses was established by simple chemical reactions.Hot hydriodicacid(HI)wasoftenusedtoreductivelyremoveoxygenfunctionalgroupsfroma molecule,andinthecaseofglucosethistreatmentgavehexane(inlowyield).Fromthisit wasconcludedthatthesixcarbonsareinanunbranchedchain.Thepresenceofanaldehyde carbonylgroupwasdeducedfromcyanohydrinformation,itsreductiontothehexaalcohol sorbitol, also called glucitol, and mild oxidation to the monocarboxylic acid, glucuronic

15.2 CarbohydratestoChemicals

acid. Somewhat stronger oxidation by dilute nitric acid gave the diacid, glucaric acid, supportingtheproposalofasixcarbonchain.

Fig.15.1.Reactionsofglucose Thefiveoxygensremaininginglucoseafterthealdehydewasaccountedforwerethoughtto be in hydroxyl groups, since a pentaacetate derivative could be made. These hydroxyl groupswere assigned,oneeach,tothelastfivecarbon atoms, because geminal hydroxyl groupsarenormallyunstablerelativetothecarbonyl compound formed by loss of water. Glucose and other saccharides are extensively cleaved by periodic acid, thanks to the abundanceofvicinaldiolmoietiesintheirstructure.Thisoxidativecleavage,knownasthe Malaprade reaction is particularly useful for the analysis of selective Osubstituted derivatives of saccharides, since ether functions do not react. The stoichiometry of aldohexosecleavageisshowninthefollowingequation.

HOCH 2(CHOH) 4CHO+5HIO 4 ——>H 2C=O+5HCO 2H+5HIO 3 Configuration of Glucose Thefourchiralcentersinglucoseindicatetheremaybeasmanyassixteen(2 4)stereoisomers havingthisconstitution.Thesewouldexistaseightdiasteromericpairsofenantiomers,and the initial challenge was to determine which of the eight corresponded to glucose. This challengewasacceptedandmetin1891bytheGermanchemistEmilFischer.

SyntheticStrategiesinChemistry 15.3

Thelastchiralcenterinanaldosechain(farthestfromthealdehydegroup)waschosenby FischerastheD/Ldesignatorsite.Ifthehydroxylgroupintheprojectionformulapointedto theright,itwasdefinedasamemberoftheDfamily.Aleftdirectedhydroxylgroup(the mirrorimage)thenrepresentedtheLfamily.Itisimportanttorecognizethatthesignofa compound's specific rotation (an experimental number) does not correlate with its configuration(DorL).Itisasimplemattertomeasureanopticalrotationwithapolarimeter. Determininganabsoluteconfigurationusuallyrequireschemicalinterconversionwithknown compoundsbystereospecificreactionpaths. FischerprojectionformulasandnamesfortheDaldosefamily(threetosixcarbonatoms) areshownbelow.

15.4 CarbohydratestoChemicals

EmilFischermadeuseofseveralkeyreactionsinthe course of his carbohydrate studies. Thesearedescribedhere Oxidation SugarsmaybeclassifiedasreducingornonreducingbasedontheirreactivitywithTollens', Benedict'sorFehling'sreagents.Ifasugarisoxidizedbythesereagentsitiscalledreducing, sincetheoxidant(Ag (+) orCu (+2) )isreducedinthereaction,asevidencedbyformationofa silvermirrororprecipitationofcuprousoxide.TheTollens'testiscommonlyusedtodetect aldehydefunctions;andbecauseofthefacileinterconversionofketosesandaldosesunder the basic conditions of this test, ketoses such as fructose also react and are classified as reducingsugars.

1.

2.

3.

SyntheticStrategiesinChemistry 15.5

Whenthealdehydefunctionofanaldoseisoxidizedtoacarboxylicacidtheproductiscalled an aldonic acid. Because of the 2º hydroxyl functions that are also present in these compounds,amildoxidizingagentsuchashypobromitemustbeusedforthisconversion (equation1).Ifbothendsofanaldosechainareoxidizedtocarboxylicacidstheproductis calledanaldaricacid.Byconvertingaldosestoitscorrespondingaldaricacidderivative,the endsofthechainbecomeidentical. Thus,ribose,xylose,alloseandgalactoseyieldachiralaldaricacidswhichare,ofcourse,not opticallyactive.Theriboseoxidationisshowninequation2below.Otheraldosesugarsmay give identical chiral aldaric acid products, implying a unique configurational relationship. Theexamplesofarabinoseandlyxoseshowninequation 3 above illustrate this result. A Fischer projection formula may be rotated by 180º in the plane of projection without changingitsconfiguration. Reduction Sodium borohydride reduction of an aldose makes the ends of the resulting alditol chain identical,HOCH 2(CHOH) nCH 2OH,therebyaccomplishingthesameconfigurationalchange producedbyoxidationtoanaldaricacid.Thus,allitolandgalactitolfromreductionofallose andgalactoseareachiral,andaltroseandtalosearereducedtothesamechiralalditol.

Derivatives of HOCH 2(CHOH) nCHO

HOBrOxidation ——> HOCH 2(CHOH) nCO 2H(anAldonicAcid)

HNO 3Oxidation ——> H2OC(CHOH) nCO 2H(anAldaricAcid)

NaBH 4Reduction ——> HOCH 2(CHOH) nCH 2OH(anAlditol)

Osazone Formation The Osazone reaction was developed and used by Emil Fischer to identify aldose sugars differinginconfigurationonlyatthealphacarbon.Theequationshowsthegeneralformof the osazone reaction, which effects an alphacarbon oxidation with formation of a bis phenylhydrazone,knownasanosazone.ApplicationoftheOsazonereactiontoDglucose andDmannosedemonstratesthatthesecompoundsdifferinconfigurationonlyatC2.

15.6 CarbohydratestoChemicals

4.

5.

Chain Shortening and Lengthening

6.

7.

Thesetwoprocedurespermitanaldoseofagivensizetoberelatedtohomologoussmaller and larger aldoses. Thus Ruff degradation of the pentose arabinose gives the tetrose erythrose.Workingintheoppositedirection,aKilianiFischersynthesisappliedtoarabinose gives a mixture of glucose and mannose. Using these reactions, we can understand the Fischer'strainoflogicinassigningtheconfigurationofDglucose.

SyntheticStrategiesinChemistry 15.7

Anomeric Forms of Glucose Twodifferentcrystallineformsofglucosewerereportedin1895.Eachofthesegaveallthe characteristic reactions of glucose, and when dissolved in water equilibrated to the same mixture.Thisequilibrationtakesplaceover aperiodofmanyminutes, andthechangein optical activity that occurs is called mutarotation . These facts are summarized in the diagram,

Asimplesolutiontothisdilemmaisachievedbyconvertingtheopenaldehydestructurefor glucoseintoacyclichemiacetal,calledaglucopyranose,asshowninthefollowingdiagram. Thelinearaldehydeistippedonitsside,androtationabouttheC4C5bondbringstheC5 hydroxyl function close to the aldehyde carbon. For ease of viewing, the sixmembered hemiacetalstructureisdrawnasaflathexagon,butitactuallyassumesachairconformation. Thehemiacetalcarbonatom(C1)becomesanewstereogeniccenter,commonlyreferredto astheanomericcarbon,andtheαandβisomersarecalledanomers.

Fig.15.2.Anomersofglucose Cyclic Forms of Monosaccharides Thepreferredstructuralformofmanymonosaccharidesmaybethatofacyclichemiacetal. Fiveandsixmemberedringsarefavoredoverotherringsizesbecauseoftheirlowangleand eclipsing strain. Cyclic structures of this kind are termed furanose (fivemembered) or

15.8 CarbohydratestoChemicals

pyranose (sixmembered), reflecting the ring size relationship to the common heterocyclic compoundsfuranandpyranshownontheright. Ribose,animportantaldopentose,commonlyadoptsafuranosestructure,asshowninthe followingillustration.Theupperbondtothiscarbonisdefinedasbeta,thelowerbondthen isalpha .

Fig.15.3.Formationofribofuranose Thecyclicpyranoseformsofvariousmonosaccharidesareoftendrawninaflatprojection knownasaHaworthformula,aftertheBritishchemist,NormanHaworth.TheseHaworth formulasareconvenientfordisplayingstereochemicalrelationships,butdonotrepresentthe trueshapeofthemolecules.Thesemoleculesareactuallypuckeredinachairconformation. Examplesoffourtypicalpyranosestructuresareshownbelow,bothasHaworthprojections andasthemorerepresentativechairconformers.

Thesizeofthecyclichemiacetalringadoptedbyagivensugarisnotconstant,butmayvary withsubstituentsandotherstructuralfeatures.Aldolhexosesusuallyformpyranoseringsand their pentose homologs tend to prefer the furanose form, but there are many counter examples.

SyntheticStrategiesinChemistry 15.9

Glycosides

Fig.15.4.Glycosideformation Acetalderivativesformedwhenamonosaccharidereactswithanalcoholinthepresenceof anacidcatalystarecalledglycosides.Thisreactionisillustratedforglucoseandmethanolin thediagrambelow.Innamingofglycosides,the"ose"suffixofthesugarnameisreplacedby "oside", and the alcohol group name is placed first. As is generally true for most aldols, glycoside formation involves the loss of an equivalent of water. Since acidcatalyzed aldolization is reversible, glycosides may be hydrolyzed back to their alcohol and sugar componentsbyaqueousacid. Glycosides abound in biological systems. By attaching a sugar moiety to a lipid or benzenoid structure, the solubility and other properties of the compound may be changed substantially.Becauseoftheimportantmodifyinginfluenceofsuchderivatization,numerous enzymesystems,knownasglycosidases,haveevolvedfortheattachmentandremovalof sugarsfromalcohols,phenolsandamines. Chemistsrefertothesugarcomponentofnaturalglycosidesastheglyconandthealcohol componentastheaglycon.Salicin,oneoftheoldestherbalremediesknown,wasthemodel forthesyntheticanalgesicaspirin.Largeclassesofhydroxylated,aromaticoxoniumcations calledanthocyaninsprovidethered,purpleandbluecolorsofmanyflowers,fruitsandsome vegetables. Peonin is one example of this class of natural pigments, which exhibit pronouncedpHcolordependence.Theoxoniummoietyisonlystableinacidicenvironments and the color changes or disappears when base is added.Thecomplexchangesthatoccur whenwineisfermentedandstoredareinpartassociatedwithglycosidesofanthocyanins.

15.10CarbohydratestoChemicals

Finally, amino derivatives of ribose, such as cytidine play important roles in biological phosphorylatingagents,coenzymesandinformationtransportandstoragematerials Disaccharides Two joined monosaccharides are called disaccharides and represent the simplest polysaccharides. They are composed of two monosaccharide units bound together by a covalentbondknownasaglycosidiclinkageformedviaadehydrationreaction,resultingin thelossofahydrogenatomfromonemonosaccharideandahydroxylgroupfromtheother. Someexamplesofdisaccharidesare; Cellobiose :4OβDGlucopyranosylDglucose Maltose :4OαDGlucopyranosylDglucose

Gentiobiose :6OβDGlucopyranosylDglucose Trehalose :αDGlucopyranosylαDglucopyranoside Cellobiose,maltoseandgentiobiosearehemiacetalstheyareallreducingsugars(oxidized byTollen'sreagent).Trehalose,adisaccharidefoundincertainmushrooms,isabisacetal, and is therefore a nonreducing sugar. Acidcatalyzed hydrolysis of these disaccharides yieldsglucoseastheonlyproduct.Enzymecatalyzedhydrolysisisselectiveforaspecific

SyntheticStrategiesinChemistry 15.11 glycosidebond,soanalphaglycosidasecleavesmaltoseandtrehalosetoglucose,butdoes notcleavecellobioseorgentiobiose.Abetaglycosidasehastheoppositeactivity. Althoughallthedisaccharidesshownherearemadeupoftwoglucopyranoserings,their propertiesdifferininterestingways.Maltose,sometimescalledmaltsugar,comesfromthe hydrolysisofstarch.Itisaboutonethirdassweetascanesugar(sucrose),iseasilydigested byhumans,andisfermentedbyyeast.Cellobioseisobtainedbythehydrolysisofcellulose. It has virtually no taste, is indigestible by humans, and is not fermented by yeast. Some bacteriahavebetaglucosidaseenzymesthathydrolyzetheglycosidicbondsincellobioseand cellulose.Thepresenceofsuchbacteriainthedigestivetractsofcowsandtermitespermits these animals to use cellulose as a food. Finally, it may be noted that trehalose has a distinctlysweettaste,butgentiobioseisbitter.Sucrose,orcanesugar,isourmostcommonly usedsweeteningagent.Itisanonreducingdisaccharidecomposedofglucoseandfructose joinedattheanomericcarbonofeachbyglycosidebonds(onealphaandonebeta). Polysaccharides polysaccharides are large highmolecular weight molecules constructed by joining monosaccharideunitstogetherbyglycosidicbonds.Theyaresometimescalled glycans .The mostimportantcompoundsinthisclass,cellulose,starchandglycogenareallpolymersof glucose. Cotton fibres are essentially pure cellulose, and the wood of bushes and trees is about50%cellulose.

Cellulose Asapolymerofglucose,cellulosehastheformula (C 6H10 O5)n where n ranges from500to5,000,dependingonthesourceofthepolymer.Theglucoseunitsincelluloseare linkedinalinearfashion,asshownbelow.Thebetaglycosidebondspermitthesechainsto stretchout,andthisconformationisstabilizedbyintramolecularhydrogenbonds.Aparallel orientationofadjacentchainsisalsofavoredbyintermolecularhydrogenbonds.Althoughan individualhydrogenbondisrelativelyweak,manysuchbondsactingtogethercanimpart great stability to certain conformations of large molecules. Most animals cannot digest celluloseasafood,andinthedietsofhumansthispartofourvegetableintakefunctionsas roughageandiseliminatedlargelyunchanged. Someanimals(thecowandtermites,forexample)harbourintestinalmicroorganismsthat breakdowncelluloseintomonosaccharidenutrientsbytheuseofbetaglycosidaseenzymes. Cellulose is commonly accompanied by a lower molecular weight, branched, amorphous

15.12CarbohydratestoChemicals

polymercalled hemicellulose .Incontrasttocellulose,hemicelluloseisstructurallyweakand is easily hydrolyzed by dilute acid or base. Also, many enzymes catalyze its hydrolysis. Hemicelluloses are composed of many Dpentose sugars, with xylose being the major component. Mannose and mannuronic acid are often present, as well as galactose and galacturonicacid. Fig.15.5.Cellulosestructure Fig.15.6.Representativepartialstructureofamylose Starch isapolymerofglucose,foundinroots,rhizomes,seeds,stems,tubersandcormsof plants, as microscopic granules having characteristic shapes and sizes. Most animals, includinghumans,dependontheseplantstarchesfornourishment.Thestructureofstarchis

SyntheticStrategiesinChemistry 15.13 more complex than that of cellulose. The intact granules are insoluble in cold water, but grindingorswellingtheminwarmwatercausesthemtoburst.Thereleasedstarchconsistsof twofractions. About20%isawatersolublematerialcalled amylose .Moleculesofamylosearelinear chains of several thousand glucose units joined by alpha C1 to C4 glycoside bonds. Amylosesolutionsareactuallydispersionsofhydratedhelicalmicelles.Themajorityofthe starchisamuchhighermolecularweightsubstance,consistingofnearlyamillionglucose units,andcalled amylopectin .Moleculesofamylopectinarebranchednetworksbuiltfrom C1toC4andC1toC6glycosidelinks,andareessentiallywaterinsoluble.

Fig.15.7.Representativepartialstructureofamylopectin

Glycogen isapolysaccharideof glucose(Glc) whichfunctionsastheprimaryshortterm energystorageinanimalcells.Glycogenistheanalogueofstarch,alessbranchedglucose

15.14CarbohydratestoChemicals polymerinplants,andiscommonlyreferredtoas animal starch ,havingasimilarstructure toamylopectin.Glycogenformsanenergyreservethatcanbequicklymobilizedtomeeta suddenneedforglucose,butonethatislesscompactthantheenergyreservesoftriglycerides (fat).Itismadeprimarilybytheliverandthemuscles,butcanalsobemadebythebrain. Theuterusalsostoresglycogenduringpregnancytonourishtheembryo. Glycogen is a highly branched polymer that is better described as a dendrimer of about 60,000 glucose residues and has a molecular weight between 10 6 and 10 7 daltons (~4.8 million).MostofGlcunitsarelinkedbyα1,4glycosidicbonds,approximately1in12Glc residuesalsomakes1,6glycosidicbondwitha second Glc ,whichresultsinthecreationof abranch.Glycogendoesnotpossessareducingend.

Fig.15.8.Glycogen

Synthetic Modification of Cellulose Cotton,probablythemostusefulnaturalfiber,is nearly pure cellulose. Crude cellulose is alsoavailablefromwoodpulpbydissolvingthelignanmatrixsurroundingit. These less desirablecellulosesourcesarewidelyusedformakingpaper.Inordertoexpandthewaysin whichcellulosecanbeputtopracticaluse,chemistshavedevisedtechniquesforpreparing solutionsofcellulosederivativesthatcanbespunintofibers,spreadintoafilmorcastin varioussolidforms.Akeyfactorinthesetransformationsarethethreefreehydroxylgroups on each glucose unit in the cellulose chain, [C 6H7O(OH) 3]n. Esterification of these

SyntheticStrategiesinChemistry 15.15 functions leads to polymeric products having very different properties compared with celluloseitself. Cellulose Nitrate ,firstpreparedover150yearsagobytreatingcellulosewithnitricacid,is the arliest synthetic polymer to see general use. The fully nitrated compound, [C6H7O(ONO2)3]n, called guncotton, is explosively flammable and is a component of smokeless powder. Partially nitrated cellulose is called pyroxylin. Pyroxylin is soluble in etherandatonetimewasusedforphotographicfilmandlacquers.Thehighflammabilityof pyroxylin caused many tragic cinema fires during its period of use. Furthermore, slow hydrolysisofpyroxylinyieldsnitricacid,aprocessthatcontributestothedeteriorationof earlymotionpicturefilmsinstorage.

Cellulose Acetate ,[C 6H7O(OAc) 3]n,islessflammablethanpyroxylin,andhasreplacedit inmostapplications.Itispreparedbyreactionofcellulosewithaceticanhydrideandanacid catalyst. The properties of the product vary with the degree of acetylation. Some chain shortening occurs unavoidably in the preparations. An acetone solution of cellulose acetate may be forced through a spinnerette to generate filaments, called acetate rayon that can be wovenintofabrics. Viscose Rayon ,ispreparedbyformationofanalkalisolublexanthatederivativethatcanbe spunintoafiberthatreformsthecellulosepolymerbyacidquenching.Thefollowinggeneral equationillustratesthesetransformations.Theproductfiberiscalledviscoserayon.

(+) NaOH H3O () (+) () (+) ROH RO Na +S=C=S ROCS 2 Na ROH cellulose viscosesolution rayon

Catalytic conversion of carbohydrates Carbohydrates are the main source of renewables used for the production of biobased products. Sucrose and starch are the major sources. Polysaccharides such as inulin are gainingimportanceasasourceoffructose.Twocarbohydratesofanimalorigin,lactoseand chitinarealsousedcommercially. Multistep reactions carried out by cascade catalysis without intermediate product recovery decrease operating time and may reduce considerably the amount of waste produced. For example, sorbitol can be obtained in onepot reaction from starchderived

15.16CarbohydratestoChemicals

polysaccharidesusingrutheniumsupportedonacidicYzeolite.Theacidicsitesofthezeolite catalyzethepolysaccharidehydrolysisyieldingtransientlyglucose,whichishydrogenatedto sorbitol on ruthenium. Similarly, 2,5Furanedicarboxylic acid, a potential substitute for terephtalicacidisobtainedinonepotreactionoverabifunctionalacidicandredoxcatalyst consisting of cobalt acetylacetonate encapsulated in sol–gel silica. In the three former examples, the reactions steps took place on heterogeneous catalysts. However, cascade catalysis without recovery of intermediate products may involve enzymatic catalysis, homogeneouscatalysisandheterogeneouscatalysis.Combinationofenzymaticandchemical stepscangiveabetteryield. Hydrogenation of glucose and derivatives Glucoseissuedfromstarchorsucrosehydrolysisishydrogenatedtosorbitol,acommodity productusedinfood,pharmaceuticalandchemicalindustriesaswellasanadditiveinmany endproducts.Mannitolandgluconicacidarethemainbyproductsofthisreaction. Fig.

15.9.Hydrogenationofglucose Catalystsallowinga100%conversionand99%selectivityarerequired.Also,theyshouldbe stableaftermanyrecyclingoperationsorforextendedperiodoftimeonstreamincontinuous reactor. Most of the industrial production is still conducted batchwise on Raney nickel catalystspromotedwithelectropositivemetalatomssuchasmolybdenumandchromium,but because of the risk of nickel or metallic promoter leaching, they tend to be replaced by carbonsupportedrutheniumcatalystswhich are also more active. However, active carbon

SyntheticStrategiesinChemistry 15.17 powdersaredifficulttohandleandrecycleinbatchoperation,thereforeacontinuousprocess withformedcarbonsupportaredesirable. Hydrogenation of glucose to sorbitol was achieved on ruthenium catalysts supported on activatedcarboncloths(ACC)obtainedbycarbonizationandCO 2activationofwovenrayon .Catalyst 0.9 wt.% Ru/ACC was loaded with ruthenium by cationic exchange or anionic adsorptionbothgivinganhomogeneousdistributionof2nmrutheniumparticlesincarbon fibers.TheACCwasclampedonasupportfittingalongtheautoclavewallsthusallowingan easyrecyclingofthecatalystsince,unlikecatalystsinpowderform,nofiltrationarerequired andthereisnoattritionorleaching. ThereisagreatinteresttoconvertC6carbohydratesavailableinlargesupplyfromstarch or sucrose into C5 and C4 polyols that are little present in biomass and find many applications in food and nonfood products. Glucose can be converted to arabitol by an oxidative decarboxylation of glucose to arabinonic acid followed by hydrogenation to arabitol.The main drawback of this reaction is the formation of deoxy products. Aqueous solutions(20wt.%)ofarabinonicacidwerehydrogenatedonRucatalystsinbatchreactor. Theselectivitywasenhancedbyaddingsmallamountsofanthraquinone2sulfonate(A2S), whichdecreasedtheformationofdeoxybyproducts.

Fig.15.10.Oxidativedecarboxylationofglucose

Dehydroxylation of carbohydrates DeoxyhexitolsconsistingofC6diols,triols,andtetrols are well suited to replace polyols derived from petrochemistry for applications in polyester and polyurethane manufacture. SorbitolwastakenasmodelmoleculetostudythehydrogenolysistoC4–C6products.To improvetheselectivitytodeoxyhexitols,catalystsandreactiontemperaturewereoptimized to favor the rupture of C–OH bonds (dehydroxylation reactions) rather than C–C bond

15.18CarbohydratestoChemicals

rupture.Copperbasedcatalysts,whichhavealowactivityforhydrogenolysisofC–Cbonds, wereemployedtotreat20wt.%aqueoussorbitolsolutionsinthetemperaturerangeof180– 2408C.Incontrast,operatinginthepresenceofpalladiumcatalystsat250 0Cunder80bar of hydrogen pressure, cyclodehydration reactions of sorbitol and mannitol occurred with formation of cyclic ethers: isosorbide, 2,5anhydromannitol, 2,5anhydroiditol, and 1,4 anhydrosorbitol. Catalytic oxidation of mono- and di-saccharides Oxidationreactionsarewidelyusedforupgradingcarbohydratestovarietiesofhighadded valuechemicalsusedindetergentsorpharmaceuticals(VitaminC).Toreplacethenongreen hypochloriteagentbyenvironmentalyfriendlyreagents,thecatalyticsystemwasimproved.

OxidationreactionswithH 2O2mediatedbymetalphthalocyaninecatalystshavealsoproved veryefficienttooxidizevariouscarbohydratesincludingtheoxidationofinsolublesubstrates suchasnativestarch.

Fig.15.11.HydrogenolysisofsorbitoltoC 6polyols Glucoseoxidationtogluconicacid,abiodegradablechelatingagentandanintermediatein food and pharmaceutical industry, was achieved with air oxidation in the presence of palladiumcatalysts.Unpromotedpalladiumcatalystswereactiveinglucoseoxidation,but therateofreactionwaslowbecauseoftheoveroxidationofPdsurface,andsideoxidation reactions decreased the selectivity. Using Pd–Bi/C catalysts (5 wt.% Pd, Bi/Pd = 0.1)

SyntheticStrategiesinChemistry 15.19

preparedbydepositionofbismuthonthesurfaceof1–2nmpalladiumparticles,therateof glucose oxidation to gluconate was 20 times higher and the selectivity at near total conversionwashighonthefreshandrecycledcatalysts.Bismuthwasassumedtoactasa cocatalyst protecting palladium from overoxidation because of its stronger affinity for oxygen.Themetalcatalyzedoxidationgavecomparableselectivityandhigherproductivity thanenzymaticglucoseoxidation. Catalytic Conversion of Polysaccharides Polysaccharides are widely available renewable polymers but it is difficult to find cost effectiveprocesstoconvertthemtovaluableendproducts.Duetoitslargeavailabilityand lowcost,nativestarchhasbeenusedforalongtime in the preparation of different end products. Tomeetspecifichydrophilicpropertiesnativestarchhasbeeneithermodifiedbyoxidation orbygraftinghydrocarbonchains .Hydrophilicstarchobtainedbypartialoxidationiswidely usedinpaperandtextileindustriesandcanbepotentiallyappliedinavarietyofapplications, e.g.,forthepreparationofpaints,cosmetics,andsuperabsorbents.Theoxidationoccursat theC6primaryhydroxylgrouporatthevicinaldiolsonC2andC3involvingacleavageof theC2–C3bondtogivecarbonylandcarboxylfunctions.

Fig.15.12.Hydrophilicstarchobtainedbypartialoxidation SeveraltransitionmetalcatalystsbasedonFe,Cu orWsalts(0.01–0.1 mol%)havebeen

proposed to activate H 2O2, which is a wellsuited oxidant from an environmental and economical point of view. However, the concentration of metal ions was quite high and heavy metals were retained by the carboxyl functionsofoxidizedstarch,whichhas good

complexingproperties.Efficientcatalyticmethodsfor native starch oxidation withH 2O2 in the presence of iron tetrasulfophthalocyanine (FePcS) were proposed .Oxidation of starch aqueous suspension in the presence of iron phthalocyanine gives both carboxylic and carbonylgroups.

15.20CarbohydratestoChemicals

Topreparemorehydrophobicstarchesforspecificapplications,partialsubstitutionofstarch withacetate,hydroxypropyl,alkylsiliconateorfattyacidestergroupsweredescribedinthe literature. This can also be achieved by grafting octadienyl chains by butadiene telomerization.

Fig.15.13.Graftingofoctadienylchainsonsucrosethroughbutadienetelomerization Thereactionwasfirstconductedwithsuccessonsucrose.Thedegreeofsubstitution(DS) obtained was controlled by the reaction time. Thus under standard conditions (0.05% Pd(OAc)2/TPPTS,NaOH(1N)/iPrOH(5/1),508C)theDSwas0.5and5after14and64h reaction time, respectively. The octadienyl chains were hydrogenated quantitatively in the presenceof0.8wt.%[RhCl(TPPTS)3]catalystinH2O/EtOH(50/10)mixtureyieldingavery goodbiodegradablesurfactant. REFERENCES: 1.www.wikipedia.org 2.www.users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates 3.www.biochemweb.org/fenteany/teaching/452/Carbohydrates.pdf 4.P.A.Jacobs,H.Hinnekens,EP0329923forSynfinaOleofina 5.M.L.Ribeiro,U.Schuchardt, Catal.Commun. ,4(2003)83 6.A.Perrard,P.Gallezot,in:J.Sowa(Ed.), CatalysisofOrganicReactions , Taylor&Francis,NewYork,2005,p.53. 7.L.Fabre,P.Gallezot,A.Perrard, J.Catal., 208(2002)247 8.B.Blanc,A.Bourrel,P.Gallezot,T.Haas,P.Taylor, GreenChem., 89(2000) 9.M.Besson,F.Lahmer,P.Gallezot,P.Fuertes,G.Fle`che, J.Catal. ,152(1999)116

SyntheticStrategiesinChemistry 15.21

10.A.Sorokin,S.KachkarovaSorokina,C.Donze´,C.Pinel,P.Gallezot, Top.Catal. ,27(2004) 67 11.A.Sorokin,S.Sorokina,P.Gallezot, Chem.Commun. ,2004,2844 12.C.Donze´,C.Pinel,P.Gallezot,P.Taylor, Adv.Synth.Catal., 344(2002)906

Chapter - 16 SOME CONCEPTUAL DEVELOPMENTS IN SYNTHESIS IN CHEMISTRY JoyceDSouza INTRODUCTION Synthesisofmaterialsplayacrucialroleindesigninganddiscoveringnewmaterialsand alsoinprovidingbetterandlesscumbersomemethodsforpreparingknownchemicals. Mostofthetenmillionorsochemicalcompoundsthatareknowntoday,canbeclassified intoarelativelysmallnumberofsubgroupsorfamilies. More than 90 percent of the compounds are organic compounds. In turn, organic compounds can be further subdivided into a few dozen major families such as the alkanes,alkenes,alkynes,alcohols,aldehydes,ketones,carboxylicacids,andamines.An importantsubsetoforganiccompoundsincludebiochemicalcompoundswithfourmajor families:carbohydrates,proteins,nucleicacids,andlipids. Becauseoftheiruniqueproperties,multicarboncompoundsexhibitextremelylarge varietyandtherangeofapplicationoforganiccompoundsisenormous.Theyformthe basisofimportantconstituentsofmanyproductsandapartfromaveryfewexceptions, they form the basis of all earthly life processes. The different shapes and chemical reactivitiesoforganicmoleculesprovideanastonishingvarietyoffunctions,likethoseof enzymecatalystsinbiochemicalreactionsoflivesystems.Theautopropagatingnatureof these organic chemicals is what life is all about. Products such as plastics, synthetic fibres,food,explosives,paintsandpigments,pharmaceuticalsandpesticidesandmany others have become readily available through the dynamic development of organic synthesis. Inorganiccompoundsaretypicallyclassifiedinto one of five major groups: acids, bases, salts, oxides, and others. A large class of compounds discussed in inorganic chemistrytextbooksarecoordinationcompounds.Examplesrangefromspeciesthatare strictly inorganic, such as [Co(NH 3)6]Cl 3, to organometallic compounds such as

Fe(C 5H5)2(ferrocene)andextendingtobioinorganiccompounds,suchasthehydrogenase enzymes.Majorclassesofinorganiccompoundsthatarestudiedundermaterialsscience tendtobepolymeric(nonmolecular)andrefractory,andoftenareofcommercialinterest

16.2SomeCon ceptualDevelopmentsinSynthesisinChemistry such as a)alloys: brass, bronze, stainless steel; b)semiconductors : silicon , gallium arsenide;c)superconductors:yttriumbariumcopperoxide. SYNTHETIC ORGANIC CHEMISTRY Synthetic organic chemistry is an applied science as it borders engineering involving design,analysis,and/orconstructionofworkforpracticalpurposes.Organicsynthesisof anovelcompoundisaproblemsolvingtask,whereasynthesisisdesignedforatarget molecule by selecting optimal reactions from optimal starting materials. Complex compoundscanhaveseveralreactionstepsthatsequentiallybuildthedesiredmolecule. The synthesis proceeds by utilizing the reactivity of the functional groups in the molecule.Forexample,acarbonylcompoundcanbeusedasanucleophilebyconverting itintoanenolate,orasanelectrophile,andthecombinationofthetwoiscalledthealdol reaction. Designing practically useful syntheses always requires conducting the actual synthesisinthelaboratory.Thescientificpracticeofcreatingnovelsyntheticroutesfor complexmoleculesiscalledtotalsynthesis. There are several strategies to design a synthesis. The modern method of retrosynthesis,developedbyE.J.Corey,startswiththetargetmoleculeandsplicesitto piecesaccordingtoknownreactions.Thepieces,ortheproposedprecursors,arefurther spliceduntilideallyinexpensiveandavailablestartingmaterialsarereached.Then,the retrosynthesisiswrittenintheoppositedirectiontogivethesynthesis.A"synthetictree" can be constructed, because each compound and also each precursor have multiple syntheses. Disconnection:Aretrosyntheticstepinvolvingthebreakingofabondtoformtwo(or more)synthons. Synthon Syntheticequivalent

SyntheticStrategiesinChemistry 16.3

Retron:Aminimalmolecularsubstructurethatenablescertaintransformations. Retrosynthetictree:Adirectedacyclicgraphofseveral(orall)possibleretrosynthesesof asingletarget. Synthon: An idealized molecular fragment. A synthon and the corresponding commerciallyavailablesyntheticequivalentareshownbelow: Target:Thedesiredfinalcompound. Transform:Theexactreverseofasyntheticreaction;theformationofstartingmaterials fromasingleproduct. Anexamplewillallowtheconceptofretrosyntheticanalysistobeeasilyunderstood.

In planning the synthesis of phenylacetic acid, two synthons are identified. A + nucleophilic "COOH" group, and an electrophilic "PhCH 2 " group. Of course, both synthons do not exist per se; synthetic equivalents corresponding to the synthons are reacted to produce the desired product. In this case,thecyanideanionisthesynthetic equivalentforthe COOHsynthon,whilebenzylbromideisthesyntheticequivalentfor thebenzylsynthon. Thesynthesisofphenylacetylenedeterminedbyretrosyntheticanalysisisthus:

1. PhCH 2Br+NaCN→PhCH 2CN+NaBr

2.PhCH 2CN+2H2O→PhCH 2COOH+NH 3

16.4SomeCon ceptualDevelopmentsinSynthesisinChemistry

STRATEGIES Functional Group Strategies :Manipulationoffunctionalgroupscanleadtosignificant reductionsinmolecularcomplexity. Stereochemical Strategies : Numerous chemical targets have distinct stereochemical demands. Stereochemical transformations (such as the Claisen rearrangement and Mitsunobu reaction) can remove or transfer the desired chirality thus simplifying the target. Structure-Goal Strategies: Directingasynthesistowardsadesirableintermediate can greatlynarrowthefocusofananalysis.Thisallowsbidirectionalsearchtechniques. Transform-based Strategies : The application of transformations to retrosynthetic analysis can lead to powerful reductions in molecular complexity. Unfortunately, powerfultransformbasedretronsarerarelypresentincomplexmolecules,andadditional syntheticstepsareoftenneededtoestablishtheirpresence. Topological Strategies :Theidentificationofoneormorekeybonddisconnectionsmay lead to the identification of key substructures : Disconnections that preserve ring structures are encouraged; Disconnections that create rings larger than 7 members are discouraged. SYNTHETIC INORGANIC CHEMISTRY Advancesininorganicchemistryhavemadesignificantcontributionstomodernliving. Forinstance,syntheticfertilizersmanufacturedfrominorganicchemicalshaveincreased worldwide crop production. Inorganic substances used to fabricate silicon chips help powertheglobalinformationage.Metalalloysareusedinautomobilesandaircraftto make them lighter and stronger. Companies use inorganic compounds to fabricate concrete,steel,andglass—materialsusedtoconstructbuildings,infrastructure,andother public works around the world. In the United States, 10 of the 11 most commonly producedchemicalsarederivedfrominorganicelements.These10inorganicchemicals (presented below in descending order of production) are used in a wide variety of applications. Sulfuric acid is used to make fertilizers, synthetic fibers, and metals.

SyntheticStrategiesinChemistry 16.5

Nitrogen is used in recovering underground petroleum deposits, in the production of ammonia, and as a blanketing material for shipping perishables such as fruits and vegetables. Oxygen is used in the production of steel and plastics, in medical applications, and in rocketry. Lime is used in the manufacture of steel and cement. Ammoniaiscombinedwithsulfuricacidtomakeammoniumsulfatethemostimportant ofthesyntheticfertilizers.Sodiumhydroxideisusedinthemanufactureofpaper,soap, detergents,andsyntheticfibers,andisalsoacaustic material used as a drain cleaner. Chlorineisusedtomanufacturevinylchlorideplastic,todisinfectdrinkingwater,andto bleach paper during manufacturing. Phosphoric acid is used to give soft drinks a tart flavorandtomakefertilizers.Sodiumcarbonateisusedintheproductionofglass,paper, andtextiles.Nitricacidisusedtomakesyntheticfibers,suchasnylon;explosives,such asnitroglycerinandTNT(trinitrotoluene);andisalsocombinedwithammoniatomake fertilizer. Because of its direct relevance to products of commerce, solid state inorganic chemistryhasbeenstronglydrivenbytechnology.Applicationsdiscoveredinthe20th century include zeolite and platinumbased catalysts for petroleum processing in the 1950’s,highpuritysiliconasacorecomponentofmicroelectronicdevicesinthe1960’s, and “high temperature” superconductivity in the 1980’s. The invention of Xray crystallography in the early 1900s by William Lawrence Bragg enabled further innovation.Althoughsomeinorganicspeciescanbeobtainedinpureformfromnature, mostaresynthesizedinchemicalplantsandinthelaboratory. Thereismuchchemicalingenuityinthesynthesis ofsolidmaterials.Whiletailor makingofmaterialsofthedesiredstructureandpropertiesremainsthemaingoalofsolid state chemistry and material science, it is not always possible. However, a rational approachtothesynthesisofsolidsmaybeevolvedwhichrequiresanunderstandingof theprinciplesofcrystalchemistry,andofthermodynamics,phaseequilibriaandreaction kinetics. Synthetic Strategies Varioustypesofchemicalreactionshavebeenusedforthesynthesisofsolidmaterials. Someofcommonreactionsemployedforthesynthesisinorganicsolidsarelistedbelow:

16.6SomeCon ceptualDevelopmentsinSynthesisinChemistry

(1)decomposition

A(s)B(s)+C(g)

(2)combination C A(s)+B(g) (s)

(3)metathetic[combining(1)and(2)above]

C +D A(s)+B(g) (s) (g) (4)addition C A(s)+B(s) (s)

C A(s)+B(l) (s)

C A(g)+B(g) (s) (5)exchange

AY +BX AX(s)+BY(s) (s) (s)

AX +BY AY(s)+BX(g) (s) (g) Morecomplexreactionsinvolvingmorethanonetype of reaction are also commonly employedinsolidstatesynthesis.Forexample,inthepreparationofcomplexoxides,itis commontocarryoutthermaldecompositionofacompoundfollowedbyoxidation(inair orO 2)essentiallyinonestep: CaMnO (s)+2CO (g) 2Ca0.5Mn0.5CO(s)+1/2O2(g) 3 2

O2 MgO(s)+Cr O (s) MgCr O (s) 2 3 2 4 Specific reagents and reaction conditions are employed to carry out various processes suchasreduction,oxidationandhalogenationinthesynthesisofsolids.

SyntheticStrategiesinChemistry 16.7

Reductionofoxides,forinstance,maybecarriedout • inanatmosphereof(flowing)pureordilutehydrogenorCO. • byheatingoxidesinargonornitrogenforthepurposeofloweringtheoxygen content • byapplicationofvacuumatanappropriatetemperature(vacuumannealingor decompositionatlowpressures) Theobviousmeansofreducingsolidcompoundsbyhydrogenisemployednotonlyfor reducingoxides,butalsohalidesandothercompounds.Thermaldecompositionofmetal halidesalsoyieldslowerhalides:

M2O3(s)+H2(g) 2MO(s)+H2O(g)

(eg.M=Fe)

ABO2.5(s)+1/2H2O(g) ABO3(s)+H2(g)

(e.g.LaCoO3)

MCl3(s)+H2(g) MCl2(s)+HCl(g)

(eg.M=Fe)

heat MX MX +1/2X (eg.M=Fe) 3 2 2 Reductionofoxidesandhalidescan alsobe accomplished by reacting with elemental carbonorwithametal. 3MCl (e.g.M=Nd,Fe) 2MCl3+M 2

3MCl4+M'(s) 3MCl3(s)+M'Cl3(g) (e.g.M=Hf,M'=Al)

M O +3M 2 3 5MO(e.g.M=Nb) Inorganic Synthetic Methods

16.8SomeCon ceptualDevelopmentsinSynthesisinChemistry

Inorganic synthetic methods can be classified roughly according to the volatility or solubilityofthecomponentreactants.Solubleinorganiccompoundsarepreparedusing themethodsoforganicsynthesis. Techniques employed in Inorganic synthesis Oven techniques: For thermally robust materials, high temperature methods are often employed. For example, bulk solids are prepared using tubular furnaces, which allow reactionstobeconducteduptoca.1100°C.Specialequipmente.g.ovensconsistingofa tantalumtubethroughwhichanelectriccurrentispassed,canbeusedforevenhigher temperaturesupto2000°C. Melt methods: If volatile reactants are involved, the reactants are often put in an ampoulethatisevacuatedandthensealed.Thesealedampouleisthenputinanovenand givena certainheattreatmenttomeltthereactants together and then later anneal the solidifiedmelt. Solution methods: Solventsareusedtopreparesolidsbyprecipitationorbyevaporation. Attimesthesolventisusedhydrothermally,i.e.underpressure at temperatures higher thanthenormalboilingpoint.Avariationonthisthemeistheuseoffluxmethods,where asaltofrelativelylowmeltingpointisaddedtothemixturetoactasahightemperature solventinwhichthedesiredreactioncantakeplace. Gas reactions: Many solids react readily with reactive gases like chlorine, iodine, oxygenetc.OthersformadductswithgaseslikeCOorethylene.Suchreactionsareoften carriedoutinatubethatisopenendedonbothsidesandthroughwhichthegasispassed Aspecialcaseofagasreactionisachemicaltransportreactionwhichentailsthe reversibleconversionofnonvolatilechemicalcompoundsintovolatilederivatives. This methodmayalsobeusedasaprocessforpurificationandcrystallizationofnonvolatile solids.Theseareoftencarriedoutinasealedampouletowhichasmallamountofa transportagent,e.g.iodineisadded.Theampouleisthenplacedinazoneoven.Thisis essentiallytwotubeovensattachedtoeachotherwhichallowsatemperaturegradientto be imposed. The volatile derivative migrates through a sealed, evacuated glass tube heated in a tubular furnace. Elsewhere in the tube where the temperature is held at a differentvalue,thevolatilederivativerevertstotheparentsolidandthetransportagentis released.Thetransportagentisthuscatalytic.Thetechniquerequiresthatthetwoendsof

SyntheticStrategiesinChemistry 16.9 tubebemaintained atdifferenttemperatures.Suchamethodcanbeusedtoobtainthe product in the form of single crystals suitable for structure determination by Xray diffraction. Air and moisture sensitive materials : Many solids are hygroscopic and/or oxygen sensitive.Manyhalidese.g.arevery'thirsty'andcanonlybestudiedintheiranhydrous formiftheyarehandledinagloveboxfilledwithdry(and/oroxygenfree)gas,usually nitrogen. Synthetic Methods A large variety of inorganic solid materials has been prepared in recent years by traditional ceramic method, which involves mixing and grinding powders of the constituent oxides, carbonates and such compounds and heating them at high temperatureswithintermediategrindingwhennecessary.Thelowtemperaturechemical routes, however, are of greater interest so as to have better control of the structure, stoichiometryandphasepurity.Noteworthychemicalmethodsofsynthesisincludethe precursor method, coprecipitation, combustion method, the gel method, topochemical methodsandhighpressuremethods.Someofthemethodsareoutlinedbelow. 1. Ceramic Method Themostcommonmethodofpreparingsolidmaterialsisbyreactionofthecomponent materials in the solid state at elevated temperatures. Several oxides, sulphides, phosphides, have been prepared by this method. Knowledge of the phase diagram is generally helpful in fixing the desired composition and conditions for synthesis. Platinum, silica and alumina containers are generally used for the synthesis of metal oxides,whilegraphitecontainersareemployedforsulphidesandotherchalcogenidesas wellaspnictides.Ifoneoftheconstituentsisvolatileorsensitivetotheatmosphere,the reactioniscarriedoutinsealedevacuatedcapsules. Most ceramicpreparations require relativelyhightemperatureswhicharegenerallyattainedbyresistanceheating.Electric arcandskulltechniquesgivetemperaturesupto3300KwhilehighpowerCO 2lasers givetemperaturesupto4300K. Theceramicmethodsuffersfromseveraldisadvantages.Theyare: 1.Whennomeltisformedduringthereaction,theentirereactionhastooccurin thesolidstate,initiallybyaphaseboundaryreactionatthepointsofcontactbetweenthe

16.10SomeConceptualDevelopmentsinSynthesisinChemistry componentsandlaterbydiffusionoftheconstituentsthroughtheproductphase.Asthe reaction progresses, diffusion paths become increasingly longer and the reaction rate slower.Theproductinterfacebetweenthereactingparticlesactsasabarrier.Thereaction canbespeededuptosomeextentbyintermittentgrindingbetweenheatingcycles. 2. There is no simple way of monitoring the progress of the reaction in the ceramicmethod.Itisonlybytrialanderror(bycarryingoutXraydiffractionandother measurements periodically) that one decides on appropriate conditions that lead to completion of the reaction. Because of this difficulty, one frequently ends up with mixturesofreactantsandproducts. 3.Separationofthedesiredproductfromthesemixturesisgenerallydifficult,if notimpossible. 4.Itissometimesdifficulttoobtainacompositionallyhomogeneousproductby theceramictechnique,evenwhenthereactionproceedsalmosttocompletion. In spite of such limitations, ceramic techniques have been widely used for the synthesis of solid materials. Mention must be made, among others, of the use of this techniqueforthesynthesisofrareearthmonochalcogenidessuchasSmSandSmSe.The method involves heating the elements, first at lower temperatures (8701170 K) in evacuatedsilicatubes;thecontentsarethenhomogenized,sealedintantalumtubesand heatedtoaround2300K. Variousmodificationsoftheceramictechniquehave been employed to overcome someofthelimitations.Oneoftheserelatestodecreasingthediffusionpathlengths.Ina polycrystallinemixtureofreactants,theindividualparticlesareapproximately10min size, representing diffusion distances of roughly 10,000 unit cells. By using freeze drying,spraydrying,coprecipitation,andsolgelandothertechniques,itispossibleto reduce the particle size to a few hundred ångströms and thus effect a more intimate mixingofthereactants. Inspraydrying,suitableconstituentsdissolvedinasolventaresprayedintheformof finedropletsintoahotchamber.Thesolventevaporatesinstantaneously,leavingbehind an intimate mixture of reactants, which on heating at elevated temperatures gives the product.

SyntheticStrategiesinChemistry 16.11

Infreezedrying,reactantsinacommonsolventarefrozenbyimmersinginliquid nitrogenandthesolventisremovedatlowpressures. Incoprecipitation,therequiredmetalcationstakenassolublesalts(e.g.nitrates)are coprecipitated from a common medium, usually as hydroxides, carbonates, oxalates, formats or citrates. In actual practice, one takes oxides or carbonates of the relevant metals,digeststhemwithanacid(usuallyHNO 3)andthentheprecipitatingreagentis added.Afterfilteringtheprecipitateanddrying,itisheatedtotherequiredtemperaturein a desired atmosphere to produce the final product. For example, tetraethylammonium oxalate has been used to prepare superconducting YBa 2Cu 408. The decomposition temperatures of theprecipitates are generally lower than the temperatures employed in theceramicmethod. 2. Combustion Method Combustion synthesis or selfpropagating hightemperature synthesis makes use of a highlyexothermicreactionbetweenthereactantstoproduceaflameduetospontaneous combustionwhichthenyieldsthedesiredproductoritsprecursorinfinelydividedform. Borides,carbides,oxides,chalcogenidesandothermetalderivativeshavebeenprepared by this method. In order for combustion to occur, one has to ensure that the initial mixtureofreactantsishighlydispersedandhashighchemicalenergy.Forexample,one mayaddafuelandanoxidizerwhenpreparingoxidesbythecombustionmethod,both these additives being removed during combustion to yield only the product or its precursor.Thus,onecantakeamixtureofnitrates(oxidizer)ofthedesiredmetalsalong withafuel(e.g.hydrazine,glycineorurea)insolution,evaporatethesolutiontodryness andheattheresultingsolidtoaround423Ktoobtainspontaneouscombustion,yielding an oxidic product in fine particulate form. Even if the desired product is not formed immediately after combustion, the fine particulate nature of the product facilitates its formationonfurtherheating.Inordertocarryoutcombustionsynthesis,thepowdered mixtureofreactants(0.1100mparticlesize)isgenerallyplacedinanappropriategas mediumwhichfavoursanexothermicreactiononignition.Thecombustiontemperature isanywherebetween1500and3500K,dependingonthereaction.Theadvantagesofthis methodarethat

16.12SomeConceptualDevelopmentsinSynthesisinChemistry

• reaction times are very short since the desired product results soon after the combustion. • agasmediumisnotalwaysnecessary. Thisissointhesynthesisofborides,silicidesandcarbideswheretheelementsarequite stableathightemperatures.Combustioninanitrogenatmosphereyieldsnitrides.Nitrides ofvariousmetalshavebeenpreparedinthismanner.Azideshavebeenusedassourcesof nitrogen.Thefollowingaresometypicalcombustionreactions:

MoSi2+7MgO MoO3+2SiO2+7Mg

WO3+C+2Al WC+Al2O3

TiO2+B2O3+5Mg TiB2+5MgO

N N2 2 Ta Ta2N TaN afterburning Mostoftheternaryorquaternaryoxidescanalsobepreparedbythismethod.Allthe superconducting cuprates have been prepared by this method, although the resulting productsinfineparticulateformhavetobeheatedtoanappropriatehightemperatureina desiredatmospheretoobtainthefinalcuprate.Table16.1liststypicalmaterialsprepared bythecombustionmethod.

SyntheticStrategiesinChemistry 16.13

Table 1. Typical materials prepared by the Combustion method

OxidesBaTiO3,LiNbO3,PbMoO4

CarbidesTiC,Mo2C,NbC

BoridesTiB2,CrB2,MoB2,FeB

SilicidesMoSi2,TiSi2,ZrSi2

PhosphidesNbP,MnP,TiP

ChalcogenidesWS2,MoS2,MoSe2,TaS2

HydridesTiH2,NdH2 3. Precursor Method Synthesis of complex oxides by the decomposition of compound precursors has been known for some time. For example, thermal decomposition of LaCo(CN) 6.5H 20 and

LaFe(CN) 6.6H 20inairreadily yieldsLaCoO 3andLaFeO 3respectively.BaTiO 3canbe preparedbythethermaldecompositionofBa[TiO(C 204)2],whileLiCrO 2canbeprepared fromLi[Cr(C 2O4)2(H 20) 2].FerritespinelsofthegeneralformulaMFe 204 (MMg,Mn,Ni, Co) are prepared by the thermal decomposition of acetate precursors of the type

M3Fe 6(CH 3COO) 17 03OH.12C 5H5N. Chromites of type MCr 204 are obtained by the decomposition of (NH 4)2M(CrO 4)2.6H 20. Carbonates of metals such as calcium, magnesium,manganese,iron,cobalt,zincandcadmiumareallisostructural,possessing thecalcitestructure.Wecanthereforepreparealargenumberofcarbonatesolidsolutions containingtwoormorecationsindifferentproportions. Carbonatesolidsolutionsareidealprecursorsfor the synthesis of monoxide solid solutions of rocksalt structure. The facile formation of rocksalt oxides by the decompositionofcarbonatesofcalcitestructureisduetotheclose(possibletopotactic) relationshipbetweenthestructuresofcalciteandrocksalt.Themonoxidesolidsolutions can be used as precursors for preparing spinels and other complex oxides. Besides monoxidesolidsolutions,anumberofternaryandquaternaryoxidesofnovelstructures

16.14SomeConceptualDevelopmentsinSynthesisinChemistry canbepreparedbydecomposingcarbonateprecursorscontainingthedifferentcationsin therequiredproportions.Anumberofternaryandquaternarymetaloxidesofperovskite andrelatedstructurescanbepreparedbyemployinghydroxide,nitrateandcyanidesolid solutionprecursorsaswell. 4. Topochemical Reactions A solid state reaction is said to be topochemically controlled when the reactivity is controlledbythecrystalstructureratherthanbythechemicalnatureoftheconstituents. The products obtained in many solid state decompositions are determined by topochemical factors, especially when the reaction occurs within the solid without the separationofanewphase.Intopotacticsolidstatereactions,theatomicarrangementin thereactantcrystalremainslargelyunaffectedduringthecourseofthereaction,except for changes in dimension in one or more directions. Dehydration of WO 3.H 20 or

MoO 3.H 20 to give WO 3 or MoO 3 is one such reaction. Dehydration of many other hydrates such as VOPO 4.2H 20 and HMoO 2PO 4.H 20 is also found to be topotactic.

Intercalationreactionsaregenerallytopotacticinnature.DecompositionofV 2O5 toform

V6O13 is a similar reaction. The reduction of NiO to nickel metal proceeds topochemically.

a)DehydrationofMo 1_x WxO3.H 2O

Mo 1_x WxO3solidsolutionscanbepreparedbytheceramicmethod(byheatingMoO 3and

WO 3 in sealed tubes at around 870 K) or by the thermal decomposition of mixed ammonium metallates. These methods, however, do not always yield monophasic products owing to the difference in volatilities of MoO 3 and WO 3. Therefore, it was soughttoprepareMo 1_x WxO3bytopochemicaldehydrationofthehydrates,theprocess being very gentle. MoO 3.H 2O and WO 3.H 2O are isostructural and the solid solutions betweenthetwohydratesarepreparedreadilybyaddingasolutionofMoO 3andWO 3in ammonia to hot 6M HNO 3. The hydrates Mo 1_x WxO3.H 2O crystallize in the same structure as MoO 3.H 2O and WO 3.H 2O with a monoclinic unit cell. The hydrate solid solutions undergo dehydration under mild conditions (around 500 K) yielding

Mo 1_x WxO3whichcrystallizesintheReO 3relatedstructureofWO 3.TheReO 3structure of MoO 3 is metastable and is produced only by topotactic dehydration under mild conditions. This preparation of ReO 31ike MoO 3 by mild chemical processing is

SyntheticStrategiesinChemistry 16.15

significant.BulkquantitiesofMoO 3intheReO 3 structurehavebeenpreparedbymild dehydrationofthehydrate. b)Reductionofperovskiteoxides

ReductionofthehightemperaturesuperconductorYBa 2Cu 307toYBa 2CuO 6 (Fig.16.7)is atopochemicalprocess.

5. Intercalation Compounds Intercalationreactionsofsolidsinvolvetheinsertionofaguestspecies(ionormolecule) intoasolidhostlatticewithoutanymajorrearrangementofthesolidstructure: x(guest)+ x[host] (guest)x[host]

where standsforavacantlatticesite.

Tungstenandmolybdenumbronzes,A xWO 3andA xMoO 3 (A=K,Rb,Cs)aregenerally preparedbyreactionofthealkalimetalswiththehostoxide.Electrochemicalmethods are alsoemployedforthesepreparations.Anovelreaction that has been employed to preparebronzeswhichareotherwisedifficulttoobtaininvolvesthereactionofoxidehost withanhydrousalkaliiodides:

16.16SomeConceptualDevelopmentsinSynthesisinChemistry

Mo 1xWxO3+y(AI)→A y.Mo 1xWxO3+y/2I 2 Atomichydrogenhasbeeninsertedintomanybinaryandternaryoxides.Recently,iodine has been intercalated into the superconducting cuprate, Bi 2CaSr 2Cu 2O8, causing an expansionofthecparameteroftheunitcell,withoutdestroyingthesuperconductivity. 6. Sol-gel Method Thesolgelmethodisawetchemicalmethodandamultistep processinvolving both chemical and physical processes such as hydrolysis, polymerization, drying and densification. The name "solgel" is given to the process because of the distinctive increaseinviscositywhichoccursataparticularpointinthesequenceofsteps.Asudden increaseinviscosityisthecommonfeatureinsolgelprocessing,indicatingtheonsetof gel formation. In the solgel process, synthesis of inorganic oxides is achieved from inorganic or organometallic precursors (generally metal alkoxides). The important featuresofthesolgeltechniquesarebetterhomogeneitycomparedwiththetraditional ceramic method, high purity, lower processing temperature, more uniform phase distribution in multicomponent systems, better size and morphological control, the possibility of preparing new crystalline and noncrystalline materials, and lastly easy preparation of thin films and coatings. The solgel method is widely used in ceramic technology.Theimportantstepsinsolgelsynthesisare: Hydrolysis. Theprocessofhydrolysismaystartwithamixtureofametalalkoxideand waterinasolvent (usuallyalcohol)attheambient or a slightly elevated temperature. Acidorbasecatalystsareaddedtospeedupthereaction.

Polymerization. ThisstepinvolvescondensationofadjacentmoleculeswhereinH 2Oand ROHareeliminatedandmetaloxidelinkagesareformed.Polymericnetworksgrowto colloidaldimensionsintheliquid(sol)state. Gelation. In this step, the polymeric networks link up to form a threedimensional networkthroughouttheliquid.Thesystembecomessomewhatrigid,characteristicofa gel. The solvent as well as water and alcohol remain inside the pores of the gel. Aggregationofsmallerpolymericunitstothemainnetworkcontinuesprogressivelyon agingthegel.

SyntheticStrategiesinChemistry 16.17

Drying. Here,waterandalcoholareremovedatamoderatetemperature(lessthan470 K),leavingahydroxylatedmetaloxidewithresidualorganiccontent.Iftheobjectiveis toprepareahighsurfaceareaofaerogelpowderwithlowbulkdensity,thesolventis removedsupercritically. Dehydration. Thisstepiscarriedoutbetween670and1070Ktodriveofftheorganic residues and chemically bound water, yielding a metal oxide with up to 20%30% microporosity. Densification. Temperatures in excess of 1270 K are used to form the dense oxide product. Many complex metal oxides are prepared by a modified solgel route without actually preparingmetalalkoxides.Forexample,atransitionmetalsaltsolutionisconvertedinto a gel by the addition of an appropriate organic reagent. In the case of cuprate superconductors,anequimolarproportionofcitricacidisaddedtothesolutionofmetal nitrates,followedbyethylenediamineuntilthesolutionattainsapHof66.5.Theblue solisconcentratedtoobtainthegel.Thexerogelisobtainedbyheatingatapproximately 420K.Thexerogelisdecomposedatanappropriatetemperaturetoobtainthecuprate. Thesolgeltechniquehasbeenusedtopreparesubmicrometremetaloxidepowders with a narrow particle size distribution and unique particle shapes (e.g. A1 2O3, TiO 2,

ZrO 2, Fe 2O3). Uniform SiO 2 spheres have been grown from aqueous solutions of colloidalSiO 2.Smallmetalclusters(e.g.nickel,copper,gold)havebeenpreparedbyin situchemicalreductionofmetalsalts.Metalceramiccomposites(e.g.NiAI 2O3,PtZrO 2) canalsobepreparedinthismanner.Byemployingseveralvariantsofthebasicsolgel technique,anumberofmulticomponentoxidesystems havebeenprepared. Typical of these are: SiO 2B2O3, SiO 2TiO 2, SiO 2ZrO 2, SiO 2A1 2O3, ThO 2UO 2 . A variety of ternaryandstillmorecomplexoxideshavebeenpreparedby this technique. Different typesofcupratesuperconductorshavealsobeenpreparedbythismethod.Theseinclude

YBa 2Cu 307,YBa 2Cu 408,Bi 2CaSr 2Cu 208andPb 2Sr 2Ca 1xYxCu 3O8. 6. Alkali Flux Method Theuseofstrongalkalinemedia,eitherintheformofsolidfluxesormolten(oraqueous) solutions,hasenabledthesynthesisofnoveloxides. The alkali flux method stabilizes higher oxidation states of the metal by providing an oxidizing atmosphere. Alkali

16.18SomeConceptualDevelopmentsinSynthesisinChemistry carbonatefluxeshavetraditionallybeenusedtopreparetransitionmetaloxidessuchas

LaNiO 3.Agoodexampleofanoxidesynthesizedinastronglyalkalinemediumisthe pyrochlore,Pb 2(Ru 2xPb x)O 7ywherePbisinthe4+state;thisoxideisabifunctional electrocatalyst. The procedure for preparation involves bubbling oxygen through a solution of lead and rubidium salts in strong KOH at 320 K. The socalled alkaline hypochloritemethodisusedinmanyinstances.Forexample,La 4NiO l0 waspreparedby bubblingCl 2gasthroughaNaOHsolutionoflanthanumandnickelnitrates.

YBa 2Cu 408hasbeenpreparedbyusingaNa 2CO 3K2CO 3fluxinaflowingoxygen atmosphere.KOHmelthasbeenusedtopreparesuperconductingBa 1xKxBiO 3. 7. Electrochemical Method Electrochemical methods have been employed to advantage for the synthesis of many solid materials. Typical of the materials prepared in this manner are metal borides, carbides,silicides,oxidesandsulphides.VanadatespinelsofformulaMV 2O4aswellas tungstenbronzeshavebeenpreparedbytheelectrochemicalroute.Tungstenbronzesare obtainedatthecathodewhencurrentispassedthroughtwoinertelectrodesimmersedina moltensolutionofthealkalimetaltungstate,A 2WO 4andWO 3;oxygenisliberatedatthe anode.Bluemolybdenumbronzeshavebeenpreparedbyfusedsaltelectrolysis.Mono sulphidesofuranium,gadolinium,thoriumandothermetalsareobtainedfromasolution of the normal valent metal sulphide and chloride in an NaC1KCI eutectic. LaB 6, is prepared by taking La 2O3 and B 2O3 in an LiBO 2LiF melt and using gold electrodes. Crystallinetransitionmetalphosphidesarepreparedfromsolutionsofoxideswithalkali metal phosphates and halides. Superconducting Ba lxKxBiO 3 has been prepared electrochemically. Althoughtheelectrochemicalmethodisold,theprocessesinvolvedinthesynthesis of various solids are not entirely understood. Generally one uses solvents whose decompositionpotentialsarehigh(e.g.alkalimetalphosphates,borates,fluorides,etc.). Changesinmeltcompositioncouldcauselimitationsincertaininstances. 8. High Pressure Methods Theuseofhighpressuresforsolidstatesynthesishasbecomeincreasinglycommonin recentyears.Withthedevelopmentofhighpressuretechnology,commercialequipment permittingsimultaneoususeofbothhighpressureandhightemperatureconditionshas

SyntheticStrategiesinChemistry 16.19 become available. For the 110 kbar pressure range,thehydrothermalmethodisoften employed.Inthismethod,thereactioniscarriedouteitherinanopenoraclosedsystem.

Intheopensystem,thesolidisindirectcontact with the reacting gas (F 2, O 2 or N 2) whichalsoservesasapressureintensifier.Agoldcontainerisgenerallyusedinthistype ofsynthesis.Thismethodhasbeenusedforthesynthesisoftransitionmetalcompounds suchasRhO 2,PtO 2andNa 2NiF 6wherethetransitionmetalisinahigheroxidationstate. Hydrothermalhighpressuresynthesisunderclosedsystemconditionshasbeenemployed forthepreparationofhighervalencemetaloxides.AninternaloxidantsuchasKClO 3is addedtothereactants,whichondecompositionunderreactionconditionsprovidesthe necessaryoxygenpressure.Forexample,pyrochloresofpalladium(IV)andplatinum(IV),

Ln 2M207, (Ln = rare earth) have been prepared by this method (970 K, 3 kbar).

(H 30)Zr 2(PO 4)3 and a family of zero thermal expansion ceramics (e.g. Ca 0.5 Ti 2P3O12 ) have also been prepared hydrothermally. Another good example is the synthesis of boratesofaluminium,yttriumandsuchmetalswhereinthesesquioxidesarereactedwith boricacid.OxyfluorideshavebeenpreparedinHFmedium[75].Zeolitesaregenerally preparedunderhydrothermalconditionsinthepresenceofalkali.Thealkali,thesilica componentandthesourceofaluminiumaremixedinappropriateproportionsandheated. The reactant mixture forms a hydrous gel which is then allowed to crystallize under pressure for several hours to several weeks between 330 and 470 K. In a typical synthesis,AI 2O3.3H 20dissolvedinconcentratedNaOHsolution(20N)ismixedwitha

1 N solution of Na 2SiO 3.9H 20 to obtain a gel (of composition . . . 2.1Na 2O Al 2O3 2.1SiO 2 60H 2O)whichisthencrystallizedtogivezeoliteA.TheNa 2O

SiO 2Al 2O3H2Osystemyieldsalargenumberofmaterialswiththezeoliticframework.

Underalkalineconditions,aluminiumispresentasAl(OH) 4anions.TheOHionsactas a mineralizing catalyst while the cations present in the reactant mixture determine the kindsofzeoliteformed. Besideswater,someinorganicsaltsarealsoencapsulatedinsomezeolites.Several zeolitestructuresarefoundintheK 2OSiO 2Al 2O3H2Osystemaswell.Li 2O,however, does not give rise to many microporous materials. Group IIA cations yield several zeoliticproducts.

16.20SomeConceptualDevelopmentsinSynthesisinChemistry

Zeolitization in the presence of organic bases is useful for synthesizing silicarich zeolites.Silicalitewithatetrahedralframeworkenclosingathreedimensionalsystemof channels(definedby10ringswideenoughtoabsorbmoleculesupto0.6nmindiameter) hasbeensynthesizedbythereactionoftetrapropylammonium(TPA)hydroxideanda reactive form of silica between 370 and 470 K. The precursor crystals have the . composition(TPA) 20.48SiO 2 H20andtheorganiccationisremovedbychemicalreaction orthermaldecompositiontoyieldmicroporoussilicalitewhichmaybeconsideredtobea newpolymorphofSiO 2.Theclathrasil(silicaanalogueofagashydrate),dodecasil1H,is prepared from an aqueous solution of tetramethoxysilane and N(CH 3)4OH; after the additionofaminoadamantane,thesolutionistreatedhydrothermallyundernitrogenfor fourdaysat470K.Theuseoftemplatecationshasenabledthesynthesisofavarietyof zeolitematerials.Cationssuchas(NMe 4)+fitsnuglyintothecages(e.g.sodalitecagesof sodaliteandSAPOorgmelinitecagesofzeolitesomega).Neutralorganicamineshave also been used (e.g. in the synthesis of ZSM5). Many new microporous materials, includingthosebasedonAlPO 4 (analogueofSiO 2),gallosilicatesandaluminogerminates

(analoguesofaluminosilicates),havebeenprepared.AlPO 4basedmaterialsareprepared bythecrystallizationofgelsformedbyaddinganorganictemplatetoamixtureofactive alumina,H 3PO 4andwateratapHof58around470K.Pressuresintherange10150 kbar are commonly used for solidstate synthesis. In the pistoncylinder apparatus consistingofatungstencarbidechamberandapistonassembly,thesampleiscontained inasuitablemetalcapsulesurroundedbyapressuretransducer(pyrophyllite).Pressureis generatedbymovingthepistonthroughtheblindholeinthecylinder.Amicrofurnace madeofgraphiteormolybdenumisincorporatedinthedesign.Pressuresupto50kbar andtemperaturesupto1800Karereadilyreachedin a volume of 0.1 cm 3 using this design. In the anvil apparatus, first designed by Bridgman, the sample is subjected to pressure by simply squeezing it between two opposed anvils. Although pressures of around200kbarandtemperaturesupto1300Karereachedinthistechnique,itisnot popular for solidstate synthesis since only milligram quantities can be handled. An extension of the opposed anvil principle is the tetrahedral anvil design, where four massively supported anvils disposed tetrahedrally ram towards the centre where the sample is located in a pyrophyllite medium together with a heating arrangement. The

SyntheticStrategiesinChemistry 16.21 multianvildesignhasbeenextendedtocubicgeometry,wheresixanvilsactonthefaces ofapyrophyllitecubelocatedatthecentre.Thebeltapparatusprovidesthebesthigh pressurehightemperature combination for solidstate synthesis. This apparatus, which wasusedforthesynthesisofdiamondssomeyearsagoisacombinationofthepiston cylinder and the opposed anvil designs. The apparatus consists of two conical pistons madeoftungstencarbide,whichramthroughaspeciallyshapedchamberfromopposite directions.Thechamberandpistonsarelaterallysupportedbyseveralsteelringsmaking it possible routinely to reach fairly high pressures (around 150 kbar) and high temperatures(approximately2300K).Inthebeltapparatus,thesampleiscontainedina noblemetalcapsule(aBNorMgOcontainerisusedforchalcogenides)andsurrounded bypyrophylliteandagraphitesleeve,thelatterservingasaninternalheater.Inatypical highpressurerun,thesampleisloaded,thepressureraisedtothedesiredvalueandthen the temperature increased. After holding the pressure for about 30 min, the sample is quenched(400Ks 1)whilestillunderpressure.Thepressureisreleasedafterthesample hascooledatroomtemperature. Highpressure methods have been used for the synthesis of several materials that cannotpossiblybemadeotherwise.Ingeneral,theformationofanewcompoundfromits componentsrequiresthatthenewcompositionhavealowerfreeenergythanthesumof thefreeenergiesofthecomponents.Pressurecanaidintheloweringoffreeenergyin differentways. (a)Pressuredelocalizesouterdelectronsintransitionmetalcompoundsbyincreasingthe magnitudeofcouplingbetweenthedelectronsonneighbouringcations,therebylowering thefreeenergy.AtypicalexampleisthesynthesisofACrO 3(A=Ca,Sr,Pb)perovskites andCrO 2. (b) Pressure stabilizes highervalence states of transition metals, thus promoting the formationofanewphase.Forexample,intheCaFeOsystemonlyCaFeO 2.5 (brown millerite) is stable under ambient pressures. Under high oxygen pressures, iron is + oxidizedtothe4 stateandhenceCaFeO 3withtheperovskitestructureisformed. (c) Pressure can suppress the ferroelectric displacement of cations, thereby adding the synthesis of new phases. The synthesis of A~MoO3 bronzes, for example, requires

16.22SomeConceptualDevelopmentsinSynthesisinChemistry

populatingtheemptydorbitalscentredonmolybdenum;atambientpressures,MoO 3is stabilizedbyaferroelectricdistortionofMoO 6octahedrauptothemeltingpoint. (d)Pressurealterssitepreferenceenergiesofcations,andfacilitatestheformationofnew 2+ 4+ phases. For example, it is not possible to synthesize A Mn O3 (A = Mg, Co, Zn) ilmenitesbecauseofthestrongtetrahedralsitepreferenceofthedivalentcations.One therefore obtains a mixture of A[AMn]O 4(spinel) + MnO 2(rutile) under atmospheric pressureinsteadofmonophasicAMnO 3.However,thelatterisformedathighpressures with a corundumtype structure in which both the A and Mn ions are in octahedral coordination. (e)Pressurecansuppressthe6s 2corepolarizationinoxidescontainingisoelectronic + 2+ 3+ Tl , Pb , Bi cations. For example, perovskitetype PbSnO 3 cannot be made at atmospheric pressure because the mixture of PbO + SnO 2 is more stable than the perovskite. Thus, solidstate reactions are generally slow under ordinary pressures even when the productisthermodynamicallystable.Pressurehasamarkedeffectonthekineticsofthe reaction, reducing the reaction times considerably, and at the same time giving more homogeneousandcrystallineproducts.Forinstance,LnFeO 3,LnRhO 3andLnNiO 3(Ln= rare earth) are prepared in a matter of hours under highpressurehightemperatue conditions,whereasatambientpressurethereactionsrequireseveraldays(LnFeO 3and

LnRhO 3)ortheydonotoccuratall(LnNiO 3).ThusLnFeO3isformedin30minat50 kbar.

Inseveral(AX)(ABX 3)nseriesofcompounds,theendmembersABX 3andA 2BX 4, having the perovskite and K 2NiF 4 structures respectively, are formed at atmospheric pressures, but not the intermediate phases such as A 3B2X7 and A 4B3X10 . Pressure facilitatesthesynthesisofsuchsolids.Sr 3Ru 2O7andSr 4Ru 3O10 areformedin15minat 20kbarand1300K. Highpressure methods have been employed in the synthesis of novel superconducting cuprates. A rudimentary example is the preparation of oxygenexcess

La 2CuO 4underhighoxygenpressure.Amoreinterestingexampleisthesynthesisofthe nexthomologuewithtwoCuO 2layers.La 2Ca lxSr xCu 2O6whichhadearlierbeenfound to be an insulator was rendered superconducting by heating it under oxygen pressure.

SyntheticStrategiesinChemistry 16.23

YBa 2Cu 408 was first prepared under high oxygen pressure, but this was soon found unnecessary. However, superconducting cuprates with infinite CuO 2 layers of the type

Ca 1xSr xCuO 2 or Sr 1xNd xCuO 2 can only be prepared under high hydrostatic pressures which help to give materials with shorter CuO bonds. It should be noted that Ca 1 xSr xCuO 2preparedatambientpressureisinsulating. 9. The pyrosol Process: a novel chemical method for depositing films Pyrolysisofspraysisawellknownmethodfordepositingfilms.Thus,onecanobtain films of oxidic materials such as CoO, ZnO and YBa 2Cu 307 by the spray pyrolysis of solutions containing salts (e.g. nitrates) of the cations. A novel improvement in this techniqueisthepyrosolprocessinvolvingthetransport and subsequent pyrolysis of a spray generated by an ultrasonic atomizer. When a high frequency (100 kHz10 MHz range) ultrasonicbeam is directed at a gasliquidinterface, a geyser is formed and the heightofthegeyserisproportionaltotheacousticintensity.Itsformationisaccompanied by the generation of a spray, resulting from the vibrations at the liquid surface and cavitationattheliquidgasinterface.Thequantityofsprayisafunctionoftheintensity. Ultrasonic atomization is accomplished using an appropriate transducer made of PZT locatedatthebottomoftheliquidcontainer.A5001000 kHz transducer is generally adequate.Theatomizedspraywhichgoesupinacolumnfixedtotheliquidcontaineris depositedontoasuitablesolidsubstrateandthenheattreatedtoobtainthefilmofthe materialconcerned.Theflowrateofthesprayiscontrolledbytheflowrateofairorany othergas.Theliquidisheatedtosomeextent,butitsvaporizationshouldbeavoided.The sourceliquidcontainstherelevantcationsinthe formofsaltsdissolvedinanorganic solvent. Organometallic compounds are often used (e.g. acetates, alkoxides, acetylacetonates,etc.).Propergasflowiscrucialtoobtainsatisfactoryconditionsfora goodliquidspray. Thepyrosolprocessisinsomewaybetweenchemicalvapourdepositionandspray pyrolysis,butthechoiceofsourcecompoundsforthepyrosolprocessislargerthanthat available for chemical vapour deposition. Films of a variety of materials have been obtainedbythepyrosolmethod.Thethicknessofthefilmscanbeanywherebetweena fewhundredangstromstoafewmicrometres.Filmsofsuperconductingcupratessuchas

16.24SomeConceptualDevelopmentsinSynthesisinChemistry

YBa 2Cu 3O7havealsobeenpreparedbythepyrosolprocess.Epitaxyhasbeenobserved infilmsdepositedontosinglecrystalsubstrates. 10. Czochralski Process for Semiconductor Materials Semiconductors with predictable, reliable electronic properties are necessary for mass production.Thelevelofchemicalpurityneededisextremelyhighbecausethepresence ofimpuritieseveninverysmallproportionscanhavelargeeffectsonthepropertiesof the material. A high degree of crystalline perfection is also required, since faults in crystal structure (such as dislocations, twins, and stacking faults) interfere with the semiconducting properties of the material. Crystalline faults are a major cause of defectivesemiconductordevices.Thelargerthecrystal,themoredifficultitistoachieve thenecessaryperfection.Currentmassproductionprocessesusecrystalingotsbetween fourandtwelveinches(300mm)indiameterwhichare grownascylindersandsliced intowafers. Because of the required level of chemical purity and the perfection of the crystal structurewhichareneededtomakesemiconductordevices,specialmethodshavebeen developedtoproducetheinitialsemiconductormaterial.Atechniqueforachievinghigh purityincludesgrowingthecrystalusingtheCzochralskiprocess.Anadditionalstepthat canbeusedtofurtherincreasepurityisknownaszonerefining.Inzonerefining,partof asolidcrystalismelted.Theimpuritiestendtoconcentrateinthemeltedregion,while the desired material recrystalizes leaving the solid material more pure and with fewer crystalline faults. In manufacturing semiconductor devices involving heterojunctions betweendifferentsemiconductormaterials,thelatticeconstant,whichisthelengthofthe repeatingelementofthecrystalstructure,isimportantfordeterminingthecompatibility ofmaterials. TheCzochralskiprocessisamethodofcrystalgrowthusedtoobtainsinglecrystals ofsemiconductors(e.g.silicon,germaniumandgalliumarsenide),metals(e.g.palladium, platinum,silver,gold),saltsandsomemanmade(orlab)gemstones.Themostimportant application may be the growth of large cylindrical ingots, or boules, of single crystal silicon. Other semiconductors, such as gallium arsenide, can also be grown by this method,althoughlowerdefectdensitiesinthiscasecanbeobtainedusingvariantsofthe Bridgemantechnique.

SyntheticStrategiesinChemistry 16.25

Highpurity,semiconductorgradesilicon(onlyafewpartspermillionofimpurities)is melteddowninacrucible,whichisusuallymadeofquartz.Dopantimpurityatomssuch asboronorphosphoruscanbeaddedtothemoltenintrinsicsiliconinpreciseamountsin ordertodopethesilicon,thuschangingitintontypeorptype extrinsicsilicon.This influencestheelectricalconductivityofthesilicon.Aseedcrystal,mountedonarod,is dippedintothemoltensilicon.Theseedcrystal'srodispulledupwardsandrotatedatthe sametime.Bypreciselycontrollingthetemperaturegradients,rateofpullingandspeed ofrotation,itispossibletoextractalarge,singlecrystal,cylindricalingotfromthemelt. Occurrence of unwanted instabilities in the melt can be avoided by investigating and visualizing the temperature and velocity fields during the crystal growth process [1]. Thisprocessisnormallyperformedinaninertatmosphere,suchasargon,andinaninert chamber,suchasquartz. Size of Crystals :Whilethelargestsiliconingotsproducedtodayare400mmindiameter and1to2metresinlength,200mmand300mmdiametercrystalsarestandardindustrial processes.Thinsiliconwafersarecutfromtheseingots(typicallyabout0.20.75mm thick) and can be polished to a very high flatness for making integrated circuits, or texturedformakingsolarcells. Impurity Incorporation :WhensiliconisgrownbytheCzochralskimethodthemeltis containedinasilica(quartz)crucible.Duringgrowththewallsofthecrucibledissolve intothemeltandCzochralskisiliconthereforecontainsoxygenimpuritieswithatypical concentrationof10 18 cm −3 .Perhapssurprisingly,oxygenimpuritiescanhavebeneficial effects. Carefully chosen annealing conditions can allow the formation of oxygen precipitates.Thesehavetheeffectoftrappingunwantedtransitionmetalimpuritiesina processknownasgettering.Additionally,oxygenimpuritiescanimprovethemechanical strength of silicon wafers by immobilising any dislocations which may be introduced duringdeviceprocessing.Ithasexperimentallybeenprovedinthe1990sthatthehigh oxygenconcentrationisalsobeneficialforradiationhardnessofsiliconparticledetectors used in harsh radiation environment ( eg. CERN's LHC/SLHC projects) [2, 3, 4]. Therefore,radiationdetectorsmadeofCzochralskiandMagneticCzochralskisiliconare considered to be promising candidates for many future highenergy physics experiments. [5][6] However, oxygen impurities can react with boron in an illuminated

16.26SomeConceptualDevelopmentsinSynthesisinChemistry environment, such as experienced by solar cells. This results in the formation of an electrically active boron–oxygen complex that detracts fromcellperformance.Module outputdropsbyapproximately3%duringthefirstfewhoursoflightexposure[7]. The Bridgman technique isamethodofgrowingsinglecrystalingotsorboules. The methodinvolvesheatingpolycrystallinematerialinacontaineraboveitsmeltingpoint andslowlycoolingitfromoneendwhereaseedcrystalislocated.Singlecrystalmaterial isprogressivelyformedalongthelengthofthecontainer.Theprocesscanbecarriedout in a horizontal or vertical geometry. It is a popular method of producing certain semiconductorcrystals,suchasgalliumarsenidewheretheCzochralskiprocessismore difficult. Purification process: Zone melting isamethodofseparationbymeltinginwhicha moltenzonetraversesalongingotofimpuremetalorchemical. Zone refining: Inzonerefining,solutesaresegregatedatoneendoftheingotinorderto purifytheremainder.Themoltenregionmeltsimpuresolidatitsforwardedgeandleaves awakeofpurermaterialsolidifiedbehinditasitmovesthroughtheingot.Theimpurities concentrate in the melt, and are moved to one end of the ingot. Zone refining was developedinBellTelephoneLaboratoriesasamethodtopreparehighpuritymaterials formanufacturingtransistors.Itsearlyusewasongermaniumforthispurpose,butitcan beextendedtovirtuallyanysolutesolventsystemhavinganappreciableconcentration differencebetweensolidandliquidphasesatequilibrium.Thisprocessisalsoknownas the Float zone process ,particularlyinsemiconductormaterialsprocessing. Zone leveling : Zonemeltingisalsousedtoconcentratetheimpuritiesforanalyticalor otherpurposes.Inzoneleveling,theobjectiveistodistributesoluteevenlythroughout thepurifiedmaterial,whichmaybesoughtintheformofasinglecrystal.Forexample, inthepreparationofatransistorordiodesemiconductor,aningotofgermaniumisfirst purifiedbyzonerefining.Thenasmallamountofantimonyisplacedinthemoltenzone, whichispassedthroughthepuregermanium.Withtheproperchoiceofrateofheating and other variables, the antimony can be spread evenly through the germanium. This techniqueisalsousedforthepreparationofsiliconforuseincomputerchips. Heaters: Avarietyofheaters canbeused forzonemelting,withtheirmostimportant characteristic being the ability to form short molten zones that move slowly and

SyntheticStrategiesinChemistry 16.27 uniformlythroughtheingot.Inductioncoils,ringwoundresistanceheaters,orgasflames arecommonmethods.Anothermethodistopassanelectriccurrentdirectlythroughthe ingotwhileitisinamagneticfield,withtheresultingmagnetomotiveforcecarefullyset tobejustequaltotheweightinordertoholdtheliquidsuspended.Zonemeltingcanbe doneasabatchprocess,oritcanbedonecontinuously,withfreshimpurematerialbeing continually added at one end and purer material being removed from the other, with impurezonemeltbeingremovedatwhateverrateisdictatedbytheimpurityofthefeed stock. ZoneRemelting:Anotherrelatedprocessiszoneremelting,inwhichtwosolutesare distributedthroughapuremetal.Thisisimportantinthemanufactureofsemiconductors, wheretwosolutesofoppositeconductivitytypeareused.Forexample,ingermanium, pentavalentelementsofgroupVsuchasantimonyandarsenicproducenegative(ntype) conductionandthetrivalentelementsofgroupIIIsuchasaluminiumandboronproduce positive(ptype)conduction.Bymeltingaportionofsuchaningotandslowlyrefreezing it, solutes in the molten region become distributed to form the desired np and pn junctions. Biocatalysis Pharmaceuticalindustryhasbeenarapidlygrowingfield.About54%ofdrugmolecules are chiral, and resolution remains an important and costeffective approach to chiral molecules.Inordertoachieveatypicalenantiomericpurityof99.5%,resolutionisoften accomplishedatthepriceofrestrictingtheoverallmaximumyieldofaprocessto50%at most.Whilemetalcatalyzedreactionshavebecomeprevalentinrecentyears,theirusein manufacturingrequiresgreatefforttoidentifyappropriatecatalysts,solventsandreaction conditions.Reproducibilityandrobustnesscanremainaproblem,especiallywithregard tosensitivitytosubstratequalityandsmallvariationinreactionparameters.Thereisa clear need for new methodologies to produce chiral molecules. Enzymes are highly efficientwithexcellentregioselectivityandstereoselectivity.Byconductingreactionsin water under ambient reaction conditions, both the use of organic solvents and energy inputareminimized.Furthermore,onecandesignprocesseswithhighenergyefficiency and safe chemistry by conducting reactions at ambient temperature under ambient

16.28SomeConceptualDevelopmentsinSynthesisinChemistry atmosphere,andatomeconomycanbeincreasedbyavoiding extensiveprotectionand deprotectionsequences. Biocatalysisisemergingasoneofthegreenesttechnologiesasaresultofrecentadvances in genomics, proteomics and pathway engineering (large scale DNA sequencing, structural biology, protein expression, high throughput screening, directed enzyme evolution and metabolic engineering). Application of the twelve principles of green chemistry can deliver higher efficiency and reduce the environmental burden during chemicalsynthesis. Asthegreenchemistrymovementgainsmomentum,freshopportunitiesofresearch anddevelopmenthavebeenopeneduptoimprovetheefficiencyofchemicalprocesses whilesimultaneouslyreducingproductioncosts.Theattentionisfocusedonreducingthe use and generation of large quantities of hazardous substances which frequently accompaniesthesynthesisofapharmaceuticalagent,ateachofthenumeroussteps,each of which involve feedstocks, reagents, solvents, and separation. The following case studiesdepictthedevelopmentsandimprovementsinthesyntheticstrategiesofvarious chemicals. CASE STUDY 1: Production of LY300164 (Talampanol) : An example of the reduction of hazardous materials is the use of a chemoenzymatic synthesis for the production of LY300164 (Talampanol) for treating epilepsy and neurodegenerative diseases. The first generation synthesis, which starts from 5allyl1,3benzodioxole 1, suffers from a low yield of 16%, and requires the use of a large amount of organic solventsandchromiumoxide,acancersuspect agent, to oxidize racemic alcohol 2 to ketone 3 (Scheme16.1).

SyntheticStrategiesinChemistry 16.29

Scheme16.1. Inordertoimproveitssynthesis,asecondgenerationroutewasdevelopedinvolvinga biocatalytic ketone reduction of 3 by Zygosaccharomyces rouxii to give (S)2 with a yield of 96% and >99.9% ee (enantiomeric excess) (Scheme 16.2). Acidcatalyzed reaction with 4nitrobenzaldehyde led to 1arylisochroman 4, which was subsequently oxidizedtoketal 5 inthepresenceofoxygen.ThefinalproductLY300164wasobtained afterformationofhydrazone 6,andcyclizationviamesylate 7 togive 8 followedbyPd catalyzednitrogroupreduction.

16.30SomeConceptualDevelopmentsinSynthesisinChemistry

Scheme16.2. Byusingachemoenzymaticapproach,notonlywastheoverallyieldimprovedto51% from16%intheoldroute,thenewsyntheticpathwayeliminatedtheuseoftransition metal oxidants and a large volume of organic solvents by judicious adjustment of oxidation states and integration of a biotransformation. For example, using the biocatalytic process, approximately 340000 L of solvents and 3000 kg of chromium wastewereeliminatedfortheproductionofevery1000kgofLY300164. CASE STUDY 2: Production of Pregabalin

SyntheticStrategiesinChemistry 16.31

Most enzymatic catalysis achieves high regio and stereoselectivity under mild conditions, allowing new and more efficient processes to be designed with significant advantagesoverchemicalapproaches.Forexample,inthefirstgenerationrouteforthe productionofpregabalin,chemicalresolutionoftheracemicγaminoacid 11 ,whichwas preparedfromtheracemiccyanodiester 9 (CNDE)afterhydrolysisanddecarboxylation, tookplaceattheendoftheprocess(Scheme3). Toobtainahighopticalpurity(99.5%) of the final API, (S)mandelic acid resolution was followed by another step of recrystallizationinTHF/H 2Owithacombinedtwostepyieldof25–29%.Asaresult,the overallyieldfortherouteisonly18–21%andover70%ofallprocessmaterialsbefore the resolution step including nickel were ultimately turned into wastes. Since the undesired enantiomer could not be recycled, this modest efficiency leads to a large amountofwastesandexcessivereactorcapacityrequirements. To overcome these issues, a second generation process using a fungal lipase was developed, where enzymatic resolution occurred at the first step and the undesired enantiomer 13 couldbeeasilyrecycled(Scheme4). Regioandstereospecifichydrolysis of 9 ledtothemonoacid 12 with45%conversionand>98%ee.Afterdecarboxylationof 12 andphasesplitting,monoester 14 wastelescopicallysubjectedtobasichydrolysisand hydrogenationtogivethefinalproduct.Simpleoperationasaresultofusingwaterasthe solvent significantly improved process efficiency. Comparing with the first generation synthesis,notonlyarealargeamountof(S)mandelicacidcompletelyeliminated,the biocatalytic process also rendered it unnecessary to use most organic solvents. This standsincontrasttoatypicalprocessfortheproduction of an API, where 80% of all wastesaresolvents. Ifspentsolventsareincineratedinsteadofbeingrecovered,thelife cycle profile and impacts are considerably increased. Moreover, recycling of the undesiredenantiomer 13 doubledboththeyield(40%vs.<21%)andthroughput.Overall, itwasprojectedthatatthepeakofmanufacturing,thebiocatalyticaqueousprocesswould annuallyeliminatetheusageofoverthousandsofmetrictonsofrawmaterialsincluding mandelicacid,CNDEandnickel,andtensmillionsofgallonsofalcoholicsolventsand THFassociatedwiththeclassicresolutionroute.

16.32SomeConceptualDevelopmentsinSynthesisinChemistry

Scheme16.3.

Scheme16.4. CASE STUDY 3: Atorvastatin Calcium AtorvastatincalciumistheactiveingredientofLipitorR,thefirstdrugwithannualsales exceeding$10B.Inthecurrentprocess,thekeychiralbuildingblockinthesynthesisof atorvastatin is ethyl (R)4cyano3hydroxybutyrate ( 15 ) with an annual demand estimatedtobeabout£440000, whichwasthenconvertedtoatorvastatinsidechain 18 uponClaisencondensation,boranechelationcontrolledreduction,bothundercryogenic

SyntheticStrategiesinChemistry 16.33 conditions,followedby protectionofthetwohydroxy groups and Nicatalyzed nitrile group hydrogenation. The final API atorvastatin was produced after Paal–Knorr condensation of 18 with a diketone (Scheme 5). A number of biocatalytic approaches havebeenreportedforthesynthesisof(R)4cyano3hydroxybutyrate 15 .Forexample, a green process has recently been reported using a twoenzyme system under neutral conditions in water (eq. 1, Scheme 16.6). In this process, the first step involves enantioselectiveenzymaticreductionofethyl4chloroacetoacetatetogive 19 followedby abiocatalyticcyanationofthechlorohydrintoproduce 15 . Alternatively, 15 was synthesized from inexpensive racemic epichlorohydrin via nitrilasecatalyzeddesymmetrizationofmeso3hydroxyglutaronitrile 20 (eq.2,Scheme 6). Byapplyinggenesitesaturationmutagenesistoimprovethestereoselectivityofthe biocatalyst, the ee of the desired product reached 99% under a 3M loading of the substrate.Thesynthesisisshortandutilizesaninexpensivestartingmaterial.Boththe ketoreductase (eq. 1, Scheme 6) and nitrilase processes (eq. 2, Scheme 6) led to significantreductionofbyproducts,wastesandorganicsolventsassociatedwithexisting chemicalroutes.Recently,amoreconciseapproachtoinstalltheatorvastatinsidechain was reported by using a microbial deoxyribose5phosphate aldolase (DERA). This enzyme catalyzes the sequential aldol condensation between one equivalent of amino aldehye 21 andtwoequivalentsofacetaldehydetoformlactol22 withexcellentee(98%) and de (97%), which was then converted to the statin side chain 18 upon oxidation, protectionandesterification. Usingthismethod,theoverallprocesstoatorvastatinwas shortenedsignificantlyandtwocryogenicstepsintheexistingprocesswereeliminated (Scheme7).Itisestimatedthathundredsofmetrictonsofrawmaterialsandsolventswill bereducedeachyearbyusingthechemoenzymaticrouteinconcomitantwithsignificant reductioninenergyconsumption.

16.34SomeConceptualDevelopmentsinSynthesisinChemistry

Scheme16.6.

SyntheticStrategiesinChemistry 16.35

Scheme16.7. Rosuvastatin : Similarapproacheshavebeenappliedtothesynthesisofthesidechainofrosuvastatin, the API of Crestor(R) (Scheme 8). By a combination of activity and sequencebased screening,aNovelDERAwasdiscoveredfromenvironmentalDNAlibrariesleadingto anenzymaticprocesstoobtainlactol 23 withvolumetricproductivityof720gL 1day 1 underanenzymeloadingof2%wt/wt.

Scheme16.8.

16.36SomeConceptualDevelopmentsinSynthesisinChemistry

CASE STUDY 4: Industrial Production of b-lactam Antibiotics : Biocatalysisisuniquelysuitedtothedevelopmentofgreenchemistryroutesforcomplex molecules, which are often labile and densely functionalized. As a result of high selectivity and mild conditions, enzymatic catalysis has been applied to the industrial productionofblactamantibiotics,whicharemostly derived from 6aminopenicillanic acid(6APA)or7aminodesacetoxycephalosporanicacid(7ADCA).Theannualworld production of 6APA and 7ADCA were estimated to be over 8000 and 600 t, respectively.Untilrecently,theywerepreparedfrompenicillinG,afermentationproduct derivedfromnonribosomalpeptidesynthase(NRPS), by chemical deacylation, where thecarboxygroupofthepenicillinGisfirstprotectedbysilylation,followedbyselective deacylationviatheimidoylchloride( 25 ),andremovaloftheprotectinggroup(Scheme 9). Themethodusesstoichiometricamountsofthesilylatingagent,andalargeamountof hazardous chemicals and solvents such as phosphorus pentachloride and dichloromethane. Scheme16.9

SyntheticStrategiesinChemistry 16.37

In contrast, enzymatic deacylation of penicillinGby penicillin G acylase was accomplishedinwateratroomtemperaturerequiringnoprotectionanddeprotectionof otherfunctionalgroups(Scheme10).Moreover,underkinetic control,animmobilized penicillinaclyasewasalsoabletocatalyzetheacylation of 6APA and 7ADCA with either Dphenylglycine methyl ester (PGA), Dphenylglycine amides (PGA) or their parahydroxylatedanalogstoproduceawiderangeofsemisyntheticblactamantibiotics suchasampicillin,amoxicillin,cefaclor,cephalexinandcefadroxil(Scheme16.10). Asa resultofthebiotransformation,rawmaterialefficiency and Efactor were significantly improvedfromthefirstgenerationofchemicalprocesses. Scheme16.10.

16.38SomeConceptualDevelopmentsinSynthesisinChemistry

CASE STUDY 5: Paclitaxel (Taxol R_) Plant secondary metabolites have provided a rich and renewable resource of natural products for drug development. However, establishing a stable supply of the active compoundsfromplantsisoftendifficult.AprominentexampleisPaclitaxel(TaxolR_), acomplexditerpenoidalkaloidoriginallyisolatedfromthebarkofthepacificyewtree Taxus brevifolia with a yield of 0.014%. In addition, isolating Paclitaxel required strippingthebarkfromtheyewtrees,thuskilling a slow growing tree in the process, whichtakesabout200yearstomature. Scheme16.11 On the other hand, the complexity of the Paclitaxel molecule makes commercial production by chemical synthesis from simple compounds impractical. As a result, a

SyntheticStrategiesinChemistry 16.39 semisynthetic process was developed starting from 10deacetylbaccatin III (Scheme 16.11), Scheme16.12. a more abundant taxoid biosynthesized from isoprenyl diphosphate and farnesyl diphosphate in the needles of the European yew tree Taxus baccata, which could be isolatedwithayieldof0.1%withoutharmtothetrees.Inthisroute,10deacetylbaccatin

16.40SomeConceptualDevelopmentsinSynthesisinChemistry

III was first acetylated and silylated. The resulting 7triethylsilyl baccatin III ( 27 ) was thencoupledtoanNacylb–lactam( 28 )toinstallaphenylisoserinesidechainattheC 13 position followed by deprotection to afford Paclitaxel. Overall, however, the semisynthetic process is still complex requiring eleven chemical transformations including the preparation of 28 and seven isolations. Consequently, an alternative processtoPaclitaxelwasdevelopedusingTaxuscellfermentationandextractionfrom culturemedium,followedbyrecrystallization. Starting from the two isoprenoid precursors, isoprenyl diphosphate and farnesyl diphosphate (Scheme 12), geranylgeranyl pyrophosphate synthetase catalyzes the coupling to give geranylgeranyl pyrophosphate, 29 , which is then cyclized and then converted to baccatin III 30 through a series of enzymatic transformationsincluding hydroxylation,acylation,oxidationandgenerationoftheoxetanering.Thesidechainin 31 was installed by enzymatic transfer of phenylisoserine. To achieve a high titer of Paclitaxelproduction,cellculturesfromvariousspeciesofTaxusanddifferentelicitors toinducePaclitaxelproductionhavebeenexamined. Forexample,methyljasmonatewas abletoenhancePaclitaxelproductionto110mg L 1every2weeksincellsuspension culture of T. media. The plant cell fermentation process led to an elimination of the elevenchemicaltransformationsandalargeamountofhazardoussolventsandwastes, which came with the semisynthesis route. Recent advances in metabolic engineering have created another opportunity to develop green processes for relatively complex molecules.Anexampleistheproductionofshikimicacidforthesynthesisofoseltamivir phosphate (Tamiflu) for treatment and prevention of in.uenza virus infections. In the currenttenstepcommercialsynthesis(Scheme16.13), thekeyintermediateisshikimic acid 32 ,whichwasproducedbyageneticallyengineeredE.colistraindeficientinboth shikimatekinaseisozymesorisolatedfromthestaranise. The metabolic engineering efforts have led to the development of an E. coli strain capable of producing the molecule with a titer of 84 g L 1 and a yield of 33% from glucose. Thisacidwassubsequentlyconvertedintoadiethylketal 33 , whichwasthen transformedtotheoseltamivirphosphateaftersidechaininstallation,reductiveopening oftheketal,basecatalyzedepoxideringclosure(34 )followedbyaziridination( 35 ).One drawbackoftheaboveprocessistheuseofthehazardousazidereagenttopreparethe

SyntheticStrategiesinChemistry 16.41 aziridineintermediate 35 .Toeliminatetheuseofazides,anazidefreechemoenzymatic synthesisofoseltamivirphosphatewasrecentlyreportedthatcomprisedfewersynthetic stepsthanthecommercialprocess. Scheme16.13.

Scheme16.14.

16.42SomeConceptualDevelopmentsinSynthesisinChemistry

Inthismethod,thekeyintermediateisaminoshikimicacid( 37 )thatwasproducedfrom glucosebyatwostepmicrobialprocessusingBacilluspumilustogeneratekanosamine 36 followedbyanengineeredE.colitogive 37 (Scheme16.14). Thenewroutehasan overall yield of 22% from aminoshikimic acid and holds great potential if the ofaminoshikimicacidcanbefurtherimproved. CASE STUDY 6: Fine Chemicals Due to the exquisite regioselectivity under mild conditions, biotransformations often offer great advantages over chemical synthesis in largescale production of fine chemicals,whichisusuallyenergyintensiveandgeneratesalargeamountofwastesand gasemissions.Thefirstexampleofenzymaticmanufactureofabulkchemicalinvolves the conversion of acrylonitrile to acrylamide, which is the monomer of widely used polyacrylamide.Thechemicalmanufacturingprocessinvolveshydrationofacrylonitrile at70–120 oCbyRaneycopperresultinginalargevolumeoftoxicwastesandHCN.In addition,theacrylamideproducedbythisfashionrequiresconsiderablepurificationasit tendstopolymerizeundertheharshreactionconditions.Acrylicacidisalsoabyproduct of chemical hydration. Many nitrile hydratases catalyze the conversion of acrylonitrile into acrylamide.For example, immobilized Rhodococcus rhodochrous J1 was able to produceacrylamideataconcentrationof400gL 1inafedbatchprocessunder10 .C. There is no need to recover residual acrylonitrile because the yield of the enzymatic conversionisalmost100%.Currentlythemicrobialprocessisoperatedatascaleof>40 000 t year 1. It is much greener and more economical than the chemical process. In additiontoacrylamide,RhodococcusrhodochrousJ1wasalsousedtoprepareavariety ofamidesinhighconcentrationandthroughputinwater(Fig.16.1). Thedrivetoward environmentallybenignsynthesisandsustainabledevelopmentinthechemicalindustry iscontributingtoagrowinginterestintheuseofrenewablefeedstocksandreductionof hazardouswastesinmanufacturing.Consequently,carbohydratessuchasglucosederived fromcornstarchorcellulosicbiomassprovideanintriguingalternativetothecurrentuse ofnonrenewablepetroleumasstartingmaterials.Thekeystothesuccessofthistransition are the elaboration of new synthetic routes, as well as the design and engineering of robust microbial biocatalysts. Two examples are the development of biocatalytic synthesesofadipicacidandcatecholfromglucose. Adipicacid,oneofthemonomers

SyntheticStrategiesinChemistry 16.43 usedinthemanufactureofnylon6,6,iscurrentlyproducedat2.2millionmetrictonsper year. The existing route to adipic acid first entails oxidation of cyclohexane, mostly derivedfrombenzene,toamixtureofcyclohexanolandcyclohexanonebyoxygenwitha cobaltcatalystatatemperatureof150–160 ºC.Thismixturewasthenconvertedtoadipic acidbyoxidationinnitricacid(Scheme16.15).

Scheme16.15

Scheme16.16

16.44SomeConceptualDevelopmentsinSynthesisinChemistry

Duetoitsmassivescale,adipicacidmanufacturehasbeenestimatedtoaccountforsome 10%oftheannualincreaseinatmosphericnitrousoxidelevels. Benzene is a carcinogen and volatile chemical and is derived from nonrenewable fossilfuels.Biocatalyticroutestoadipicacidhavebeenreportedinwhichglucosewas converted to cic,cismuconic acid followed by hydrogenation (Scheme 16.16). The biosyntheticpathwayleadingtotheproductionofcic,cismuconicacidwasassembledby expressingKlebsiellapneumoniaearoZencoded3dehydroshikimatedehydratase,aroY encoded protocatechuate decarboxylase, 44c and Acinetobacter calcoaceticus catA encoded catechol 1,2dioxygenase in a 3dehydroshikimatesyntheszing E. coli strain. TheresultingheterologousbiocatalystE.coliWN1/pWN2.248wasabletoproduce37 gL 1ofcic,cismuconicacid( 42 )in22%yieldfromglucoseinonepotviaintermediates 39 , 40 and 41 (Scheme16.16).Catalytichydrogenationofcic,cismuconicacidaffords adipicacidin97%yield.

Scheme16.17. In the above metabolic pathway, catechol is an intermediate in the route to cic,cis muconic acid. As a result, this technology may also be applied to the production of catechol, which is manufactured at an annual volume of 25000 metric tons as an important chemical building block for flavors, pharmaceuticals, agrochemicals,

SyntheticStrategiesinChemistry 16.45 polymerizationinhibitorsandantioxidants.Althoughsomecatecholisdistilledfromcoal tar,phenolisthestartingmaterialfortheproductionofthemajorityofcatechol,andwas obtained by Hocktype air oxidation of benzenederived cumene (pathway A, Scheme 17). 47 Directmicrobialsynthesisofcatecholfromglucoseresultedinonly5%yielddue to its high toxicity to the production host. The issue was circumvented by microbial production of a less toxic intermediate protocatechuate using E. coli KL3/pWL2.46B followedbychemicaldecarboxylationinwater(pathwayB,Scheme16.17). Bythisstrategy,theoverallyieldofcatecholfromglucosewasimprovedto43%. As illustratedinmicrobialsynthesesofadipicacidandcatechol,manufactureofchemicals ofmajorindustrialimportancefromrenewablefeedstocksoffersignificantenvironmental advantagesandeconomicopportunities.Asinglegeneticallyengineeredmicrobeisable tocatalyzetheconversionofglucoseinwateratnearambientpressureandtemperature. The spectrum of chemicals that could be synthesized from glucose can be further expandedbytheconstructionofheterologousbiocatalysts and integration of microbial andchemicaltransformations. Scheme16.18.

16.46SomeConceptualDevelopmentsinSynthesisinChemistry

Research aimedatincreasingsyntheticefficiency andovercomingproducttoxicityand isolationproblemsarecriticaltomovebiocatalyticsynthesesfromproofofconceptinto practicalroutestocompetewithexistingchemicalroutesbasedonpetroleum.Another example using metabolic engineering is microbial production of 1,3propanediol. The emergenceof anew1,3propanediol(PDO)basedpolyesterhasdramaticallyincreased thedemandforPDOinrecentyears.Onceafinechemical,PDOisnowbecomingabulk chemicalasitsproductionvolumewillincreasetoalevelofonemilliontonsannually. Historically,PDOwasproducedfrompetroleumchemicalsfromtwodifferentchemical processes.Oneusespropyleneasthestartingmaterial,whichwascatalyticallyoxidized to acrolein, followedby hydration to 3hydroxypropionaldehyde (pathway A, Scheme 18). An alternative process starts from ethylene oxide obtained from oxidation of ethylene (pathway B, Scheme 18). Ethylene oxide was then converted to 3 hydroxypropionaldehydebyhydroformylationunderhighpressure. Theconversionof 43 toPDOrequireshydrogenationoverarubidiumornickelcatalystunderhighpressure. To lower the cost of PDO and reduce the negative environmental impact of its production,arecombinantE.colistrainwasdevelopedtoproducePDO from glucose. TheE.colistrainwasmodifiedtocontaingenesfromSaccharomycescerevisiae,which is capable of producing glycerol from glucose, and Klebsiella pneumoniae, which containsthemetabolicpathwayofglyceroltoPDO(Scheme19). Scheme16.19.

SyntheticStrategiesinChemistry 16.47

Theengineeredstrainreliesonapredominantlyheterologouscarbonpathwaythatdiverts carbon from dihydroxyactone phosphate (DHAP) to PDO. Two genes from yeast encoding glycerol 3phosphate dehydrogenase (dar1) and glycerol 3phosphate phosphatase (gpp2) directs glycolysis pathway to glycerol. Two genes from K. pneumoniae encoding glycerol dehydratase (dhaB13) and 1,3propanediol oxidoreductase (dhaT) then transform glycerol to PDO. Significant modifications were then made to the base strain by extensive metabolic engineering to improve the fermentationproductivity. Additionalgenesfromdhaoperonwereincorporatedintothe engineeredstraintostabilizeglyceroldehydrataseviareactivation.Moreover,anE.coli endogenousoxidoreductase(yqhD)wasfoundtobesuperiortodhaTinprovidinghigh PDO titer (¡«130 g L1). As a result, the yield of PDO production in onepot from glucosewaseventuallyimprovedto51%(gg1). In contrast to the chemical processes to PDO, the biocatalytic process uses a renewable resource and was run at close to room temperature without added pressure. The bioprocess to PDO also reduces the energy consumption by 40% and green gas emissionsby20%.AsofDecember2006,shipmentsof corn sugarderived PDO have beeninitiatedfroma100millionlbperyearplant. Thesuccessfuldevelopmentofabio basedPDOprocessshowedthatitispossibletoproducebulkchemicalsinamorecost effective way than the chemical processes while adhering to the principles of green chemistry. CASE STUDY 7: Polymers Another recent application of biocatalysis for green chemistry development is the productionofpolymericmaterials.Enzymaticpolymerization,i.e.,invitrosynthesisof polymers using isolated enzymes, offers a number of advantages over conventional chemical methods that often require harsh conditions and the use of toxic reagents. Benefitingfromhighregioandenantioselectivitiesundermildconditions,anenzymatic approachprovidesanewsyntheticstrategyfortheproductionofnovelpolymers,which areotherwisedif.culttogetbychemicaltransformations. Enzymaticpolymerizationhas beenappliedtothesynthesisofanumberofpolymerssuchaspolyesters,polycarbonates, polysaccharides,polyurethanes,polyaromaticsandvinylpolymersusingoxidoreductases, transferasesandhydrolases.

16.48SomeConceptualDevelopmentsinSynthesisinChemistry

For example, a simple, environmentallyfriendly, and versatile method was recently developed to produce polyolcontaining polyesters through selective lipasecatalyzed condensationpolymerizationbetweendiacidsandreducedsugarpolyolssuchassorbitol (Scheme20). Insteadofusingorganicsolvents,themonomersadipicacid,glyceroland sorbitolweresolubilizedwithinbinaryorternarymixtures,andnopreactivationofadipic acidwasneeded.Thedirectcondensationofadipicacidandsorbitolwasperformedin bulkat90 oCfor48husingimmobilizedlipaseBfromCandidaAntartica(Scheme20).

Theproduct,poly(sorbityladipate),hadanaveragemolecularweightof10880(Mn)and

17030(Mw)respectively. Scheme16.20 Byreplacingafractionofsorbitolwith1,8octanediol,copolyestersofadipicacid,1,8 octanediol,andsorbitolwereobtainedinthemolarratioof50:35:15withanMwof >100 000. Similar results were also obtained with glycerol in place of sorbitol as the naturalpolyol(notshown).Thecondensationreactionswithgycerolandsorbitolbuilding blocksproceededwithhighregioselectivitywithoutprotection–deprotectionoffunctional groups.Althoughthepolyolmonomerscontainthreeormorehydroxygroups,onlytwo are highly reactive in enzymatic polymerization. Therefore instead of obtaining highly crosslinkedproducts,thehighregioselectivityleadstolightlybranchedpolymerswhere

SyntheticStrategiesinChemistry 16.49 thedegreeofbranchingvarieswiththereactiontimeandmonomerstoichiometry.The mildreactionconditionsalsofacilitatethepolymerization of chemically and thermally sensitive molecules. Alternative chemical polymerization would necessitate the use of stoichiometricamountofcouplingagents.Thebiocatalyticapproachisalsoversatilefor simultaneous polymerization of lactones, hydroxyacids, cyclic carbonates, cyclic anhydrides, amino alcohols, and hydroxythiols as a result of high regioselectivity. Another example is enzymecatalyzed ringopening polymerizations of lactones and cyclic carbonates, which offer number of advantagesoverchemicalmethodsincluding mildreactionconditionsandimprovedpropagationkineticsand/ormolecularweights. The ringopening polymerization of pentadecalactone and trimethylene carbonate catalyzedbythelipasePS30andNovozyme435inbulkat70 oCaffordedhighaverage molecular weight and moderate dispersity products (not shown). Enzymatic polymerization also allows structural control by regioselective incorporation of multifunctionalinitiatorssuchascarbohydrates.Intheringopeningpolymerizationofe caprolactone or trimethylene carbonate with ethyl glucopyranoside as an initiator catalyzedbyporcinepancreaticlipase,thepolymerizationoccurredselectivelyfromthe 6hydroxylposition(Scheme21). Sinceboth ethyl glucopyranoside and the monomers are liquids, the reaction could be carried out in the absence of solvents. The novel amphiphilic product was prepared in onepot without the need of protection and deprotectionchemistry.Hydrolaseswerealsoabletocatalyzetransesterificationreactions betweenamonomerandapolymerorbetweentwohomopolyestersthatdifferinmain chain structure. For example, Novozym435 catalyzed transesterification between polypentadecalactone (4300 g mol1) and polycaprolactone (9200 g mol1) resulted in random copolymers within one hour. In addition to catalyzing metalfree transesterificationundermildconditions,lipasesendowtransesterficationreactionswith remarkableselectivity,allowingthepreparationofblockcopolymersthathaveselected block lengths. Recent advances in biocatalysis have also made an in road in the manufacture of biodegradable plastics such as polyhydroxyalkanoates (PHA) and polylactides(PLA). Polyhydroxyalkanoatesareaclassofnaturalpolyestersproducedby many bacteria in the form of intracellular granulates as a carbon and energy reserve. Sincetheinitialdiscoveryofpoly3hydroxybutyrate(PHB),morethan130hydroxyacid

16.50SomeConceptualDevelopmentsinSynthesisinChemistry monomercompositionshavebeenidentifiedthathavebeenclassifiedasshortchainand mediumchain hydroxyalkanoates. Once extracted from bacterial cells, the biobased polymersdisplaydiversematerialpropertiesthatrangefromthermoplasticstoelastomers depending upon their monomer unit composition. The environmental benefit of using biodegradable PHA and the utilization of abundant, renewable starch as a starting material for its synthesis provide an appealing alternative to the commonly used petroleumderivedplastics.OfallthePHAs,PHBcopolymersarethemostextensively studied. The biosynthetic pathway of polyhydroxybutyrate consists of three enzymatic reactions (Scheme 22).Thiolase catalyzes the condensation of two molecules of acetyl CoA to form one molecule of acetoacetylCoA, which is then reduced to (R)3 hydroxybutyrylCoA by aNADPHdependent acetoacetylCoA dehydrogenase. The crucial polymerization of 44 is catalyzed by a PHB synthase with concomitant regenerationofcoenzymeA(Scheme16.22).

Scheme16.22. PHBhomopolymerisastiffandbrittlematerial.ThehighmeltingtemperatureofPHB limitstheabilitytoprocessthebiopolymer. Incorporationof3hydroxyvalerateintoPHB rendersapoly(3hydroxybutyrateco3hydroxyvalerate)blockcopolymer 48 ,whichcan beprocessedatalowertemperaturewhilestillretainingexcellentmechanicalproperties. The monomer (R)3hydroxyvalerylCoA 47 isproduced fromcondensationofacetyl CoAandpropionylCoAto3ketovalerylCoA( 46 )followedbyreductioncatalyzedby acetoacetylCoAdehydrogenase(Scheme23).Withtheadvancesingenomesequencing projects,novelPHAsynthaseswithdifferentactivities and substrate specificities have

SyntheticStrategiesinChemistry 16.51 beenidentified. Togetherwithmetabolicengineeringofdiverse,heterologouspathways forefficientbiosynthesisofthemonomersubstrates from renewable starting materials, high yielding production of polyhydroxyalkanoates has been achieved via microbial fermentation. Accumulation of PHA in genetically engineered Pseudomonas reached 90%ofthecellweight. Typicalmolecularweightofthepolymerrangesfrom50000to1 000 000 Da. Recent research efforts have been devoted to the production of PHA in transgenicplants. DirectproductionofPHAinplantshasthepotential to dramatically lower the manufacturing cost, and reduce the burden of plastic wastes as a result of biodegradabilitytoenvironmentallyfriendlyproducts.

Scheme16.23. CONCLUSIONS The new development in synthetic chemistry is green chemistry, and the principles involved have created newer opportunities to develop new technologies to improve chemicalprocesses.Arecentstudyshowsthatamongthe1039chemicaltransformations

16.52SomeConceptualDevelopmentsinSynthesisinChemistry analyzedforthesynthesisof128drugmolecules, chemicalacylationsareoneofthemost common transformations which, however, are generally inefficient. Development of catalytic, low waste acylation methods would significantly improve the environmental performance of many syntheses. The discovery of new regioselective and catalytic oxidationswouldgreatlyincreaseflexibilityinsyntheticdesign,asillustratedinthegreen andbiocatalyticsynthesisofpharmaceuticalsandfinechemicals.Biotransformationsare uniquely suited to deliver high stereo and regioselectivity in water at ambient temperatureandatmosphere,andareabletocatalyzereactionsthatarechallengingfor traditional chemistry. The key is to integrate synthetic chemistry, biological transformations and process development and biocatalysis is positioned to be a transformationaltechnologyforchemicalproductionwithimprovedsyntheticefficiency. REFERENCES 1.JamesA.Schwarz,CristianContescu&AdrianaContescu,Chem.Rev.,(1995) 95477 2.NingqingRan,LishanZhao,ZhenmingChenb&JunhuaTao,GreenChem., 10(2008)361. 3.C.N.R.Rao,MaterialsScienceandEngineering,BI8(1993)1. 4.AbrahamSzöke,WilliamG.Scott&JánosHajdu,FEBSLetters553(2003)18. 5.TheWayofSynthesis:EvolutionofDesignandMethodsforNaturalProducts TomasHudlicky,JosephineW.Reed,2007,WileyVCH. 6.SynthesisofInorganicMaterials,UlrichSchubert,NicolaHüsing,2005,WileyVCH. 7.Härkönenetal.,Nucl.Instr.andMeth.A541(2005)202. 8.leksicetal.,Ann.ofNYAcademyofSci.972(2002)158. 9.Lindströmetal.,Nucl.Instr.andMeth.A466(2001)308andcitedliteraturetherein. 10.C.N.R.RaoMaterialsScienceandEngineeringB,Volume18,Issue1,1993,1. 11.InsightsintoSpecialityInorganicChemicals,DavidThompson,1995,WileyVCH. 12.OrganicChemistryPrinciplesandIndustrialPractice,MarkMGreen,HaroldA Wittcoff,2003,WileyVCH.

Chapter - 17 COMPUTATIONAL BASICS UNDERLYING SYNTHETIC STRATEGIES S.Sabiah KEY WORDS: Computation,Syntheticstrategy,Synthesisdesign,Computer assistedsynthesis INTRODUCTION: Syntheticstrategyisanimportantsteppingstoneforscientificcommunity.Ithasbeen foundsocrucialandinnovativeinthefourmainstreamofresearchnamely, 1. ObservationalScience 2. ExperimentalScience 3. TheoreticalScience 4. ComputationalScience Sincethetopicofchoiceisoncomputational,Iwouldliketodrawfewlinesaboutthe computationalscience. Computational science (or scientific computing )isthefield of study concerned with constructing mathematical models and numerical solution techniquesandusingcomputerstoanalyzeandsolvescientific,socialscientificand engineering problems. In practical use, it is typically the application of computer simulation and other forms of computation to problems in various scientific disciplines. It uses everything that scientists already know about a problem and incorporatesitintoamathematicalproblemwhichcanbesolved.Themathematical model which then develops gives scientists more information about the problem. ComputationalScienceisbeneficialfortwomainreasons: 1. Itisacheapermethodofconductingexperiments. 2. Itprovidesscientistswithextrainformationwhichhelpsthemtobetter planandhypothesizesaboutexperiments. Duetothesereasons,computationhasgainedmuchattentioninalmostallfieldsof sciencewhicharelistedbelowundercomputationaldisciplines. COMPUTATIONAL DISCIPLINES • Bioinformatics • Cheminformatics • Chemometrics • Computationalbiology

17.2 ComputationalBasicsUnderlyingSyntheticStrategies

• Computationalchemistry • Computationaleconomics • Computationalelectromagnetics • Computationalengineering • Computationalfluiddynamics • Computationalmathematics • Computationalmechanics • Computationalphysics • Computationalstatistics • Environmentalsimulation • Financialmodeling • Geographicinformationsystem(GIS) • Highperformancecomputing • Machinelearning • Networkanalysis • Numericalweatherprediction • Patternrecognition Since the main focus of the chapter is on synthetic strategies in chemistry , it is likely to be elaborated towards chemical aspects. In the field of chemistry, we all knowthatthebeginningoftheorganicsynthesisisstartedwiththepreparationofurea byWohlerin1828.Itisonlyin1967thatasystematicanalysisofsynthesistowards the direction of computerassisted synthesis design (CASD) was reported by E. J. Corey[1].Sincethenthecomputationalmethodsinchemistryhasbeen growingin manydimensionsandeveryscientificpaperpaysattentiontocomputationalsupport in addition to the experimental evidences. It has also become a useful way to investigatematerialsthataretoodifficulttofindortooexpensivetopurchase.Ithelps chemiststomakepredictionsbeforerunningtheactualexperimentssothattheycan be better prepared for making observations. Hence, we can say that computational chemistryispartlyassistingthebasicchemicalproblemsdealwithsynthesis,structure andspectroscopyofmaterialsandindeedimportanttodiscussfurther. Computational Chemistry Computational chemistry is a branch of chemistry that generates data, which complements experimental data on the structures, properties and reactions of substances.Itcaninsomecasespredicthithertounobservedchemicalphenomena.It

SyntheticStrategiesinChemistry 17.3 iswidelyusedinthedesignofnewdrugsandmaterials.Thecalculationsarebased primarilyonSchrödinger’sequation(eqn.1).

…………….. eqn 1 ThesymbolψisamathematicalfunctionthatcalculatesthestrengthofthedeBroglie waveatvariouspositionsinspace.Therestofthecomponentsareasfollows: h=Planck'sconstant m=themassoftheparticle ∆=apartialdifferentialoperatorcalledtheLaplacianoperator V=thepotentialenergy =psi,thewavefunction i=thesquarerootof1 Thefinalformofequation 1isHψ=Eψ WhereH=HamiltonianOperator;E=totalenergyofthesystem • Computationalchemistryisparticularlyusefulfordeterminingmolecular propertieswhichareinaccessibleexperimentallyandforinterpreting experimentaldata • Withcomputationalchemistry,onecancalculate: o electronicstructuredeterminations o geometryoptimizations o frequencycalculations o transitionstructures o proteincalculations,i.e.docking o electronandchargedistributions o potentialenergysurfaces(PES) o rateconstantsforchemicalreactions(kinetics) o thermodynamiccalculationsheatofreactions,energyofactivation • Therearethreemaintypesofcalculations: 1. Ab Initio: (Latin for "from scratch") a group of methods in which molecular structures can be calculated using nothing but the Schroedingerequation,thevaluesofthefundamentalconstantsandthe atomicnumbersoftheatomspresent

17.4 ComputationalBasicsUnderlyingSyntheticStrategies

2. Semiempirical: techniques use approximations from empirical (experimental)datatoprovidetheinputintothemathematicalmodels 3. Molecular mechanics: uses classical physics to explain and interpret thebehaviorofatomsandmolecules Currently, there are two ways to approach chemistry problems: computational quantum chemistry and non-computational quantum chemistry. Computational quantum chemistry is primarily concerned with the numerical computation of molecularelectronicstructuresby abinitio andsemiempiricaltechniquesandnon computationalquantumchemistrydealswiththeformulationofanalyticalexpressions forthepropertiesofmoleculesandtheirreactions.Examplesofsuchpropertiesare structure(i.e.theexpectedpositionsoftheconstituentatoms),absoluteandrelative (interaction) energies, electronic charge distributions, dipoles and higher multipole moments, vibrational frequencies, reactivity or other spectroscopic quantities, and crosssectionsforcollisionwithotherparticles. Themethodsemployedcoverbothstaticanddynamicsituations.Inallcasesthe computer time increases rapidly with the size of the system being studied. That systemcanbeasinglemolecule,agroupofmoleculesorasolid.Themethodsare thusbasedontheories whichrangefromhighlyaccurate, but are suitable only for smallsystems,toveryapproximate,butsuitableforverylargesystems.Theaccurate methodsusedarecalledabinitiomethods,astheyarebasedentirelyontheoryfrom first principles. The less accurate methods are called empirical or semiempirical becausesomeexperimentalresults,oftenfromatomsorrelatedmolecules,areused alongwiththeory. Therearetwodifferentaspectstocomputationalchemistry: • Computationalstudiescanbecarriedoutinordertofindastartingpointfora laboratorysynthesis,ortoassistinunderstandingexperimentaldata,suchas thepositionandsourceofspectroscopicpeaks. • Computationalstudiescanbeusedtopredictthepossibilityofsofarentirely unknownmoleculesortoexplorereactionmechanismsthatarenotreadily studiedbyexperimentalmeans. Toperformthesecomputationalsimulationsorcalculations,supercomputerswith highperformancecomputingfacilityisneeded.

SyntheticStrategiesinChemistry 17.5

Impact of High Performance Computing Theamountofchemicalinformationisquitelargeandcallsforcomputerasthemain sourceforstorage.Withpresently • 17millionknowncompounds • 500000newcompoundseachyear • 600000chemistryrelatedpublicationsannually Anoverviewandaccesstoallthisinformationcanonlybemaintainedbyelectronic means. Thus,databasesonchemicalinformationplayamajorroleinpresentdayresearchand development.Databasesprovideaccessto • literatureon17millioncompounds • factualdataon7millionorganiccompounds • severalmillionreactions • 140000experimental3Dstructures • 250000spectra SOFTWARES Theuseofcomputersastoolsinchemistrydatesbacktolate1950s.Theadoptionof FORTRAN as a scientific programming language is the beginning of these studies[2].Onlyfromaround1980–1990,theuserhadaccesstoavarietyofnetwork basedresourcesfromasinglepointofuse.Amoresophisticatedexampleistheuseof JavatodisplaydigitalspectralinformationderivedfromanNMRspectrometer,[3] and Java is a computer language developed by Sun Microsystems for writing programstorunonthewebwithinabrowser.Itisusedtolinkregionsofthespectrum tospecificatomsorresiduesina3Dmolecularobject.Thelinkscanbebidirectional, i.e.clickingonaspecifiedatomwillhighlightthespectral region containing peaks associatedwiththatatom. Chemistshavebeensomeofthemostactiveandinnovativeparticipantsinthisrapid expansion of computational science. Some common computer software used for computationalchemistryincludes: • Gaussian03 • GAMESS • MOPAC • Spartan

17.6 ComputationalBasicsUnderlyingSyntheticStrategies

• Sybyl Nowletusmovetowardstheuseofthesecomputationalbasicsforsyntheticstrategy especially in basic organic chemistry. Once we know how it works, it can be extendabletoothermaterials. 1. IN ORGANIC SYNTHESIS When a chemist wants to synthesize a compound of interest, it is called target molecule and he looks in to the target to identify any known fragment called substructure is present whose synthesis is already available. For example, in the synthesis of the Ibogamine, chemist perceives the presence of an indole nucleus, whosesynthesisisknown(Chemicalstructuresareshowninsidethebox).So,hemay planthesynthesissimilartoIndoleasoutlinedinScheme1.

N

N N H H Ibogamine Indole Scheme 1

Synthesis of Ibogamine Synthesis of Indole

N + NH2 O + NH2 N N H H O

N

N H N H Ibogamine Indole Aknown startingmaterial isverysimilartothetargetandtheproblemis'reduced'to find the reactions which will convert it to the target. Despite the similarity of the startingmaterialandthetarget,thenumberofstepstoobtainthetargetmaybehighin certain cases which triggered the venture of using a computer to solve synthesis problems [4]. The number of reactions being very high (several thousands), the computershouldbeabletostoreallthesereactions.Itwouldneverforgetthemandit

SyntheticStrategiesinChemistry 17.7 could generate all the possibilities. For this purpose, Corey proposed a general approachwhichisdiscussed. Corey’s Approach Starting from the target, the program finds its precursors, then each precursor becomesatargetandnewprecursorsaregenerated;theprocessisthenrepeateduntil commercialorsimpleproductsaregenerated.Thisapproachiscalled retrosynthetic or antithetic, since it is the reverse of the synthesisaspracticedatthe bench.This approachmaybevisualizedbya 'synthesistree' (Scheme2).Thisapproachshould, formally, be 'easily' programmed: it necessitates 'only' writing a program able to generate the precursors of a target and this program is repeated again and again. Numerousprogramshavebeenwrittenafterthisinitialreportandseveralreviewson thissubjecthavebeenpublished[5]. Scheme 2

Target

Firstlevel P1 P2 P3 ofprecursors

Secondlevel P4 P5 P6 P7 P8 ofprecursors Theabove tree pointsoutlinetheessentialproblemslikehowtodescribeamolecule andhowtodescribeareaction,thatis,howtogenerateaprecursor.Letusconsider theclassicalDielsAlderreaction, Scheme 3

diene dienophile cyclohexene

1 2 3 The above equation corresponds to writing of the reaction in the synthetic (or forward)direction:reacting 1with 2, underappropriateconditions,generates 3.Inthe approach proposedby Corey, theprogram worksbackward, from the target to the precursor:intheretrosyntheticorantitheticdirection.Sotheprogramhastosearch the substructure 3inthetargetand,ifitispresent,itreplacesitbytheprecursor(s)

17.8 ComputationalBasicsUnderlyingSyntheticStrategies substructure(s),here1+2.Todescribethereactionintheretrosyntheticdirection, the term of 'transform' hasbeenproposed[6]andtheuseofadoublelined arrow indicatesthisoperation(Scheme4): Scheme 4

cyclohexene diene dienophile Thisdescriptionmaylooksimpleforacomputer,butfailsforcertaincircumstances. Forexample, Scheme 5

5 4 If4isthetargetthentheprecursorwouldbe5,whichisanimpossiblesolutiondueto theallenicfunctioninafivememberedring.But,computerpredictsscheme5asa retrosyntheticapproachwhichisnotapplicabletolaboratorysynthesis.Similarly,if the computer is taught the reduction of a keto group (7) to form an alcohol (6), described in scheme 6, the backward mode by the transform of scheme it will generate7asapossibleprecursorfor6.Thisisagainawrongsolution,indeedthereis another keto group which will generally be reduced, so that, in the laboratory the resultwouldnotbe6but8(Scheme6). Scheme 6

OH O OH

O O OH 6 7 8 Thesetwoexamplesclearlyindicatethatthecomputershouldknowtodiscardwrong solutions for a synthetic strategy. Hence there should be a way to settle communicationbetweenthecomputerandthechemistforthefollowingaspects: I. Descriptionofmolecules II. Descriptionofreactions

SyntheticStrategiesinChemistry 17.9

III. Pruningthesynthesistree Theseaspectsarediscussedintheupcomingsection. I. Description of Molecules (a) Use of Connectivity Table The molecules of interest are first described by connectivity tables which list in severalarraystheatomsandthebondsofthetarget.Anexampleisgivenwithatom labeling(bold)andnatureofbonds(normalnumbers)forcompoundA. 2 O 1 1 C 3 3 4 5 2 H3C C N 4 compundA ThentheconnectivityisgivenbyatomictableandbondtableasdescribedinTables 17.1and17.2.Thesetablesarenotgiveninthisformtothecomputer.Actuallythe chemistdrawsthestructureonthescreenusingamouse[7],andaprogramtakescare oftransformingthisgraphicstructureintoaconnectivitytable. Table17.1.AtomictableforcompoundA Atomnumber Atomtype Neighbors Bonds Numberofhydrogen number number atoms 1 C 234 123 0 2 O 1 1 0 3 C 1 2 3 4 C 15 34 0 5 N 4 4 0 Thisdescriptionisa topological one,butwhenachemistlooksatatargethedoesnot only see a succession of atoms and bonds. He automatically perceives structural features,suchasfunctionalgroups,rings,stereocenters,andtheirrelativepositions. Theinformationiscentraltofindingthesolutionsoftheproblem.Thecomputeralso needstobetaughtthischemicalperceptionofthetarget.So,whenthedrawingofthe target is done and the connectivity tables have beenestablished, before starting the retrosyntheticprocess,theprogramsearchesforthesecharacteristicsandstoresthem

17.10ComputationalBasicsUnderlyingSyntheticStrategies inabinaryarraywhichisakindofidentitycardofthetarget. Anexampleofapartial binaryarrayforstructureAisgiveninTable17.3. Table17.2.BondtableforcompoundA Bondnumber Atom1 Atom2 Bondtype 1 1 2 2 2 1 3 1 3 1 4 1 4 4 5 3

Table17.3.BinaryarrayforcompoundA

Groups C N O CH CH 2 CH 3 Ketone Nitrile Cyclic atom atom number 1 1 0 0 0 0 0 1 0 0 2 0 0 1 0 0 0 0 0 0 3 1 0 0 0 0 1 0 0 0 4 1 0 0 0 0 0 0 1 0 5 0 1 0 0 0 0 0 0 0 With the topological tables and this analytical one, the program gains a chemical perceptionofthetarget. (b) Use of Matrices In 1971 Ugi and Gillespie [8] proposed the concept of 'BE' (for bondelectron) matricestodescribemolecules.'BE'matriceswerethentransformedintoconnection tables. In this model the element (i, j) ofthematrixcorrespondstothebondorder betweenatomsiand j (l=singlebond,2=doublebond,...)andthediagonalelements (i,i)describethefreevalenceelectronsofatomi.CompoundAisdescribedbythe matrixofequation(1):

SyntheticStrategiesinChemistry 17.11

1 2 3 4 5 1 02110 2 22000 3 10000 (1) 4 10003 5 00031 (c) Use of Numerical Linear Notation In1971,Hendricksonproposedalogicaldescriptionofstructuresandreactionsbya simplemathematicalmodel[9]whichhasbeendevelopedandusedintheprogram SYNGEN. This mainly focuses on carbon skeleton as shown for a three carbon systemofcompoundB.Intheoriginalsystemfourkindsofattachmentstoanycarbon aredescribedandcounted:Hforattachmentofhydrogen,orelectropositiveatoms;R forσbondtoanothercarbon;ΠforπbondtoanothercarbonandZforabond (σor π)toanelectronegativeheteroatom(N,O,S,X).TheH,R,ΠandZarenotations usedintheprogram.Howmanynumbersofsuchattachmentsisdescribedby h, σ ,π, z,respectively.Thefunctionality( f)atacarbonsiteisdefinedasthesumofzandπ, andthecharacter (c) ofacarbonsiteisc=l0σ+ f. InSYNGENthefunctionalgroupsoneachcarbonatomareabstractedwiththetwo digits z and π; for atom 2 of Scheme 7, the zπ list is 11, for atom 3 it is 30. 1 Cl Scheme 7 H2CC 2 C N 3 CompoundB

atom σσσ z πππ f c

1 1 0 1 1 11

2 2 1 1 2 22

3 1 3 0 3 13

II. Coding of a Particular Reaction The three main systems described above lead to three main methods for coding reactionsinCASDprograms.Thedescriptionofreactionsleadsalsototwofamilies ofCASDprograms:empiricalandnonempirical.Theempiricalprogramsarebased

17.12ComputationalBasicsUnderlyingSyntheticStrategies onknownreactionlibraries.Theadvantageofsuchanapproachisthattheprograms predict syntheses which have great chances of feasibility provided that specific structuralfeaturesdonotstronglyinterferewithoneoftheproposedreactions.Onthe other hand, these programs cannot suggest totally new synthesis reactions. Further, thenumberofreactionstocodeisverylarge;theoretically,allknownreactionsshould becodedinthereactionfiles. Inthenonempiricalprogramsreactionsarecodedinalogical,mathematicalor general way, in order to describe the maximum of reactions with a minimum of principles.Theaimisalsotohaveasystemabletoproposenewsyntheses,evennew reactionsiftheyhavenotyetbeendescribedintheliterature (a) The Transform Approach As indicatedabove,the word'transform'is employed to describe a reaction in the retrosyntheticdirection.TheaimofaCASDprogramistogeneratetheprecursorsof atarget.Todothis,aprogramhasthreemaintaskstoperform: Searchforasubstructurewhichdescribesthetransform(calledsynthonorretron) Generationoftheprecursors Evaluationofthevalidityofthesolutions. Themaindifferencesbetweenthevariousprogramscomefromtheevaluationstep, which may be more or less accurate. In SOS program, a full graphic interface has beendevelopedtoinputtransformandevaluationtests.Forexample,theuserdraws theretronanditsprecursorwiththemouse(Fig.17.1).Letusconsidertheexampleof the DielsAlder reaction. Symbol A stands for any atom in order to generalize the reaction.Sincethedienemayreactwithadoubleoratriplebond,onemayusethe simple/doublebondinthetargetanddouble/triplebondintheprecursor(bondswith dottedlines).

SyntheticStrategiesinChemistry 17.13

Fig.17.1.RepresentationofanactivewindowfromSOSprogram Toenteratest,thechemistselectsthisoptioninthereactionmenuandindicatesthat thetestmustbeappliedonthetarget.Theatomsandthebondsarenumberedandthe usermayenterthetestbyclickingonthebuttonsatthebottomofthescreen.Thetest allowsforasolutionsuchasthatifatom4issp 2andbond3isafusedbondthenthe reaction is impossible. This program also allows graphical tests: the user draws a substructureandtheactiontoperformifitispresentinthescrutinizedtarget. In SOS, coding of reactions by means of mechanisms (i.e., elimination, nucleophilicaddition,nucleophilicsubstitution,etc.)allowedchemiststogeneralize reactions,toreducethenumberofreactionsinthefiles,andtoextendaschemetoa newcaseevenifithasnotbeendescribedintheliterature. (b) Be-matrices Approach. UgiandDugundjidevelopedamathematicalmodelofconstitutionalchemistry[10]. This model is based on the concept of isomerism of molecules which has been extendedtoensemblesofmolecules.Forexample,atheoreticalreaction:A+B C + D can be seen as the conversion of an ensemble of molecules (A + B) into an isomericensemble(C+D).Asanextension,thediscoveringofasynthesis:Target  Precursor1 Precursor2 :::: Startingmaterials,maybedonebygenerationof isomers. Thedescriptionofmoleculesbymeansofmatricesallowsonetodescribe reactionsbyadditionsofmatrices;letustaketheadditionofHBronadoublebond (equation2).

17.14ComputationalBasicsUnderlyingSyntheticStrategies

1 1 2 2 H2C CH2 H2C CH2 (2) H Br H Br 4 3 4 3 Let B be the bematrix for the starting materials and E the bematrix for the end products(Scheme8). Scheme 8

1 2 3 4 1 2 3 4

1 0200 1 0101

2 2000 2 1010

3 0001 3 0100

4 0010 4 1000

B E Thetransformation of B intoE isdefined by the reactionmatrixRofequation(3) suchthat:B+R=Ewhereoffdiagonalentries Rij = Rji =0,±l,±2,±3,indicatethe bonds made or broken. This method is conceptually attractive because the retro synthesisissimply:B=ER.

1 2 3 4

1 010+1

R = 2 10+10 (3) 3 0+101

4 +1010

(c) The Numerical Approach The numerical approach is dealt by different ways by different programs. The SYNGENprogramisbasedupontheconceptofhalfreactions; the formation of a bondmaybeseenastwolinkedhalfreactionsoneachsideofthebond(Scheme9).

SyntheticStrategiesinChemistry 17.15

Scheme 9

C C CC C C γγγβββ ααα ααα βββ γγγ

CC C- +C C C γγγβββ ααα ααα βββ γγγ Nucleophilic Electrophilic Halfreaction Halfreaction Onlythreeatomsareconsideredaroundthebondwhichisformed,becausemorethan three carbons which change in a halfreaction are virtually never found [11]. The nucleophiliccentersaregiveninScheme10,andtheelectrophiliconesinScheme11. Scheme 10 ααα βββ γγγ - C C C C

Z CC C C Z

H C CC C C C Thus, combining these three nucleophilic halfreactions with the three electrophilic halfreactionsproducesninepossiblefullconstructionreactions.Areactionmayalso bedescribedbytwoletters,thefirstforthebondmade,secondforthatbroken. Scheme 11 ααα βββ γγγ

CZ C

H CC C CH

C CC Z C C C FortheMichaelreaction(Scheme12),thisconceptyieldsforatom2:formationofan σbond(+R)andlossofhydrogen(H),thereactiononthiscarbonisdesignatedby RH;foratom3:formationofanσbondandlossofaπbond:RΠandforatom4:H Π.

17.16ComputationalBasicsUnderlyingSyntheticStrategies

Scheme 12 O O

CCC CH C 11112222333344445555

O O

C CH + C CC

11112222333344445555 Sincetherearefourkindsofbond(H,R,Π,Z),thereare16possibleunitexchanges per carbon. This code allows a simple classification of reactions, for example ZH representsoxidationreactions(CH→COH),HZrepresentsreduction,theoppositeof thepreviousone;additionreactionsonacarboncarbondoublebondare:HΠ,RΠ, ZΠ, etc. This systematic definition of organic reactions provided a basis for developingCOGNOS,aprogramfororganizingandretrieving reactions in a large database[12]. III. Pruning the Synthesis Tree Thedescriptionswhicharegivenaboveconcernthecodingofonereactionandhowa precursormaybegenerated.Inabasicretrosynthesisprogram,eachreactionofthe file(orofthefiles)isappliedinturnforgeneratingprecursorsandbuilding,stepby step,thesynthesistree.Asindicated,thenumberofsolutionsfoundmaybelargeand itisnecessarytodevelopmethodsthatarestrategies,inordertoreducethistreeand totrytoselectthebestsolutions. Thesearchofthekeystepinasynthesisis,rather,afundamentalstep.Whenit hasbeenfound,onemaysaythatageneralplanofthesynthesishasbeenfound.This searchhasbeendoneinaverysimplewayintheSASprogram:thisprogramsimply deletedoneorseveralbondsinthetarget,suggestingideasofsynthesis.Forexample, inthecaseofellipticine(CompoundD),itsuggestedthatseveralinternalDielsAlder reactions are involved. Solutions similar to E and F have been subsequently and independently found experimentally by others groups of researchers [13] (Scheme 13).

SyntheticStrategiesinChemistry 17.17

Scheme 13

N NH

N H D E

NH NH N H N H

F G TheotherStrategiesalsoincludestartingmaterialorientedstrategy,topological Strategies,stereochemicalStrategiesandTacticalCombinationsofTransforms Strategyforpruningthesynthesistreefororganiccompounds.Theideaderived from this is also applicable to synthesis of Inorganic metal complexes, Energetic materials, Bio-molecules and drug design which are briefly describedinthefollowingsection. 2. INORGANIC MATERIALS (a) Structure Identification: Inprinciple,systemsforrepresentingorganiccompoundscanbeappliedtoinorganic substances,asfarascompoundsareconcernedinwhichtheatomsareconnectedby covalent bonds. The Chemical Abstracts Service (CAS) registry system uses the notion of a mixture to accommodate substances without structure, e.g., alloys, whereascovalentlybondedinorganiccompoundsrepresentedbytheconnectiontable (CT) based system of CAS. Salts are represented as mixtures of the possibly structuredionsconstitutingthesalt. In organic compounds an atom is generally connected to at most four neighbouringatomswhereasininorganiccompoundslargercoordinationnumbersis found and, consequently, a more involved stereochemistry is encountered. Furthermore, one has to deal with more complicated types of chemical bonds and sometimestherearewelldefinedsubstancesforwhichitisnotobvioushowtodrawa structureatall.Whereasthesubjectsoforganicchemistryareexclusivelycovalently bondedcompoundsofcarbon,inorganiccompoundsfurthermorecompriseionically

17.18ComputationalBasicsUnderlyingSyntheticStrategies bondedsubstanceslikesalts,alloysandglasses,differentkindsofsolutions,minerals, etc.Thereforearepresentationofinorganiccompoundsmustprovidesomemeansto deal not only with covalently bonded compounds but also with substances which belongtothoseothertypes[14].Thus,thegeneralizationleadstheconceptofmulti componentsystemsinwhicheachcomponentconsists of oneorseveral fragments. The fragments turn may be structured or not, i.e., it may be possible to draw a structure diagram for them or not. In the structure storage system of the Gmelin Database, compounds without a structure are taken into account by a tabular representation, the inorganic structure tables (1ST).ThehierarchyisshowninFig. 17.4.

Figure 4 Substance

Component1 Componentn

Fragment1 Fragmentn

Yes No Structure?

CT IST (b) Methodology Molecular mechanics (MM) calculations have become increasingly important in understanding the structures and steric interactions that occur in inorganic, bioinorganic, and organometallic compounds. Since 1984, the growing use of the MMmodelininorganicchemistryhasbeendocumentedinanumberofreviews[15]. Metal complexes that have been treated with MM typically involve one metal and multidentateorganicligands.Thecommonfunctionalgroupsfoundintheseligands include amines, imines, pyridines, amino acids, alcohols, ethers, carboxylic acids, thiols, sulfides, and phosphines. Many MM models, developed originally for

SyntheticStrategiesinChemistry 17.19 application to organic molecules, have the capability to treat the ligand part of the metal complex. These organic MM models have been extended to include metal ligandinteractionswiththeadditionofrelativelyfewterms.Variousapproachesare distinguishedbythetypesofmetaldependentinteractionthatareused. The two bonded methods in common use are the valence force field (VFF) methodandthepointsonasphere(POS)method.Inbothmethodsthemetalion,M, isformallyconnectedtotheliganddonoratoms,L.InanormalorganicMMprogram, thisconnectionwillcreateMLbonds,MLXandLMLbondangles,MLXX andXMLXtorsions,andMXnonbondedinteractions.Theuserdecideswhich oftheseinteractionswillbeusedinthecalculationthroughachoiceofparameters. TheVFF andPOS methodsdifferonlyinthetreatmentofLM Langles. Inthe VFFmethod,LMLbondangleinteractionsareusedtodefineageometryprefer enceaboutthemetalcenter.InthePOSmethod,LMLbondangleinteractionsare notusedandgeometrypreferenceaboutthemetalcenterderivesprimarilyfrom1,3 vanderWaalsinteractionsbetweenthedonoratoms.Thethirdmethodisnonbonded methodwherethemetalionisnotformallyconnectedtotheliganddonors;themetal ligand complex is modeled with a collection of pairwise electrostatic and van der Waalsinteractions.Thethreemethodswhicharecommonlyusedfordenotingthese interactionsaresummarizedinTable17.4. Table17.4.MethodstoextendMMmodelsformetalcomplexes No Interaction Bonded Non-bonded VFF POS Ionic 1 MLstretch yes yes no 2 MLXbend yes yes no 3 LMLbend yes no no 4 LLnonbonded no yes yes 5 MLXXtorsion yes yes no 6 LMLXtorsion no no no 7 MLnonbonded no no yes 8 MXnonbonded no no yes Apartfromthis,thegeometryoptimizationandfrequencyanalysiswillgiveanidea aboutthestabilityofthemoleculesforlaboratorysynthesis.Itisalsopossibletoget structurereactivity relationship for metal complexes. The concept of structure

17.20ComputationalBasicsUnderlyingSyntheticStrategies reactivity relationship implies that changes in structure should be quantitatively reflectedinsomemeasurablereactivityparametersassociatedwiththemolecule.For metalcomplexes,thecapacityoftheligandstructuretoinfluencechemicalproperties is measurable in terms of reactivity parameters such as stability constants, rates of liganddissociation,andreductionpotentials.Theinfluenceofstructureonchemical reactivitycanoftenberationalizedintermsofstericandelectroniccomponentswhich module the synthesis of a particular compound. Same way, the structurereactivity playsimportantroleinconstructingenergeticmaterialswhichisoutlinedbelow. 3. ENERGETIC MATERIALS Energeticmaterialsencompassdifferentclassesofchemicalcompositionsoffueland oxidantthatreactrapidlyuponinitiationandreleaselargequantitiesofforce(through thegeneration ofhighvelocity product species) or energy (inthe form of heat and light).Theseparticularfeatureshavebeenadvantageouslyemployedinawidevariety ofindustrialandmilitaryapplications,butoftentheseutilizationshavenotbeenfully optimized, mainly due to the inability to identify and understand the individual fundamentalchemicalandphysicalstepsthatcontroltheconversionofthematerialto itsfinalproducts.Theconversionofthematerialisusuallynottheresultofasingle stepreaction,orevenasetofafewsimpleconsecutivechemicalreactions.Rather,it is an extremely complex process in which numerous chemical and physical events occur in a concerted and synergistic fashion, and whose reaction mechanisms are stronglydependentonawidevarietyoffactors. Also, these processes often occur under extreme conditions of temperature and pressures, making experimental measurement difficult. These are but a few of the complexities associated with studies of reactions of energetic materials that make resolving the individual details so difficult. These difficulties have required the developmentofavarietyofinnovativetheoreticalmethods,modelsandexperiments designedtoprobedetailsofthevariousphenomenaassociatedwiththeconversionof energeticmaterialstoproducts[16]. Thehightimeandpecuniary costsassociatedwiththesynthesisorformulation, testing and fielding of a new energetic material has called for the inclusion of modelingandsimulationintotheenergeticmaterialsdesignprocess.Thishasresulted in growing demands for accurate models to predict properties and behavior of notionalenergeticmaterialsbeforecommittingresourcesfortheirdevelopment.For example, in earlier times, extensive testing and modification of proposed candidate

SyntheticStrategiesinChemistry 17.21 materialsformilitaryapplicationscouldtakedecadesbeforethematerialwasactually fielded, in order to assure the quality and consistent performance of the Predictive modelsthatwillallowforthescreeningandeliminationofpoorcandidatesbeforethe expenditure of time and resources on synthesis and testing of advanced materials promisesignificanteconomicbenefitinthedevelopmentofanewmaterial. 4. BIO-MOLECULES Computational bio-modeling refers to a type of artificial life research concerned with building computer simulations of biological systems (biomodeling). The immediate goal is to understand how biological entities such as cells or whole organisms,develop,workcollectively,andsurviveinchangingenvironmentsusinga purelycomputationalmodel.Inordertomeetthischallengeweneedtoestablishthe methodologiesandtechniquesthatwillenableustogainasystemlevelunderstanding ofbiologicalprocesses.Thesekindsofmodelsholdgreatpromisefornewdiscoveries inawidevarietyofbiologicalsystems.Onceanexecutablemodelhasbeenbuiltofa particularsystem,itcanbeusedtogetaglobaldynamicpictureofhowthesystem responds tovarious perturbations. In addition, preliminary studies can be quickly performedusingexecutablemodels,savingvaluablelaboratorytimeandresourcesfor onlythemostpromisingavenues. One way of approaching this problem is to use ideas and methods originally developed in computer science,mainly insoftware and systemsengineering (andin particularvisuallanguagesandformalverification)toconstruct,simulateandanalyze biologicalmodels.Thebenefitofmolecularmodelingisthatitreducesthecomplexity of the system, allowing many more particles (atoms) to be considered during simulations. The types of biological activity that have been investigated using molecular modeling include protein folding, enzyme catalysis, protein stability, conformational changes associated with biomolecular function, and molecular recognitionofproteins,DNA,andmembranecomplexes. Softwares for bio-models  SPiM TheStochasticPiMachine(SPiM)isasimulatorforthestochasticpicalculusthat can be used to simulate models of Biological systems. The machine has been formallyspecified,andthespecificationhasbeenprovedcorrectwithrespectto thecalculus.

17.22ComputationalBasicsUnderlyingSyntheticStrategies

 ScatterWeb.NET SDK ScatterWeb.NETSDKisanewapproachtoworkingwithwirelesssensor networks.Ithidesthecomplexityofembeddedprogrammingandmakesiteasyto handleobjectsrepresentingwirelesssensors. 5. DRUG DESIGN Drug design is presently the most prominent and visible application ofinformation processing in chemistry. Clearly this is due to the large scientific and economic investmentnecessaryforthedevelopmentofanewdrug.Effortsaremadetocombine all possible means for developing an understanding of the relationships between structure and biological activity. Consequently, this is an area where quantum mechanicalandmolecularmechanicscalculationsareeffectivelyemployed. CONCLUSIONS The collected examples explain the use of computational methods in synthesis of organic compounds and also extendable to inorganic and energetic materials. Researchers are focusing on creating the computational tools that will enable biologists and others working in the life sciences to better understand and predict complex processes in biological systems, which could revolutionize our understanding of disease, and lead to new and faster insights into entirely novel therapiesandbettervaccines. REFERENCES 1. E.J.Corey, PureAppl.Chern., 14(1967)19. 2. S.1.Salley, J.Am.Chem.Soc., 89(1967)6762. 3. H.S.Rzepa,"ChemistryandtheWorldWideWeb",chapterin"TheInternet: AGuideforChemists",Ed.S.Bachrach,AmericanChemicalSociety,1995. Seehttp://www.ch.ic.ac.uk/java/HyperSpec 4. E.J.CoreyandW.T.Wipke, Science, 166(1969)178. 5. M.BersohnandA.Esack, Chern.Rev., 76(1976)269. 6. E.J.Corey,R.D.Cramer,III,andW.J.Howe, J.Am.Chem.Soc., 94(1972) 440 7. S.Hanessian,J.Franco,G.Gagnon,D.Laramee,andB.Laruche, J.Chem.Inf.Cornput.Sci., 30(1990)413. 8. I.UgiandP.Gillespie, Angew.Chem.,Int.Ed.Engl., 10(1971)914. 9. J.B.Hendrickson, J.Am.Chem.Soc. ,93(1971)6847.

10. I.DugundjiandI.Ugi, Top.Curr.Chem., 39(1973)19.

SyntheticStrategiesinChemistry 17.23

11. J.B.Hendrickson,D.L.Grier,andA.G.Toczko, J.Am.Chem.Soc., 107 (1985)5228. 12. J.B.HendricksonandT.L.Sander, Chen.Eur.J., 1(1995)449. 13. E.DifferdingandL.Ghosez, TetrahedronLett., 26(1985)1647. 14. N.Allinger,EncyclopediaofComputationalChemistry,15(1997)1320. 15. N.Allinger,EncyclopediaofComputationalChemistry,15(1997)1580. 16. P.PolitzerandJ.Murray,TheoreticalandComputationalChemistry: EnergeticMaterials,Part1.Decomposition,CrystalandMolecularProperties, 2003,ElsevierPubs.

Abbreviations NMR–NuclearMagneticResonance GAMESSTheGeneralAtomicandMolecularElectronicStructureSystem MOPACMolecularOrbitalPACkage SYNGENSYNthesisGENerator SOSSimulatedOrganicSynthesis SASSimulatedAnalyticalSynthesis