DOCSLIB.ORG

Free Radical Cyclisations

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Free Radical Cyclisations rl zel-rf4 FREE RADICAL CYCLISATIONS A THESIS PRESENTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the UNIVERSITY OF ADELAIDE by T0NY LAWRENCE, B.Sc. (Hons. ) Departnrent of Organic Chemistry 1980 +: CONTENTS SUMMARY I STATEMENT II ACKNOhILEDGEMENTS III INTRODUI]TÏ()N 1 Background chemìstry 2 Outljne of the research project 21 Rates of cyclisation 27 Energies of activation 30 HEX.s.EN-1-YL RADICAL 32 2-METHYLHEX-5-EN-1-YL and 3-METHYLHEX-5-EN-1-YL RADICALS 43 2,z-DII4ETHYLHEX- s-EN- L-YL and 3,3-DIMETHYLHEX-s-EN-1-YL RADICALS 60 2,2,s.IRIMETHYLIIEX- 5- EN - 1.YL RADI CAL 78 2,2-DIIIETHYLPtNT-4-EN-1-YL and 3,3-DIMETHYLPENT-4-EN-1-YL RADICALS 97 OCT.7.ENE.2-YL RADICAL 108 4-METHYLHEX.5.EN.l-YL RADI CAL 118 HEX-s-YN-l-YL, HEPT-6-YN-l-YL and OCT-7-YN-l-YL RADICALS 131 5,6-EP0XYIjEXIìN- 1-YL RADI CAL r52 AFTËRl^IARD 159 EXPERIMENTAL SECI ION 163 Methods of reduction r64 Methods of analysis 17I Syntheti c rnethods 176 RE FERENCES 244 I SUMMARY The fol'ìowing free radicals were generated in solutíon by the reaction of tri-n-butyltin hydrÍde with the correspond'ing bromocompounds: Hex-5-en-1-yl, 2-Methyl hex-S-en-1-yl, 3-Methy'l hex-5-en-1-yl' 4-Methyl hex-5-en-1-yl, 2,2-Dimethy'l hex-5-en-1-yl, 3-3-Dimethyl hex-5-en-1-yl, 2,?-,5-Trimethyì hex-5-en-1-y'1, 2,2-Dimethyl pent-4-en-1-y1, 3,3-Dimethyl pent-4-en-1-yl, 1-Methylhept-6-en-1-y1, Hex-S-yn-1-y1, Hept-6-yn-1-yl' Oct-7-yn-l-yl, and 5,6-Epoxyhexan-1-yl . Under careful'ly controlled corditions each brcmide was reduced with tri-n-butyltin hydride in benzene, and the relative yieìcls of cycìic and acJ¿clic hyd:"ocarbon products were deiernrined by gas'liquid cltroma- tograph'ic ana.lysis. The rates of intramolecular cyclisation of each radical relative to its rate of hydrogen atom abstraction from tri-n-butyltin hydride were determined over a range of temperatures and reagent concentrations. Energies of activation were calcujated. Structures of the cyclic transìtion states were studied by analysing the ci's and lttans isomer djstributions of 1,2-d'imethyl- and jons 1,3-dìnrethy'lcycl oal kanes o whi ch were formed by i ntramol ecul ar acldi t of mononrethylaì kenyì radicals. Where ìt appeared possible, atternpts urere made to re1ate these s'Ludies to earlier work in free radical cyclisation. The ex'isting explanat'ions for the selective exo-cyclisation of the hex-5-en-1-yì radjcal were evaluated in the light of present observations. Summary of the results: (1) To the extent that they do cyclise the above radicals undergo selective and irreversible øxo-cyc'lisation, with the sole except- ion of the 2,2,5-trimethylhex-5-en-1-yl radical whjch undergoes both ¿xo- and øndo-cyc'lisatjons at comparable rates. (2) Methy'lsubstitution at C2, or C3, or C4 increases the rate of intramolecular cyc'lìsation of the hex-5-en-1-yl radical in djrect relaticnsh'ip to the extent of the tnethyì induced gauche inter- act'ions. Lowered enthalpy of activatjon is observed with these rate enhancements without significant changes in the entropy of acti vati on . (3) Intramolecular cycl isat,íon of the 2,2,S-trimethy'l hex-5-en-1-y'l radical contradicts the hypothes'is that selective 1,5-cycl isation of the hex-5-en-1-yl rad'ical js caused by through space inter- actions of hydrogens at C2 and C6, which bar the format'ion of t.he transjt'ion state for 1,6-cyclisation. (4) Mononrethyl substi tut'ion of the hex-5-en- 1-y'l rad i cal at ei ther C2,or C3,or C4 exerts control' in the selective formatjon of ci.t or üan^ stereo'isomers of dimethy'lcycl opentane. Thi s i s consistent wjth a chaìr-ljke cycl'ic conformation of the transition state in whìch the.: methyl group ìs predonlinantly in an equatorial ori entation . (5) The following free radicals do not undergo intramolecular cyc'l ì sation : 2,2-Dinrethyl pent-4-en- 1-y1, 3, 3-Dimethyl pent-4-en- 1- yl , and 5,6-Epoxyhc'xan-1-yl . II STATEMENT This thesis contains no material previously submitted for a degree or diploma in any University, and to the best of my knowledge and belief, contains no material previous'ly published or written by another person, except where due reference is made in the text. TONY LAWRENCE. III ACKNOI^JLEDGEMTNTS At its best the work presented in this thesis js but a small addition to the results of research in free radical cyclisations, which were available to me during my studjes. 0f particular relevance were the reports jn free radical cycfisa- tions by K.U. Ingold, C. lnlallirg, M. Juìia, and A.L.J. Beckwith. I have endeavoured to do my work as well as they did theirs. This project was supervised by Professor A.L.J. Beckwith, and by Dr. G.E. Gream during the absence of Professor Beckwith in t979. The whole manuscript rvas proof-read by Dr. R.H. Prager. Throughout jts duratjon thjs work was supported by a Commonweal th Postgraduate Scholarshi p. 1 INTRODUCTI ON Backqround chemistry This is a study of carbon radicals. In all cases the unpaired electron is vlhol'ly centred on one carbonl There is no deiocaljsation of the unpa'ired electron onto neìghbouring groups by conjugation. There is no l¡eteroatom within the molecule which could change the free spin density on the radical carbon. l,rlith but one exception all are primary carbon radicals, one beìng a secondary carbon radical. The followjng radicals were investigated in the course of this work. I o a a a ilt * To the e.xte-nt that.Lt. dou ex,í,tt, el..¿ettton de..LocæLtsctlirtn bq hgputcctttiugalion i,s tty.(i]¿c(1¡ .{o lx*,e /¿igt+L(ic.rutt.in{Luer.c.e u¡ttsn thrz next'(-t''s o{ 'l'hi-t LaltLl¿' 3 There are many ways to generate such free spin.bearing reactive inter- medíates in so]ution. Some of the methods used to generate hex-S-en-l-y'l radica'l - the first one listed above - involved the following reactions: (1) Thermoìysis of di-6-heptenoyl perox'ide1. -Þ 2 (2) Reduction of 6-mercapto-1.-hexene with triethy'lphosphite H (3) Kolbe's electrolysis of 6-heptenoic acid 3 + c0, 4 4 (4) Reaction of 6-bromohex-1-ene with sodjum naphthalene (5) Reaction of di-6-heptenoyì peroxide with copper acetates. (6) Reduction cl't 6-chloro or 6-bromohex-1-ene with tri-n-butyl tin hydride6'31. r a l- n Bu SnH + nBu3SnBr + RH (7) Reaction of hex-S-enyl-1-mercuric bromjde with sodjunr borohydricle - reductjve demercurationT. (S) Reduction of hex-5-enyl -1-mercuric bromide with tri-n-buty'lt'in hydr i cleB . Reductìon of aìky1 halides with tri-n-butyltìn hydride js a simpìe and efficient nethod for generatìng free radicals in solution. It is the method of choice, and the only method empìoyed for generating free radicals in thìs work. Some twenty years ago it was known that organotin hyclrides react w'ith al kyl hal ides9'tO On rep'ìacing the aì ky'l halogen vrith tlre t'in hydrogen, thus producing organotin halides and hydrocarbons. In 1962-64 Kuivila and co-workers undertool< mechanist'ic stuCies in the reduction of aìkyl halides by tri-n-butyltìn hydridell-13. Their investigations led them to the conclusjon that the reduction of aìkyl hal'ides by tin hydrìde involves free rad'ical reactjve intermedjates. 0n the basjs of their observations they proposed the following free radi cal cha'in mechani rnl13 . 5 'l.vu,ti¡tiion Initiator + R-X---c> R' Pnopagation R' + BurSnH --Þ RH + Bu3Sñ BurSñ + R-X -_ÞR'+ Bu3SnX TunLwLLon R'+ R'-> p-o R' + Bu3Sh _-Þ> BurSn-R InneÍivø ytrtoduct6 BurSñ + BurSh .---> BurSn-Sn Bu 3 Non-stabilized radicals of the hex-5-en-1-y'l type are hi9h1y reactive. Their free spin bearing carbons are not sterically crolded, and such radicals t^Jil1 undergo coupling reactions at diffusion controlled rates. In a steady state process, where the concentration of free radicals is low and does not change with the reaction time, the extent of coupling reactjons is likewise very low. Once generated a free radical jn solution may undergo one or more of the fol'lovring reactions: {1) Disproportionation (2) Intermolecular hydrogen atom abstraction (3) Intramolecular hydrogen atom abstractÍon (4) Intramolecular rearrangement When a radical in solutjon undergoes two or more irreversjble intermolecular reactions with reactants of identjcal concentrations the relative yield of each product is dependent only on the rate constattt of its formation. Likeu'ise the relative amounts of products frorn two or more irreversible intramolecular reactions of the sarne reactive intermediate are direct'ly related to their respec'bive rate constants. Reduction of bromoal kenes with tri-n-buty'ltin h-vdride i s a steady state process - the concentration of the free radical reactive inter- nlediates is constant throughout the reaction. Here the extent of intrarno.lecular rearrangements depend only on tinle and the reaction rate constani:s. l'ligh conceirtrations 6 of reactants lead to an increased number of intramo'lecular collisions per unjt time, and to relative'ìy higher yields of products from irreversjble intermolecular react'ions.
Load more

Recommended publications
  • An Indicator of Triplet State Baird-Aromaticity
    inorganics Article The Silacyclobutene Ring: An Indicator of Triplet State Baird-Aromaticity Rabia Ayub 1,2, Kjell Jorner 1,2 ID and Henrik Ottosson 1,2,* 1 Department of Chemistry—BMC, Uppsala University, Box 576, SE-751 23 Uppsala, Sweden; rabia.ayub@kemi.uu.se (R.A.); kjell.jorner@kemi.uu.se (K.J.) 2 Department of Chemistry-Ångström Laboratory Uppsala University, Box 523, SE-751 20 Uppsala, Sweden * Correspondence: henrik.ottosson@kemi.uu.se; Tel.: +46-18-4717476 Received: 23 October 2017; Accepted: 11 December 2017; Published: 15 December 2017 Abstract: Baird’s rule tells that the electron counts for aromaticity and antiaromaticity in the first ππ* triplet and singlet excited states (T1 and S1) are opposite to those in the ground state (S0). Our hypothesis is that a silacyclobutene (SCB) ring fused with a [4n]annulene will remain closed in the T1 state so as to retain T1 aromaticity of the annulene while it will ring-open when fused to a [4n + 2]annulene in order to alleviate T1 antiaromaticity. This feature should allow the SCB ring to function as an indicator for triplet state aromaticity. Quantum chemical calculations of energy and (anti)aromaticity changes along the reaction paths in the T1 state support our hypothesis. The SCB ring should indicate T1 aromaticity of [4n]annulenes by being photoinert except when fused to cyclobutadiene, where it ring-opens due to ring-strain relief. Keywords: Baird’s rule; computational chemistry; excited state aromaticity; Photostability 1. Introduction Baird showed in 1972 that the rules for aromaticity and antiaromaticity of annulenes are reversed in the lowest ππ* triplet state (T1) when compared to Hückel’s rule for the electronic ground state (S0)[1–3].
    [Show full text]
  • Cycloalkanes, Cycloalkenes, and Cycloalkynes
    CYCLOALKANES, CYCLOALKENES, AND CYCLOALKYNES any important hydrocarbons, known as cycloalkanes, contain rings of carbon atoms linked together by single bonds. The simple cycloalkanes of formula (CH,), make up a particularly important homologous series in which the chemical properties change in a much more dramatic way with increasing n than do those of the acyclic hydrocarbons CH,(CH,),,-,H. The cyclo- alkanes with small rings (n = 3-6) are of special interest in exhibiting chemical properties intermediate between those of alkanes and alkenes. In this chapter we will show how this behavior can be explained in terms of angle strain and steric hindrance, concepts that have been introduced previously and will be used with increasing frequency as we proceed further. We also discuss the conformations of cycloalkanes, especially cyclo- hexane, in detail because of their importance to the chemistry of many kinds of naturally occurring organic compounds. Some attention also will be paid to polycyclic compounds, substances with more than one ring, and to cyclo- alkenes and cycloalkynes. 12-1 NOMENCLATURE AND PHYSICAL PROPERTIES OF CYCLOALKANES The IUPAC system for naming cycloalkanes and cycloalkenes was presented in some detail in Sections 3-2 and 3-3, and you may wish to review that ma- terial before proceeding further. Additional procedures are required for naming 446 12 Cycloalkanes, Cycloalkenes, and Cycloalkynes Table 12-1 Physical Properties of Alkanes and Cycloalkanes Density, Compounds Bp, "C Mp, "C diO,g ml-' propane cyclopropane butane cyclobutane pentane cyclopentane hexane cyclohexane heptane cycloheptane octane cyclooctane nonane cyclononane "At -40". bUnder pressure. polycyclic compounds, which have rings with common carbons, and these will be discussed later in this chapter.
    [Show full text]
  • Alkenes and Alkynes
    02/21/2019 CHAPTER FOUR Alkenes and Alkynes H N O I Cl C O C O Cl F3C C Cl C Cl Efavirenz Haloprogin (antiviral, AIDS therapeutic) (antifungal, antiseptic) Chapter 4 Table of Content * Unsaturated Hydrocarbons * Introduction and hybridization * Alkenes and Alkynes * Benzene and Phenyl groups * Structure of Alkenes, cis‐trans Isomerism * Nomenclature of Alkenes and Alkynes * Configuration cis/trans, and cis/trans Isomerism * Configuration E/Z * Physical Properties of Hydrocarbons * Acid‐Base Reactions of Hydrocarbons * pka and Hybridizations 1 02/21/2019 Unsaturated Hydrocarbons • Unsaturated Hydrocarbon: A hydrocarbon that contains one or more carbon‐carbon double or triple bonds or benzene‐like rings. – Alkene: contains a carbon‐carbon double bond and has the general formula CnH2n. – Alkyne: contains a carbon‐carbon triple bond and has the general formula CnH2n‐2. Introduction Alkenes ● Hydrocarbons containing C=C ● Old name: olefins • Steroids • Hormones • Biochemical regulators 2 02/21/2019 • Alkynes – Hydrocarbons containing C≡C – Common name: acetylenes Unsaturated Hydrocarbons • Arene: benzene and its derivatives (Ch 9) 3 02/21/2019 Benzene and Phenyl Groups • We do not study benzene and its derivatives until Chapter 9. – However, we show structural formulas of compounds containing a phenyl group before that time. – The phenyl group is not reactive under any of the conditions we describe in chapters 5‐8. Structure of Alkenes • The two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°. 4 02/21/2019 Structure of Alkenes • Figure 4.1 According to the orbital overlap model, a double bond consists of one bond formed by overlap of sp2 hybrid orbitals and one bond formed by overlap of parallel 2p orbitals.
    [Show full text]
  • Stereospecific Fragmentations in the Mass Spectra of Stereoisomeric Isoindoloquinazolines
    Stereospecific Fragmentations in the Mass Spectra of Stereoisomeric Isoindoloquinazolines Vladimir V. Ovcharenko and Kalevi Pihlaja Structural Chemistry Group, Department of Chemistry, University of Turku, Turku, Finland Ge´za Sta´jer Institute of Pharmaceutical Chemistry, University of Szeged, Szeged, Hungary The mass spectrometric behavior of stereo- and regioisomeric, partially saturated isoindolo- quinazolines was studied by positive-ion electron ionization (EI) and fast-atom bombardment (FAB/LSIMS) mass spectrometry combined with collision-induced dissociation (CID). A highly stereospecific retro-Diels-Alder process was observed in the cyclohexene-fused isomers under the EI conditions, and a corresponding (although less specific) fragmentation was observed in their FAB spectra. In the absence of RDA fragmentations, regio- and stereoisomers of the cyclohexane-fused heterocycles could be distinguished based on their FAB/CID spectra. (J Am Soc Mass Spectrom 2003, 14, 1049–1056) © 2003 American Society for Mass Spectrometry arious condensed-skeleton heterocycles contain- behavior of these compounds under the EI and FAB ing up to three rings can be prepared starting conditions is reported in the present study. Vfrom stereoisomeric aminobicycloalkanecar- The choice of experimental methods was dictated by boxylic acids by applying a retro-Diels-Alder (RDA) the following considerations. Mass-spectrometric RDA decomposition as a part of the synthetic strategy. For fragmentations often proceed in a regio- or stereospe- instance, the reaction of 3-aminobicyclo[2.2.1]hept-5- cific manner [4, 5]. The structure-specificity of RDA ene-2-carboxylic acid with formylbenzoic acid followed phenomena have previously been studied by us in the by a mild thermolysis of the pentacyclic intermediate mass spectra of stereoisomeric polycondensed hetero- afforded the novel 1,3-oxazino[2,3-a]indole-2,6-dione cycles using various ionization methods, such as elec- [1].
    [Show full text]
  • Reactions of Alkenes and Alkynes
    05 Reactions of Alkenes and Alkynes Polyethylene is the most widely used plastic, making up items such as packing foam, plastic bottles, and plastic utensils (top: © Jon Larson/iStockphoto; middle: GNL Media/Digital Vision/Getty Images, Inc.; bottom: © Lakhesis/iStockphoto). Inset: A model of ethylene. KEY QUESTIONS 5.1 What Are the Characteristic Reactions of Alkenes? 5.8 How Can Alkynes Be Reduced to Alkenes and 5.2 What Is a Reaction Mechanism? Alkanes? 5.3 What Are the Mechanisms of Electrophilic Additions HOW TO to Alkenes? 5.1 How to Draw Mechanisms 5.4 What Are Carbocation Rearrangements? 5.5 What Is Hydroboration–Oxidation of an Alkene? CHEMICAL CONNECTIONS 5.6 How Can an Alkene Be Reduced to an Alkane? 5A Catalytic Cracking and the Importance of Alkenes 5.7 How Can an Acetylide Anion Be Used to Create a New Carbon–Carbon Bond? IN THIS CHAPTER, we begin our systematic study of organic reactions and their mecha- nisms. Reaction mechanisms are step-by-step descriptions of how reactions proceed and are one of the most important unifying concepts in organic chemistry. We use the reactions of alkenes as the vehicle to introduce this concept. 129 130 CHAPTER 5 Reactions of Alkenes and Alkynes 5.1 What Are the Characteristic Reactions of Alkenes? The most characteristic reaction of alkenes is addition to the carbon–carbon double bond in such a way that the pi bond is broken and, in its place, sigma bonds are formed to two new atoms or groups of atoms. Several examples of reactions at the carbon–carbon double bond are shown in Table 5.1, along with the descriptive name(s) associated with each.
    [Show full text]
  • II. Nomenclature Rules for Alkenes 1. the Parent Name Will Be the Longest
    1 Lecture 9 II. Nomenclature Rules For Alkenes 1. The parent name will be the longest carbon chain that contains both carbons of the double bond. Drop the -ane suffix of the alkane name and add the –ene suffix. Never name the double bond as a prefix. If a double bond is present, you have an alkene, not an alkane. alkane + -ene = alkene 2. Begin numbering the chain at the end nearest the double bond. Always number through the double bond and identify its position in the longest chain with the lower number. In the older IUPAC rules the number for the double bond was placed in front of the stem name with a hyphen. Under the newer rules, the number for the double bond is placed right in front of “ene”, with hyphens. We will use the newer rules for specifying the location of pi bonds. 1 2 3456 H3CCHCH CH 2 CH2 CH3 hex-2-ene (newer rules) 2-hexene (older rules) 3. Indicate the position of any substituent group by the number of the carbon atom in the parent (longest) chain to which it is attached. CH 1 2 345 3 H3CCHCHCHCH2 CH CH3 6 7 CH3 Numbering is determined by the double bond, not the branches, because the double bond has 5,6-dimethylhept-3-ene (newer rules) higher priority than any alkyl branch. 5,6-dimethyl-3-heptene (older rules) 4. Number cycloalkenes so that the double bond is 1,2 (number through the double bond). Number in the direction about the ring so that the lowest number is used at the first point of difference.
    [Show full text]
  • Module 4 Alkenes and Alkynes Lecture 7 Alkenes 4.1 Introduction Hydrocarbons That Contain Carbon-Carbon Double Bond Are Called Alkenes (Also Called As Olefins)
    NPTEL – Biotechnology – Cell Biology Module 4 Alkenes and Alkynes Lecture 7 Alkenes 4.1 Introduction Hydrocarbons that contain carbon-carbon double bond are called Alkenes (also called as Olefins). Many alkenes are found in plant and animals. It has three sp2 orbitals that lie in a plane with angles of 120°. One of the carbon–carbon bonds in a double bond is σ-bond, formed by the overlap of a sp2 orbital of one carbon with a sp2 orbital of the other carbon. The second carbon–carbon bond in the double bond is formed from side-to-side overlap of the remaining p-orbitals of the carbons. These two p-orbitals must be parallel to each other to achieve maximum orbital-orbital overlap. Therefore, all six atoms of the double- bond system are in the same plane (Figure 1). Since there is maximum side-to-side overlap, rotation about a double bond does not occur. H H C C H H Figure 1 Each C-H σ-bond is formed by overlap of a sp2 hybrid orbital of carbon atom with the 1s orbital of a hydrogen atom. The C-H bond length in ethylene is slightly shorter than the C-H bond in ethane because the sp2 orbital in ethylene has more s character that attracts the electrons even more strongly. The C=C bond in ethylene is much shorter than the C-C bond in ethane, partly because the σ-bond of ethylene is formed from sp2 orbitals and partly because both σ- and π-bonds are attracting the atoms together (Figure 2).
    [Show full text]
  • Chem 150, Spring 2015 Unit 5
    Chem 150, Spring 2015 Unit 5 - Hydrocarbons Organic Chemistry and Hydrocarbons • Organic chemistry is the chemistry of carbon ✦ Carbon is unique among the elements in its ability to bond to itself and other elements in so many different ways. ✦ It was originally thought that only living systems could make organic molecules. ✦ Friedrich Wöhler demonstrated in 1828 that this was not the case. Chem 150, Unit 5: Hydrocarbons 2 Organic Chemistry and Hydrocarbons • Organic chemistry is the chemistry of carbon ✦ Carbon is unique among the elements in its ability to bond to itself and other elements in so many different ways. ✦ It was originally thought that only living systems could make organic molecules. ✦ Friedrich Wöhler demonstrated in 1828 that this was not the case. Chem 150, Unit 5: Hydrocarbons 2 Organic Chemistry and Hydrocarbons • Organic chemistry is the chemistry of carbon ✦ Carbon is unique among the elements in its ability to bond to itself and other elements in so many different ways. ✦ It was originally thought that only living systems could make organic molecules. ✦ Friedrich Wöhler demonstrated in 1828 that this was not the case. Chem 150, Unit 5: Hydrocarbons 2 9.1 The Special Properties of Carbon Chem 150, Unit 5: Hydrocarbons 3 Tetrahedral Geometry • When a carbon atom has four single bonds, the arrangement of atoms forms a tetrahedron. • The VSEPR model (Valence-shell electron pair repulsion model) predicts this geometry. Chem 150, Unit 5: Hydrocarbons 4 Lewis and Wedge-Dash Structures • Lewis structures are used even though they show bonds at right angles to one another. • To show the 3-dimensional structure chemists show perspective with wedges and dashed lines.
    [Show full text]
  • Alkenes and Alkynes
    04 Alkenes and Alkynes Carotene and carotene-like molecules are partnered with chlorophyll in nature to assist in the harvest of sunlight. In autumn, green chlorophyll molecules are destroyed and the yellows and reds of carotene and related molecules become visible. The red color of tomatoes comes from lycopene, a molecule closely related to carotene. See Problems 4.37 and 4.38. Inset: A model of b-carotene. (Charles D. Winters) KEY QUESTIONS 4.1 What Are the Structures and Shapes of Alkenes and HOW TO Alkynes? 4.1 How to Name an Alkene 4.2 How Do We Name Alkenes and Alkynes? CHEMICAL CONNECTIONS 4.3 What Are the Physical Properties of Alkenes and Alkynes? 4A Ethylene, a Plant Growth Regulator 4.4 Why Are 1-Alkynes (Terminal Alkynes) 4B Cis–Trans Isomerism in Vision Weak Acids? 4C Why Plants Emit Isoprene IN THIS CHAPTER, we begin our study of unsaturated hydrocarbons. An unsaturated hydro- carbon is a hydrocarbon that has fewer hydrogens bonded to carbon than an alkane has. There Alkene An unsaturated are three classes of unsaturated hydrocarbons: alkenes, alkynes, and arenes. Alkenes contain one hydrocarbon that contains or more carbon–carbon double bonds, and alkynes contain one or more carbon–carbon triple a carbon–carbon double bonds. Ethene (ethylene) is the simplest alkene, and ethyne (acetylene) is the simplest alkyne: bond. Alkyne An unsaturated H H hydrocarbon that contains a C C H C C H carbon–carbon triple bond. H H Ethene Ethyne (an alkene) (an alkyne) 108 Alkenes and Alkynes 109 ChemicalCheChmical i l ConnectionsConnect io ns 4A ETHYLENE, A PLANT GROWTH REGULATOR O Ethylene occurs only in trace amounts in nature.
    [Show full text]
  • Photocyclization of Arylethylenes: Mechanism and Scope of the Reactions
    Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 105, No. 6, December 1993, pp. 603-610. Printed in India. Photocyclization of arylethylenes: Mechanism and scope of the reactions R LAPOUYADE Photophysique et Photochimie Mol6culaire, UA du CNRS N ~ 348, Universit6 Bordeaux I, 33405 Talence, France Abstract. The photocyclization of three structural types of arylethylenes is systematically investigated in order to provide new pathways to regiospecifically substituted and/or partially hydrogenated polycyclic aromatic hydrocarbons. The scope of the photoformation of acenaphthenes from ~t-arylotefins,in amine solutions, is discussed: when the double bond is acyclic, only the naphthyl and the pyrenyl derivatives are photoreactive, but when the double bond is in a cycle, all the compounds which lead to the trans double bond photocyclize. The amine-mediated 1,3-H-shift, discovered with the preceding series, also occurs with 4a,4b-dihydrophenanthrene, formed by irradiation of 1,2-diarylethylenes, and leads to dihydrophenanthrenes. The photocyclization of 2-vinylbiphenyls is stereoselective from the singlet excited state but not from the triplet, where a fast equilibrium between E and Z configurations precedes the adiabatic cyclization. The radical cation of 2-vinylbiphenyls cyclizes to a phenanthrene radical cation which initiates a protic catalysis to the related fluorenes or, in presence of 2,6-di-t-butypyridine or water, leads stoichiometrically to phenanthrenes. Keywords. Arylethylenes; photocyclization; amine-mediated proton shift; polycyclic aromatic hydrocarbons. 1. Introduction Photocyclization of arylethylenes is the preferred method for synthesis of many different polycyclic aromatic hydrocarbons (PAH). There is an overwhelming amount of literature, recently surveyed (Laarhoven 1983; Mallory and Mallory 1984; Liu et al 1991), on the photocyclodehydrogenation of 1,2-diarylethylenes (stilbene-like) into angularly fused PAH (phenanthrene-like).
    [Show full text]
  • Klein 9-10 Alkenes Alkynes
    Grossman, CHE 230 Chapters 9–10. Additions across C=C π Bonds. We’ve learned a number of different substitution and elimination reactions of alkyl halides. We’ve learned that compounds with lone pairs are nucleophiles, and alkyl halides are electrophiles. Now we’re going to learn the reactions of a different class of nucleophiles: the C=C π bond in alkenes and alkynes. 10.1. Reactivity of Alkenes. The C=C π bond of an alkene is higher in energy than the C—H or C—C σ bond. As a result it can act as a nucleophile towards various electrophilic species. When a C=C π bond acts as a nucleophile, one of the C atoms takes the π electrons away from the other C and uses them to make a bond to the electrophile. The other C atom either becomes electron- deficient: The top C takes the pair of electrons from the π H H+ bond and uses it to make a σ bond to the H. The lower C has a pair of electrons taken away and becomes electron-deficient. or it gets a pair of electrons from the electrophile at the same time as the electrophile is getting a pair of electrons from the π bond: The top C takes the pair of electrons from the π bond and uses it to make a σ bond to one Br, Br Br displacing the other Br as a leaving group. Br+ Br– The electrophilic Br uses a lone pair to make a bond to the lower C to replace the π electrons that were taken away.
    [Show full text]
  • Heterogeneous Catalytic Hydrogenation of Aromatic Compounds: I
    Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1971 Heterogeneous catalytic hydrogenation of aromatic compounds: I. Deuteration and hydrogen exchange of mono- and polyalkylbenzenes, II. Selectivity in the reduction of diarylalkanes, III. Competitive hydrogenolysis of arylmethanols David Herbert Bohlen Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Bohlen, David Herbert, "Heterogeneous catalytic hydrogenation of aromatic compounds: I. Deuteration and hydrogen exchange of mono- and polyalkylbenzenes, II. Selectivity in the reduction of diarylalkanes, III. Competitive hydrogenolysis of arylmethanols " (1971). Retrospective Theses and Dissertations. 4944. https://lib.dr.iastate.edu/rtd/4944 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact digirep@iastate.edu. 72-5179 BOHLEN, David Herbert, 1938- HETEROGENEOUS CATALYTIC HYDROGENATION OF AROMATIC COMPOUNDS: I. DEUTERATION AND HYDROGEN EXCHANGE OF MONO- AND POLYALKYLBENZENES, II. SELECTIVITY IN THE REDUCTION OF DIARYLALKANES, III. COMPETITIVE HYDROGENOLYSIS OF ARYLMETHANOLS. Iowa State University, Ph.D., 1971 Chemistry, organic University
    [Show full text]