Chapter 9 Formation of Alkenes and Alkynes
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Elimination Reactions E1, E2, E1cb and Ei
Elimination Reactions Dr. H. Ghosh Surendranath College, Kol-9 ________________________________________________ E1, E2, E1cB and Ei (pyrolytic syn eliminations); formation of alkenes and alkynes; mechanisms (with evidence), reactivity, regioselectivity (Saytzeff/Hofmann) and stereoselectivity; comparison between substitution and elimination. Substitution Reactions Elimination Reactions Elimination happens when the nucleophile attacks hydrogen instead of carbon Strong Base favor Elimination Bulky Nucleophile/Base favor Elimination High Temperature favors Elimination We know- This equation says that a reaction in which ΔS is positive is more thermodynamically favorable at higher temperature. Eliminations should therefore be favoured at high temperature Keep in Mind---- Mechanism Classification E1 Mechanism- Elimination Unimolecular E1 describes an elimination reaction (E) in which the rate-determining step is unimolecular (1) and does not involve the base. The leaving group leaves in this step, and the proton is removed in a separate second step E2 Mechanism- Elimination Bimolecular E2 describes an elimination (E) that has a bimolecular (2) rate-determining step that must involve the base. Loss of the leaving group is simultaneous with removal of the proton by the base Bulky t-butoxide—ideal for promoting E2 as it’s both bulky and a strong base (pKaH = 18). Other Organic Base used in Elimination Reaction These two bases are amidines—delocalization of one nitrogen’s lone pair on to the other, and the resulting stabilization of the protonated amidinium ion, E1 can occur only with substrates that can ionize to give relatively stable carbocations—tertiary, allylic or benzylic alkyl halides, for example. E1-Elimination Reaction not possible here The role of the leaving group Since the leaving group is involved in the rate-determining step of both E1 and E2, in general, any good leaving group will lead to a fast elimination. -
Open PS Thesis - Clara Capparelli
The Pennsylvania State University The Graduate School Department of Material Science and Engineering INFLUENCE OF CARBON SPACERS AND ALKYL PENDANT CHAINS ON THE STABILITY OF QUATERNARY AMMONIUM CATIONS FOR ANION EXCHANGE MEMBRANES A Thesis in Material Science and Engineering by Clara Capparelli 2015 Clara Capparelli Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science August 2015 The thesis of Clara Capparelli was reviewed and approved* by the following: Michael A. Hickner Associate Professor of Materials Science and Engineering Thesis Advisor James Runt Professor of Polymer Science T.C. Mike Chung Professor of Material Science and Engineering Suzanne Mohney Professor of Material Science and Engineering and Electrical Engineering Chair, Intercollege Graduate Degree Program in Material Science and Engineering *Signatures are on file in the Graduate School iii ABSTRACT Proton and anion exchange membranes are of great importance in the function of fuel cells, one of the most promising technologies for renewable energy conversion. Proton exchange membrane fuel cells (PEMFC) have been studied extensively in the past couple of decades, and there have been tremendous advances in the development of these systems, especially in industries such as automotive and portable power. Anion exchange membranes (AEM) have caught the attention of scientists because they would allow for the development of fuel cells without costly precious metal catalysts, among other advantages. Efforts are being made in developing long-lived and high performance AEMs for fuel cell applications. Primarily, the focus in AEM research has been membrane stability. It has been observed that AEMs are not as stable as the state-of-the-art NAFION® PEM and demonstrations of cell performance beyond 1000 hours is rare. -
Recent Advances in the Direct Nucleophilic Substitution of Allylic
SHORT REVIEW ▌25 Recentshort review Advances in the Direct Nucleophilic Substitution of Allylic Alcohols through SN1-Type Reactions AlejandroSN1 Reactions of Allylic Alcohols Baeza,* Carmen Nájera* Departamento de Química Orgánica and Instituto de Síntesis Orgánica, University of Alicante, Apdo.99, 03080 Alicante, Spain Fax +34(965)903549; E-mail: [email protected]; E-mail: [email protected] Received: 03.10.2013; Accepted after revision: 06.11.2013 Abstract: Direct nucleophilic substitution reactions of allylic alco- hols are environmentally friendly, since they generate only water as a byproduct, allowing access to new allylic compounds. This reac- tion has, thus, attracted the interest of the chemical community and several strategies have been developed for its successful accom- plishment. This review gathers the latest advances in this methodol- ogy involving SN1-type reactions. 1 Introduction 2SN1-Type Direct Nucleophilic Substitution Reactions of Allylic Alcohols 2.1 Lewis Acids as Catalysts Alejandro Baeza was born in Alicante (Spain) in 1979. He studied 2.2 Brønsted Acids as Catalysts chemistry at the University of Alicante and he received his M.Sc. (2003) and Ph. D. degrees (2006) from here under the supervision of 2.3 Other Promoters Prof. José Miguel Sansano and Prof. Carmen Nájera. He was a post- 3 Conclusions and Outlook doctoral researcher in Prof. Pfaltz’s research group (2007–2010). In 2010 he returned to Alicante and joined the research group of Prof. Key words: S 1 reaction, allylic substitution, carbocations, allylic N Carmen Nájera. His main research interests focus on the development alcohols, green chemistry of new environmentally friendly methodologies, especially in asym- metric synthesis. -
Bsc Chemistry
Subject Chemistry Paper No and Title 05, ORGANIC CHEMISTRY-II (REACTION MECHANISM-I) Module No and Title 15, The Neighbouring Mechanism, Neighbouring Group Participation by π and σ Bonds Module Tag CHE_P5_M15 CHEMISTRY PAPER :5, ORGANIC CHEMISTRY-II (REACTION MECHANISM-I) MODULE: 15 , The Neighbouring Mechanism, Neighbouring Group Participation by π and σ Bonds TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. NGP Participation 3.1 NGP by Heteroatom Lone Pairs 3.2 NGP by alkene 3.3 NGP by Cyclopropane, Cyclobutane or a Homoallyl group 3.4 NGP by an Aromatic Ring 4. Neighbouring Group Participation on SN2 Reactions 5. Neighbouring Group Participation on SN1 Reactions 6. Neighbouring Group and Rearrangement 7. Examples 8. Summary CHEMISTRY PAPER :5, ORGANIC CHEMISTRY-II (REACTION MECHANISM-I) MODULE: 15 , The Neighbouring Mechanism, Neighbouring Group Participation by π and σ Bonds 1. Learning Outcomes After studying this module, you shall be able to Know about NGP reaction Learn reaction mechanism of NGP reaction Identify stereochemistry of NGP reaction Evaluate the factors affecting the NGP reaction Analyse Phenonium ion, NGP by alkene, and NGP by heteroatom. 2. Introduction The reaction centre (carbenium centre) has direct interaction with a lone pair of electrons of an atom or with the electrons of s- or p-bond present within the parent molecule but these are not in conjugation with the reaction centre. A distinction is sometimes made between n, s and p- participation. An increase in rate due to Neighbouring Group Participation (NGP) is known as "anchimeric assistance". "Synartetic acceleration" happens to be the special case of anchimeric assistance and applies to participation by electrons binding a substituent to a carbon atom in a β- position relative to the leaving group attached to the α-carbon atom. -
Reaction Kinetics in Organic Reactions
Autumn 2004 Reaction Kinetics in Organic Reactions Why are kinetic analyses important? • Consider two classic examples in asymmetric catalysis: geraniol epoxidation 5-10% Ti(O-i-C3H7)4 O DET OH * * OH + TBHP CH2Cl2 3A mol sieve OH COOH5C2 L-(+)-DET = OH COOH5C2 * OH geraniol hydrogenation OH 0.1% Ru(II)-BINAP + H2 CH3OH P(C6H5)2 (S)-BINAP = P(C6 H5)2 • In both cases, high enantioselectivities may be achieved. However, there are fundamental differences between these two reactions which kinetics can inform us about. 1 Autumn 2004 Kinetics of Asymmetric Catalytic Reactions geraniol epoxidation: • enantioselectivity is controlled primarily by the preferred mode of initial binding of the prochiral substrate and, therefore, the relative stability of intermediate species. The transition state resembles the intermediate species. Finn and Sharpless in Asymmetric Synthesis, Morrison, J.D., ed., Academic Press: New York, 1986, v. 5, p. 247. geraniol hydrogenation: • enantioselectivity may be dictated by the relative reactivity rather than the stability of the intermediate species. The transition state may not resemble the intermediate species. for example, hydrogenation of enamides using Rh+(dipamp) studied by Landis and Halpern (JACS, 1987, 109,1746) 2 Autumn 2004 Kinetics of Asymmetric Catalytic Reactions “Asymmetric catalysis is four-dimensional chemistry. Simple stereochemical scrutiny of the substrate or reagent is not enough. The high efficiency that these reactions provide can only be achieved through a combination of both an ideal three-dimensional structure (x,y,z) and suitable kinetics (t).” R. Noyori, Asymmetric Catalysis in Organic Synthesis,Wiley-Interscience: New York, 1994, p.3. “Studying the photograph of a racehorse cannot tell you how fast it can run.” J. -
![[3+2]-ANNULATION REACTIONS with NITROALKENES in the SYNTHESIS of AROMATIC FIVE-MEMBERED NITROGEN HETEROCYCLES Vladimir A. Motorn](https://docslib.b-cdn.net/cover/7083/3-2-annulation-reactions-with-nitroalkenes-in-the-synthesis-of-aromatic-five-membered-nitrogen-heterocycles-vladimir-a-motorn-757083.webp)
[3+2]-ANNULATION REACTIONS with NITROALKENES in the SYNTHESIS of AROMATIC FIVE-MEMBERED NITROGEN HETEROCYCLES Vladimir A. Motorn
237 [3+2] - ANNULATION REACTIONS WIT H NITROALKENES IN THE SYNTHESIS OF AROMATIC FIVE - MEMBERED NITROGEN HETEROCYCLES DOI: http://dx.medra.org/ 10.17374/targets.2020.23.2 37 Vladimir A. Motornov, Sema L. Ioffe, Andrey A. Tabolin * N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, 119991 Moscow, Russia (e - mail: [email protected]) Abstract. [3+2] - A nnulation reactions are widely used for the synthesis of aromatic heterocycles. In recent years they have become attractive for the preparation of medicinally relevant heterocycles due to their broad substrate scope and availability of starting materials . A nnulations with nitroalkenes may lead to different products due to the ability of the nitro - group to act both as activating group and as leaving group. Ultimately this gives rise to the synthesis of multifunctional heterocyclic compounds, including nitro - substituted ones. The present review covers various annulation re actions with nitroalkenes leading to five - membered nitrogen - containing heterocyclic rings. Oxidative annulation, annulation/elimination and self - oxidative annulation pathways are discussed. Contents 1. Introduction 2. Classification of nitroalkene - b ased annulation reactions 3. A nnulations with nitroalkenes in the synthesis of five - membered rings 3.1. Synthesis of pyrroles 3.1.1. Barton - Zard pyrrole synthesis 3.1.2. Annulation with enamines 3.1.3. Annulation with azomethine ylides 3.2. Synthesis of pyrazoles 3.2.1. Nitroalkene - diazo comp o unds [3+2] - cycloadditions 3.2.2. Oxidative annulation of nitroalkenes with hydrazones 3.3. Synthesis of imidazoles and imidazo[1,2 - a]pyridines 3.4. Synthesis of indolizines and related heter ocycles 3.5. -
Chapter 23: Substituted Hydrocarbons and Their Reactions
736-773_Ch23-866418 5/9/06 3:37 PM Page 736 CHAPTER 23 Substituted Hydrocarbons and Their Reactions Chemistry 2.b, 2.d, 2.h, 3.a, 3.g, 8.c, 10.a, 10.b, 10.e I&E 1.b, 1.c, 1.j What You’ll Learn ▲ You will recognize the names and structures of several important organic functional groups. ▲ You will classify reactions of organic substances as sub- stitution, addition, elimina- tion, oxidation-reduction, or condensation and predict products of these reactions. ▲ You will relate the struc- tures of synthetic polymers to their properties. Why It’s Important Whether you are removing a sandwich from plastic wrap, taking an aspirin, or shooting baskets, you’re using organic materials made of substituted hydrocarbons. These com- pounds are in turn made of molecules whose atoms include carbon, hydrogen, and other elements. Visit the Chemistry Web site at chemistrymc.com to find links about substituted hydrocarbons and their reactions. The spooled threads shown in the photo are made from large organ- ic molecules called polymers. 736 Chapter 23 736-773_Ch23-866418 5/9/06 3:37 PM Page 737 DISCOVERY LAB Making Slime Chemistry 10.b n addition to carbon and hydrogen, most organic substances con- Itain other elements that give the substances unique properties. In this lab, you will work with an organic substance consisting of long carbon chains to which many ϪOH groups are bonded. How will the properties of this substance change when these groups react to form bonds called crosslinks between the chains? Safety Precautions Do not allow solutions or product to contact eyes or exposed skin. -
Reactions of Aromatic Compounds Just Like an Alkene, Benzene Has Clouds of Electrons Above and Below Its Sigma Bond Framework
Reactions of Aromatic Compounds Just like an alkene, benzene has clouds of electrons above and below its sigma bond framework. Although the electrons are in a stable aromatic system, they are still available for reaction with strong electrophiles. This generates a carbocation which is resonance stabilized (but not aromatic). This cation is called a sigma complex because the electrophile is joined to the benzene ring through a new sigma bond. The sigma complex (also called an arenium ion) is not aromatic since it contains an sp3 carbon (which disrupts the required loop of p orbitals). Ch17 Reactions of Aromatic Compounds (landscape).docx Page1 The loss of aromaticity required to form the sigma complex explains the highly endothermic nature of the first step. (That is why we require strong electrophiles for reaction). The sigma complex wishes to regain its aromaticity, and it may do so by either a reversal of the first step (i.e. regenerate the starting material) or by loss of the proton on the sp3 carbon (leading to a substitution product). When a reaction proceeds this way, it is electrophilic aromatic substitution. There are a wide variety of electrophiles that can be introduced into a benzene ring in this way, and so electrophilic aromatic substitution is a very important method for the synthesis of substituted aromatic compounds. Ch17 Reactions of Aromatic Compounds (landscape).docx Page2 Bromination of Benzene Bromination follows the same general mechanism for the electrophilic aromatic substitution (EAS). Bromine itself is not electrophilic enough to react with benzene. But the addition of a strong Lewis acid (electron pair acceptor), such as FeBr3, catalyses the reaction, and leads to the substitution product. -
AROMATIC NUCLEOPHILIC SUBSTITUTION-PART -2 Electrophilic Substitution
Dr. Tripti Gangwar AROMATIC NUCLEOPHILIC SUBSTITUTION-PART -2 Electrophilic substitution ◦ The aromatic ring acts as a nucleophile, and attacks an added electrophile E+ ◦ An electron-deficient carbocation intermediate is formed (the rate- determining step) which is then deprotonated to restore aromaticity ◦ electron-donating groups on the aromatic ring (such as -OH, -OCH3, and alkyl) make the reaction faster, since they help to stabilize the electron-poor carbocation intermediate ◦ Lewis acids can make electrophiles even more electron-poor (reactive), increasing the reaction rate. For example FeBr3 / Br2 allows bromination to occur at a useful rate on benzene, whereas Br2 by itself is slow). In fact, a substitution reaction does occur! (But, as you may suspect, this isn’t an electrophilic aromatic substitution reaction.) In this substitution reaction the C-Cl bond breaks, and a C-O bond forms on the same carbon. The species that attacks the ring is a nucleophile, not an electrophile The aromatic ring is electron-poor (electrophilic), not electron rich (nucleophilic) The “leaving group” is chlorine, not H+ The position where the nucleophile attacks is determined by where the leaving group is, not by electronic and steric factors (i.e. no mix of ortho– and para- products as with electrophilic aromatic substitution). In short, the roles of the aromatic ring and attacking species are reversed! The attacking species (CH3O–) is the nucleophile, and the ring is the electrophile. Since the nucleophile is the attacking species, this type of reaction has come to be known as nucleophilic aromatic substitution. n nucleophilic aromatic substitution (NAS), all the trends you learned in electrophilic aromatic substitution operate, but in reverse. -
S.T.E.T.Women's College, Mannargudi Semester Iii Ii M
S.T.E.T.WOMEN’S COLLEGE, MANNARGUDI SEMESTER III II M.Sc., CHEMISTRY ORGANIC CHEMISTRY - II – P16CH31 UNIT I Aliphatic nucleophilic substitution – mechanisms – SN1, SN2, SNi – ion-pair in SN1 mechanisms – neighbouring group participation, non-classical carbocations – substitutions at allylic and vinylic carbons. Reactivity – effect of structure, nucleophile, leaving group and stereochemical factors – correlation of structure with reactivity – solvent effects – rearrangements involving carbocations – Wagner-Meerwein and dienone-phenol rearrangements. Aromatic nucleophilic substitutions – SN1, SNAr, Benzyne mechanism – reactivity orientation – Ullmann, Sandmeyer and Chichibabin reaction – rearrangements involving nucleophilic substitution – Stevens – Sommelet Hauser and von-Richter rearrangements. NUCLEOPHILIC SUBSTITUTION Mechanism of Aliphatic Nucleophilic Substitution. Aliphatic nucleophilic substitution clearly involves the donation of a lone pair from the nucleophile to the tetrahedral, electrophilic carbon bonded to a halogen. For that reason, it attracts to nucleophile In organic chemistry and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which a leaving group(nucleophile) is replaced by an electron rich compound(nucleophile). The whole molecular entity of which the electrophile and the leaving group are part is usually called the substrate. The nucleophile essentially attempts to replace the leaving group as the primary substituent in the reaction itself, as a part of another molecule. The most general form of the reaction may be given as the following: Nuc: + R-LG → R-Nuc + LG: The electron pair (:) from the nucleophile(Nuc) attacks the substrate (R-LG) forming a new 1 bond, while the leaving group (LG) departs with an electron pair. The principal product in this case is R-Nuc. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged. -
Subject Index
455 Subject Index Aminohydroxylation, 364 a-Aminoketone, 281 4-Aminophenol, 18 A Aminothiophene, 158 Abnormal Claisen rearrangement, 1 a-Aminothiophenols, 184 Acrolein, 378 Ammonium ylide, 383 2-(Acylamino)-toluenes, 245 Angeli-Rimini hydroxamic acid Acylation, 100, 145, 200 synthesis, 9 Acyl azides, 98 ~-Anomer, 225 Acylium ion, 145, 149, 175, 177 Anomeric center, 211 a-Acyloxycarboxamides, 298 Anomeric effect, 135 a-Acyloxyketones, 17 ANRORC mechanism, 10 a-Acyloxythioethers, 327 Anthracenes, 51 Acyl transfer, 17, 42, 228, 298, 305, Anti-Markovnikov addition, 219 327,345 Amdt-Eistert homologation, 11 AIBN, 22, 23, 415 Aryl-acetylene, 66 Alder ene reaction, 2 Arylation, 253 Alder's endo rule, Ill 0-Aryliminoethers, 67 Aldol condensation, 3, 14, 26, 34, 2-Arylindoles, 38 69, 130, 147, 172, 305, Aryl migration, 31 340,396,412 Autoxidation, 69, 115, 118 Aldosylamine, 8 Auwers reaction, 13 Alkyl migration, 16, 132, 315, 443 Axial, 347 Alkylation, 144, 145 Azalactone, I 00 N-Aikylation, 162 Azides, 125, 330 Alkylidene carbene, 151 Azirine, 6, 7, 281 Allan-Robinson reaction, 4, 228 Azulene, 310 Allene, 119 1t-Allyl complex, 414 B Allylation, 213, 414 Baeyer-Drewson Allylstannane, 213 indigo synthesis, 14 Allylsilanes, 349 Baeyer-Villiger oxidation, 16, 53 Alper carbonylation, 6 Baker-Venkataraman Alpine-borane®, 262 rearrangement, 17 Aluminum phenolate, 149 Balz-Schiemann reaction, 354 Amadori rearrangement, 8 Bamberger rearrangement, 18 Amide acetal, 74 Bamford-Stevens reaction, 19 Amides, 28, 67,276,339, 356 Bargellini reaction, 20 Amidine, -
Solvating Alkylamine Hofmann Elimination in Zeolites Through Cooperative Adsorption Han Chen† and Omar A
Solvating Alkylamine Hofmann Elimination in Zeolites Through Cooperative Adsorption Han Chen† and Omar A. Abdelrahman †,‡* † Department of Chemical Engineering, University of Massachusetts Amherst, 686 N. Pleasant Street, Amherst, MA 01003, USA ‡ Catalysis Center for Energy Innovation, University of Delaware, 150 Academy Street, Newark, DE 19716, USA *Corresponding Author: [email protected] Abstract. A kinetic investigation of the vapor phase Hofmann elimination of tert-butylamine over H- ZSM-5 reveals a carbocation mediated E1-like mechanism, where isobutene and ammonia are exclusively produced over Brønsted acid sites. Hofmann elimination kinetics are found to be insensitive to Al content or siting, varying only with alkylamine carbocation stability (rtertiary > rsecondary > rprimary). Under conditions of complete tert-butylamine surface coverage, experimentally measurable apparent kinetics are directly equivalent to the intrinsic kinetics of the rate determining unimolecular surface elimination. The direct measurement of elementary step kinetics served as a water-free reactive probe, providing a direct measurement of the impact of water on solid Brønsted acid catalyzed chemistries at a microscopic level. Over a range of temperatures (453‒513 K) and tert-butylamine partial pressures (6.8×10-2‒6.8 kPa), water reversibly inhibits the rate of Hofmann elimination. Despite expected changes in aluminosilicate hydrophobicity, the water-induced inhibition is found to be insensitive to Al content, demonstrated to be due to one water molecule per Brønsted acid site. Regardless of the significant reduction in the rate of Hofmann elimination, kinetic interrogations and operando spectroscopic measurements reveal that the coverage of TBA adsorbed on H-ZSM-5 is unaltered in the presence of water.