Mechanisms in Organic Reactions (Tutorial Chemistry Texts) ( PDF

《Mechanisms in Organic Reactions (Tutorial Chemistry Texts) ( PDF》由会员分享，可在线阅读，更多相关《Mechanisms in Organic Reactions (Tutorial Chemistry Texts) ( PDF（210页珍藏版）》请在凡人图书馆上搜索。

1、Cover images 0 Murray Robertson/visual elements 1998-99, taken from the 109 Visual Elements Periodic Table, available at www.chemsoc.org/viselements ISBN 0-85404-642-9 A catalogue record for this book is available from the British Library 0 The Royal Society of Chemistry 2004 All rights reserved Apa

2、rt-from any fair dealing for the purposes of research or private study, or criticism or reviews as permitted under the terms of the UK Copyright, Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in

3、 Ivriting of The Royal Society of Chemistry, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization ou

4、tside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 OWF, UK Registered Charity No

5、. 207890 For further information see our web site at www.rsc.org Typeset in Great Britain by Alden Bookset, Northampton Printed and bound by Italy by Rotolito Lombarda Preface The wonderful complexity of organic chemistry involves thousands of different reactions which allow the synthesis and interc

6、onversions of mil- lions of compounds, some of great complexity. The key to understanding this vital branch of chemistry is the concept of the reaction mechanism. This book starts with a discussion of how covalent bonds break and form, and how these bond-breaking and bond-forming processes pro- vide

7、 the basis of reaction mechanisms. The principles governing how to make sensible suggestions about possible mechanisms are set out, and the distinction is made between elementary reactions, which involve just one step, and stepwise reactions which have more than one step and involve the production o

8、f intermediates that react further. Chapter 2 covers kinetics, which provides useful information about react- ion mechanisms, and allows us to distinguish between possible mecha- nisms in many cases. Elementary reactions do not involve intermediates, but go through a transition state. Although this

9、transition state cannot be isolated, it can be studied in various ways which provide insights into the reaction mechanism, and this forms the subject matter of Chapter 3. This is followed by three chapters on the most important intermediates in organic chemistry: anions, radicals and cations. A fina

10、l chapter on molecular reactions concerns thermal and photochemical processes. The concepts of frontier orbitals and the aromatic transition state allow us to predict which reactions are “allowed” and which are “forbidden”, and provide insights into why most reactions of practical interest involve m

11、ulti-step processes. Where common names are used for organic compounds, the systematic name is given as well at the first mention. Common names are widely used in the chemical literature, in industry and commerce, and there is a great divergence in the use of systematic as opposed to non-systematic

12、nomen- clature in the English-speaking world. I thank many colleagues for helpful comments and advice, particularly Mr Martyn Berry and Professor Alwyn Davies FRS who have read the entire manuscript and whose suggestions for changes have improved the text in numerous places. I would also like to tha

13、nk my wife Pat for her support and forbearance over the past three years. Enjoy the book! Richard A. Jackson University of Sussex iii ED IT0 R-1 N -C H I EF EXECUTIVE EDITORS EDUCATIONAL CONSULTANT Projessor E W Abel Professor A G Duvies Professor D Phillips Prqfessor J D Woollins Mr M Berry This se

14、ries of books consists of short, single-topic or modular texts, concentrating on the fundamental areas of chemistry taught in undergraduate science courses. Each book provides a concise account of the basic principles underlying a given subject, embodying an independent- learning philosophy and incl

15、uding worked examples. The one topic, one book approach ensures that the series is adaptable to chemistry courses across a variety of institutions. TITLES IN THE SERIES TITLES IN THE SERIES Stereochemistry D G Morris Reactions and Characterization of Solids Main Group Chemistry W Henderson d- and f-

16、Block Chemistry C J Jones Structure and Bonding J Burrett Functional Group Chemistry J R Hanson Organotransition Metal Chemistry A F Hill Heterocyclic Chemistry M Sainsbury Atomic Structure and Periodicity J Barrett Thermodynamics and Statistical Mechanics Basic Atomic and Molecular Spectroscopy Org

17、anic Synthetic Methods J R Hanson Aromatic Chemistry J D Hepworth, D R Waving and M J Waring Quantum Mechanics for Chemists D 0 Hayward Peptides and Proteins S Doonan Biophysical Chemistry A Cooper Natural Products: The Secondary Metabolites J R Hanson Maths for Chemists, Volume I, Numbers, Function

18、s and Calculus M Cockett and G Doggett Maths for Chemists, Volume TI, Power Series, Complex Numbers and Linear Algebra A4 Cockett and G Doggett Inorganic Chemistry in Aqueous Solution Organic Spectroscopic Analysis J Barrett R J Anderson, D J Bendell and P W Groundwater Mechanisms in Organic Reactio

19、ns R A Jackson S E Dann J M Seddon and J D Gale J M Hollas Nucleic Acids S Doonan Further information about this series is available at ivww.rsc.org/tct Order and enquiries should be sent to: Sales and Customer Care, Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambrid

20、ge CB4 OWF, UK Tel: +44 1223 432360; Fax: +44 1223 426017; Email: sales(alrsc.org Contents 1.1 Elementary and Stepwise Reactions 1.2 Bond Making and Bond Breaking 1.3 Molecularity 1.4 Formulating Mechanisms 1.5 Why Study Mechanisms? 2.1 Rates and Rate Constants 2.2 Conclusions about Mechanism that c

21、an be Drawn from Kinetic Order 2.3 The Dependence of Rate of Reaction on Temperature: Arrhenius Parameters 2.4 Primary Kinetic Isotope Effects 3.1 Early and Late Transition States 3.2 Solvent Effects 3.3 Electronic Effects of Substituents in Polar Reactions 3.4 Steric Effects 3.5 Stereochemistry 4.1

22、 Acids and Bases 4.2 Bases and Nucleophiles 4.3 Carbanions 2 4 11 13 16 20 25 34 37 45 47 48 58 60 73 77 77 vi Contents 5.1 Formation of Carbocations 95 5.2 Evidence for Cations 96 5.3 Reactions of Carbocations 99 5.4 Electrophilic Rearrangements involving Migration 108 of C to 0 or N 5.5 Acid-catal

23、ysed Reactions 110 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 Formation of Free Radicals Destruction of Radicals (Termination) Detection of Radicals as Reaction Intermediates Electron Spin Resonance (ESR) Structure and Stability of Free Radicals Radical Chain Reactions Atmospheric Reactions Non-chain Radical R

24、eactions 118 121 122 124 128 135 143 144 7.1 Frontier Orbitals 151 7.2 The Aromatic Transition State 159 7.3 Application of the Idea of the Aromatic Transition State to Pericyclic Reactions 163 7.4 Photochemical Reactions 170 7.5 Molecular Reactions with Non-cyclic Transition States 172 What Is a Me

25、chanism? The chemical structure of most organic compounds is well established. Spectroscopic methods and X-ray crystallography show that individual atoms in a molecule are connected, usually by covalent bonds. Bond lengths are often known to within about +1 pm (0.01 A) and bond angles to within k 1.

26、 Molecular models and graphics programs give a good picture of the overall shape of the molecule, including possible interactions between atoms that are not covalently bonded to each other. These structures correspond to energy minima. Molecules can acquire extra energy by collisions, and this energ

27、y may cause distortions of bond lengths or angles by small amounts. However, the bond lengths and angles will tend to return to the equilibrium values. However, if the distortions become too great, one or more covalent bonds may break, and new bonds may be formed, either within the molecule or with

28、a new molecule with which the first has collided. A chemical reaction has occurred, and when equilibrium is reached, one or more new molecules will be produced, which may be stable or may undergo further reactions. The energy required to break covalent bonds may be provided thermally by molecular co

29、llisions, which give a range of molecular energies, providing some molecules with enough energy to react. At higher temperatures, more molecules will have sufficient energy to react, so the reaction will be faster. Alternatively, the energy can be provided in other ways, especially by visible or UV

30、light. Absorption of a photon by a molecule causes electronic excitation, and the excited state of the molecule may then undergo reactions which cannot be carried out thermally. 1 2 Mechanisms in Organic Reactions I =I Elementary and Stepwise Reactions Reactions are of two types. In the reacting mol

31、ecule or molecules are transformed into products directly, without the formation of intermediates. In a , one or more inter- mediate species are produced, which react further to give the products. A stepwise reaction can be split up into two or more elementary reactions. As an elementary reaction pr

32、oceeds, the Gibbs free energy increases up to a maximum value and then goes down to a value corresponding to that of the products. The position of highest energy is called the , and is a key feature of the reaction; most of the experimental information about chemical reactions relates to the transit

33、ion state and will be discussed in the next two chapters. In a stepwise reaction, at least one of the products of the first elementary reaction reacts further in a second elementary reaction. This may be followed by further elementary reactions until the reaction is complete. Any molecules produced

34、in the course of a stepwise reaction which react further and are not present at the end of the reaction are known as . Intermediates are discussed in more detail in Chapters 4, 5 and 6. Figure 1.1 shows free energy diagrams for an elementary reaction (1.1 a) and for a stepwise reaction with two step

35、s (1.1 b). What Is a Mechanism? 3 Transition state i Transition states Reaction coordinate (1.la) Elementary reaction e.g. HO- + CH3-Br _.* HO-CH3 + Br- Reaction coordinate (1. lb) Stepwise reaction e.g. Me$-Br _+Me$+ + Bi Me$+ + -OH - Meg-OH Figure 1.1 Free energy diagrams for elementary and stepwi

36、se reactions An example of an elementary reaction (1.1 a) is the SN2 displacement of a bromide ion from bromomethane by the hydroxide anion. The reaction is thermodynamically favourable (negative AGO) and takes place when a hydroxide ion collides with a bromomethane molecule. A bond starts to form b

37、etween the oxygen atom and the carbon atom at the same time as the carbon-bromine bond breaks. Because of electron repulsion, in the early stages of the reaction the energy released by the formation of the new bond is not quite as much as the energy required to break the C-Br bond, so the free energ

38、y increases and eventually reaches a maximum at the transition state, before decreasing to the value appropriate to the products. No intermediate is involved; the reaction proceeds smoothly from reagents through the transition state directly to products. In contrast, the hydrolysis of tert-butyl bro

39、mide (2-bromo-2- methylpropane) occurs in a stepwise manner (reaction 1.lb). In the first slow step, the C-Br bond breaks, with the bromine atom taking both electrons from the bond and leaving as a negatively charged bromide ion. The remainder of the molecule is the positively charged tert-butyl cat

40、ion (2-methylprop-2-yl cation). This is a highly reactive intermediate, which reacts rapidly with the hydroxide ion to form the corresponding alcohol. 4 Mechanisms in Organic Reactions These two examples raise an important point about mechanisms. The two reactions are similar from the point of view

41、of reagents and products, yet are known to have different mechanisms. Thus we cannot determine mechanisms merely by knowing the starting materials and products; we need further information. The remainder of this chapter is devoted to writing sensible possible mechanisms for a new reaction, and the r

42、emainder of the book concerns the methods that we can use to distinguish between the various mechanistic possibilities. 1.2 Bond Making and Bond Breaking Inspection of the structure of the reagents and products tells us which covalent bonds have been broken and formed during the reaction. 1.2.1 Bond

43、 Breaking Covalent bonds, which involve two electrons, can be broken in two ways, (homo = same), when one electron is retained by each fragment, or (hetero = different), when both electrons go to one of the fragments. Heterolysis is more likely if the two atoms in the bond have different electronega

44、tivities, and in polar solvents which stabilize charges. Electronegativity is a measure of the power of an atom or a group of atoms to attract electrons from other parts of the same molecule: fluorine is the most electronegative element; cesium is the most electropositive. A covalent bond between tw

45、o different elements is polarized in the direction +X-Y-, where Y is the more electronegative element. In a heterolysis, the bond will almost always break in the direction which will leave both bonding electrons on the more electronegative atom. If the original molecule has no net charge, this will

46、give an anion centred on the more electronegative element and the cation centred on the less electronegative element. The electronegativities of elements commonly found in organic compounds are listed in Table 1.1. What Is a Mechanism? 5 H 2.1 Li Be B C N 0 F 1 .o 1.5 2.0 2.6 3.0 3.4 4.0 0.9 1.3 1.6

47、 1.9 2.2 .2.6 3.2 K Br 0.8 3.0 I 2.7 I Na Mg Al Si P S CI Table 1 .l Electronegativities of elements commonly found in organic compoundsa aL numbers = electropositive; high numbers = electronegative The first stage of the SN1 reaction in reaction (l.lb) is a heterolysis involving a neutral molecule

48、dissociating into a cation and an anion. Heterolysis will take place exclusively in the direction indicated in reaction (1.2a), with no contribution from (1.2b). For the heterolysis shown in (1.2a), the electrons originate in the covalent bond; both move to the bromine atom during the reaction. Homo

49、lysis is more likely for weak covalent bonds and for bonds containing atoms with similar electronegativities. Heating di-tert-butyl 6 Mechanisms in Organic Reactions peroxide affords an example of homolysis (equation 1.2). The central 0-0 bond breaks homolytically to give two tert-butoxyl radicals.

50、Since one electron from the two shared electrons in the covalent bond goes to each atom, no charge is created during an homolysis. 1.2.2 Bond Formation This is the reverse of bond breaking. The electrons involved in the new bond may both come from one of the reagents; this is the reverse of heteroly