MECHANISM -$ $.NDTHmw -I NORGANIC ,t&:m$*+?%. - - ,- .>. - CHEMISTRY 2 Ihomas H. Lowry Kathleen Schuelier Richardson , , MECHANISM AND THEORY ORGANIC CHEMISTRY THOMAS H. LOWRY Smith College and KATHLEEN SCHUELLER RICHARDSON The Ohio State University HARPER& ROW,P UBLISHERS New York, Hagerstown, San Francisco, London To Nancy and Frank Sponsoring Editor: John A. Woods Special Projects Editor: Carol J. Dempster Project Editor: Brenda Goldberg Designer: T. R. Funderburk Production Supervisor: Will C. Jomarrbn Compositor: William Clowes & Sons Limited Printer and Binder: Halliday Lithograph Corporation Art Studio: Vantage Art Inc. Copyright 0 1976 by Thomas H. Lowry and Kathleen Schueller Richardson All rights reserved. Printed in the United States of America. No part of this book may be used or reproduced in any manner whatsoever without written permission except in the case of brief quotations embodied in critical articles and reviews. For information address Harper & Row, Publishers, Inc., 10 East 53rd Street, New York, N. Y. 10022. Library of Congress Cataloging in Publication Data Lowry, Thomas H. Mechanism and theory in organic chemistry. Includes bibliographical references and index. 1. Chemistry. Physical organic. I. Richardson, Kathleen Schueller, joint author. 11. Title. QD476.1A68 547'. 1'3 75-43926 ISBN 0-06-044082-1 CONTENTS Preface ix 1 THE COVALENT BOND 1 1.1 MODELOSF CHEMICABLO NDING1 - 1.2 MOLECULAORR BITALS9 1.3 HYBRIDO RBITALS2 0 1.4 DELOCALIZEDB ONDING2 7 P 1.5 AROMATICITY28 PROBLEMS4 0 Appendix 1 : Hybrid Orbitals 43 Appendix 2: Molecular Orbital Theory 50 2 SOME FUNDAMENTALS OF PHYSICAL ORGANIC CHEMISTRY 57 2.1 STEREOCHEMIST5R7Y 2.2 LINEARF REE-ENERGRYE LATIONSHIP6S0 2.3 THERMOCHEMIST7R1Y 2.4 SOLUTIONS8 4 2.5 KINETICS9 0 2.6 INTERPRETAOTF RIOATNE CONSTANTS94 2.7 ISOTOEPFEF ECTS 105 PROBLEMS1 11 vi Contents Appendix 1 : Derivation of the Transition State Theory Expression for a Rate Constant 11 3 Appendix 2: The Transition State Theory of Isotope Effects 120 3 ACIDS AND BASES 124 3.1 BR~NSTEADC IDSA ND BASES 124 3.2 STRENGTHOFS WEAKB R~NSTBEADS ES 129 3.3 STRENGTHOFS WEAKB RDNSTEADC IDS 138 3.4 SUBSTITUENETF FECTSO N STRENGTHOSF BR~NSTEADC IDSA ND BASES 150 /' 3.5 LEWISA CIDSA ND BASES 163 PROBLEMS1 68 4 BIMOLECULAR SUBSTITUTION REACTIONS 170 4.1 SN1a nd SN2 SUBSTITUTIOMNE CHANISM1S7 1 4.2 STEREOCHEMISOTFR TYH E SN2 REACTION1 74 4.3 THES OLVENTS, UBSTRATEN,U CLEOPHILAEN, D LEAVINGG ROUP 4 177 4.4 BIMOLECULNAUR CLEOPHILSIUCB STITUTIAOTN S ULFUR 194 4.5 BIMOLECULAERLE CTROPHILSIUCB STITUTIOATN SATURATECDA R- BON 203 PROBLEMS2 10 ' UNIMOLECULAR SUBSTITUTIONS AND RELATED REACTIONS 213 d INTRAMOLECULAR REARRANGEMENTS 268 6.1 1,2-SHIFTISN CARBENIUIMON S 268 @ CARBONIUIMON S 288 6.3 MIGRATIONTOS CARBONYCLA RBON3 16 @ REARRANGEMETNOT E LECTRON-DEFICIENNITT ROGENAN D OXY- GEN 318 PROBLEMS3 32 7 ADDITION AND ELIMINATION REACTIONS 337 7.1 ELECTROPHILAIDCD ITIONTO DOUBLEAN D TRIPLBEO NDS 337 7.2 1, 2-ELIMINATIORNE ACTIONS3 55 7.3 NUCLEOPHILAICD DITIOTNO MULTIPLBEO NDS 377 Contents vii 7.4 ELECTROPHILAIRCO MATISCU BSTITUTIO3N7 9 7.5 NUCLEOPHILAICR OMATISCU BSTITUTIO3N9 5 PROBLEMS3 99 8 REACTIONS OF CARBONYL COMPOUNDS 402 8.1 HYDRATIOANN D ACID-BASEC ATALYSIS4 03 8.2 OTHERS IMPLEA DDITIONS4 16 8.3 ADDITIONF OLLOWEBDY ELIMINATION42 4 8.4 ADDITIONO F NITROGENNU CLEOPHILE4S3 2 8.5 CARBOXYLAICCI DD ERIVATIVE4S3 9 8.6 ENOLS,E NOLATESA,N D ADDITIONO F CARBONN UCLEOPHILTEOS C - 0 449 PROBLEMS45 9 9 RADICAL REACTIONS 462 9.1 CHARACTERISTOIFC SO RGANIFCR EER ADICALS4 62 9.2 RADICARLE ACTIONS47 5 9.3 FREE-RADICASLU BSTITUTION49S7 9.4 RADICAALD DITIONSA ND ELIMINATION5S0 6 9.5 REARRANGEMEONF TRSA DICALS5 17 PROBLEMS52 4 Appendix 1: Chemically Induced Dynamic Nuclear Polarization (CIDNP) 527 10 PERTTJRBATION THEORY AND SYMMETRY 538 10.1 PERTURBATITOHNE ORY53 8 10.2 SYMMETRY54 1 10.3 INTERACTIOBNEST WEENM OLECULES55 2 10.4 APPLICATIOONF PERTURBATIOTNH EORYA ND SYMMETRTYO .~r SYSTEMS5 59 PROBLEMS56 6 11 THE THEORY OF PERICYCLIC REACTIONS 568 11 .1 DEFINITIONS56 9 11 .2 PERTURBATITONH EORINY P ERICYCLRICE ACTIONS5 79 11.3 CORRELATIODNIA GRAMASND PERICYCLSICE LECTIORNU LES 581 11.4 INTERACTIODNI AGRAMASN D THE GENERALIZEWD OODWARD- HOFFMANPNE RICYCLSICE LECTIORNU LES 596 11.5 AROMATIACN D ANTIAROMATTICR ANSITSIOTANT ES 602 11.6 COMPARISONOF THE WOODWARD-HOFFMAANNND DEWAR- ZIMMERMAPENR ICYCLSICE LECTIORNU LES 6 11 11 .7 CORRELATIOOFN E LECTRONSICT ATES 61 7 PROBLEMS6 23 12 APPLICATIONS OF THE PERICYCLIC SELECTION RULES 626 12.1 CYCLOADDITIO6N2S6 12.2 ELECTROCYCRLEICA CTIONS6 45 viii Contents 12.3 SIGMATRORPIECA CTIONS65 7 PROBLEM6S7 7 PHOTOCHEMISTRY 681 13.1 LIGHTA BSORPTION68 1 13.2 UNIMOLECULPAHRO TOPHYSICPARLO CESSE6S8 7 13.3 BIMOLECULPAHRO TOPHYSICPARLO CESSE6S9 3 13.4 PHOTOCHEMIRCEAALC TIONS70 6 PROBLEMS72 9 Index 731 PREFACE This book is intended as a text for undergraduate and first-year graduate students who have completed a one-year course in organic chemistry. Its aim ., is to provide a structure that will help the student to organize and interrelate 1 the factual information obtained in the earlier course and serve as a basis for 'L study in greater depth of individual organic reactions and of methods by which \ , chemists obtain information about chemical processes. The primary focus of the book is on reaction mechanisms, not only because knowledge of mechanism is essential to understanding chemical processes but also because theories about reaction mechanisms can explain diverse chemical phenomena in terms of a relatively small number of general principles. It is this latter capability of mechanistic theory which makes it important as an organizing device for the subject of organic chemistry as a whole. In treating mechanisms of the important classes of organic reactions, we have tried to emphasize the experimental evidence upon which mechanistic ideas are built and to point out areas of uncertainty and controversy where more work still needs to be done. In this way we hope to avoid giving the impression that all organic mechanisms are well understood and completely agreed upon but instead to convey the idea that the field is a dynamic one, still very much alive and filled with surprises, excitement, and knotty problems. The organization of the book is traditional. We have, however, b2en' selective in our choice of topics in order to be able to devote a significant portion of the book to the pericyclic reaction theory and its applications and to include a chapter on photochemistry. The pericyclic theory is certainly the most important development in mechanistic organic chemistry in the past ten years. Because it is our belief that x Preface the ideas and method of thinking associated with the pericyclic theory will have an increasing impact in both organic and inorganic chemistry in the future, we have given a more detailed discussion of its purely theoretical aspects than has heretofore been customary in books of this kind. This discussion includes both the Woodward-Hoffmann approach and the Dewar-Zimmerman aromaticity approach and makes the connection between them. Our treatment requires as background a more sophisticated understanding of covalent bonding than is ordinarily given in introductory courses; we have therefore included an exten- sive presentation of bonding theory. It begins at a basic level with a review of familiar concepts in Chapter 1 and introduces in Chapter 10 the terminology and ideas needed to understand the pericyclic theory and its ramifications. The treatment is qualitative throughout. Although quantitative molecular orbital calculations are not needed for our purposes, Appendix 2 to Chapter 1 sum- marizes the molecular orbital calculation methods in general use. The Hiickel MO method is covered in sufficient detail to allow the reader to apply it to simple systems. Another innovation in this text is the use of three-dimensional reaction coordinate diagrams, pioneered by Thornton, More O'Ferrall, and Jencks, in the discussions of nucleophilic substitutions, eliminations, and acid catalysis of carbonyl additions. We hope that the examples may lead to more widespread use of these highly informative diagrams. A chapter on photochemistry provides a discussion of photophysical processes needed as background for this increasingly important area of chemistry and treats the main categories of light-induced reactions, The text assumes elementary knowledge of the common organic spectro- scopic techniques. Nevertheless, we have included a description of the recently developed method of chemically induced dynamic nuclear polarization (CIDNP), which has already proved to be of great importance in the study of radical reactions and which has not yet found its way into books covering spectroscopy of organic compounds. Problems of varying difficulty have been included at the ends of the chapters. Some problems illustrate points discussed in the text, but others are meant to extend the text by leading the student to investigate reactions, or even whole categories of reactions, which we have had to omit because of limitations of space. References to review articles and to original literature are given for all problems except those restricted to illustration of points that the text discusses in detail. Problems that represent significant extensions of the text are included in the index. The book is extensively footnoted. It is neither possible nor desirable in a book of this kind to present exhaustive reviews of the topics taken up, and we have made no effort to give complete references. We have tried to include references to review articles and monographs wherever recent ones are avail- able, to provide key references to the original literature for the ideas discussed, and to give sources for all factual information presented. The text also contains numerous cross references. The amount of material included is sufficient for a full-year course. For a one-semester course, after review of the first two chapters, material may be chosen to emphasize heterolytic reactions (Chapters 3-8), to cover a broader Preface xi range including radicals and photochemistry (selections from Chapters 3-8 plus 9 and 13), or to focus primarily on pericyclic reactions (Chapters 10-12). In selecting material for a one-semester course, the following sections should be considered for possible omission : 3.5, 4.4, 4.5, 5.6, 6.3, 7.3, 7.5, 8.3, 9.5, 10.4, 11.6, 11.7. MTe would like to thank the following people for reviewing parts of the manuscript and for providing helpful comments: Professors D. E. Applequist, C. W. Beck, J. C. Gilbert, R. W. Holder, W. P. Jencks, J. R. Keeffe, C. Levin, F. B. Mallory, D. R. McKelvey, N. A. Porter, P. v. R. Schleyer, J. Swenton, and T. T. Tidwell. We are particularly grateful to Professor N. A. Porter, who reviewed and commented on the entire manuscript. We owe special thanks to Professor Charles Levin for many enlightening discussions and to Carol Demp- ster for essential help and encouragement. Thomas H. Lowry Kathleen Schueller Richardson