Physical Organic Chemistry–3. Plenary Lectures Presented At The Third Iupac Conference On Physical Organic Chemistry, Montpellier, France, 6–10 September, 1976

Organizing Committee Chairman: A. Fruchier Members: P. Geneste G. Lamaty C. Moreau J.P. Roque U.K. Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 0BW, England U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A. CANADA Pergamon of Canada Ltd., 75 The East Mall, Toronto, Ontario, Canada Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street, Rushcutters Bay, N.S.W. 2011, Australia AUSTRALIA FRANCE FEDERAL REPUBLIC OF GERMANY Pergamon Press SARL, 24 rue des Ecoles, 75240 Paris, Cedex 05, France Pergamon Press GmbH, 6242 Kronberg-Taunus, Pferdstrasse 1, Federal Republic of Germany Copyright© 1977 International Union of Pure and Applied Chemistry All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the copyright holders The contents of this book appear in Pure and Applied Chemistry, Vol. 49, No. 7 British Library Cataloguing in Publication Data IUPAC Conference on Physical Organic Chemistry, 3rd, Montpellier, 1976 Physical organic chemistry 3. 1. Chemistry, Organic - Congresses 2. Chemistry, Physical and theoretical - Congresses I. Title II. Fruchier, A III. International Union of Pure and Applied Chemistry. Organic Chemistry Division IV. Société chimique de France V. Société de chimie physique 547'. Γ3 QD476 77-30275 ISBN 0-08-021197-6 Printed in Great Britain by A. Wheat on & Co. Ltd., Exeter International Union of Pure and Applied Chemistry (Organic Chemistry Division) in conjunction with La Société Chimique de France and La Société de Chimique Physique Physical Organic Chemistry — 3 Plenary lectures presented at the Third IUPAC Conference on Physical Organic Chemistry Montpellier, France, 6 - 10 September, 1976 Symposium Editor: A. Fruchier Université des Sciences et Techniques du Languadoc PERGAMON PRESS OXFORD · NEW YORK · TORONTO · SYDNEY · PARIS · FRANKFURT Pure & Appl0 Chem.3 Vol. 49, pp. 963 - 968. Pergamon Press, 1977. Printed in Great Britain. EQUILIBRIUM ACIDITIES OF CARBON ACIDS F. G. Bordwell Department of Chemistry, Northwestern University, Evanston, Illinois, 60201, USA Abstract - A scale of absolute equilibrium acidities for organic acids in aimetnyl sulfoxide solution is presented, and these acidities are compared with: va) apparent acidities in solvents of low dielectric constant, (bj absolute acidities in water, and (c) relative acidities in the gas phase. In dimethyl sulfoxide the carbon acid acetophenone is weaker than its oxygen analog, benzoic acid, by almost fourteen orders of magnitude, but the Hammett p for acetophenone s is only one order of magnitude greater than for benzoic acids. This shows that large changes in the stabilities of ions in side chains may be accompanied by relatively small changes in p, and that the success of the Hammett equation rests, in part, on its relative insensitivity to energy changes. INTRODUCTION For the past 40 years organic chemists have been using a scale developed by McEwen as a guide to the relative acidities of weak acids and the relative stabilities of anions derived therefrom (1). Following a method developed by Conant and Wheland in ether (2), McEwen constructed his scale by determining, in a semiquantitative manner, the position of equilibrium in benzene solution between two weak acids, HA and Hin, one of which forms a colored (indicator) anion, In". HA + M+In" C H m * * M+A" + Hin Ina solvent of low dielectric constant, such as benzene, the metallic salts, M+In" and MA