Chapter 3. Part (i) Reaction mechanisms

Abstract

This year an extensive coverage of acid-base catalysis is included since this has not been fully reported recently. Similarly, since deuterium isotope effects were dealt with comprehensively last year this topic is covered only briefly on this occasion.Acidity Functions and Molecular Basicity.-New acidity-function data reported this year includes values of H , for HClO, in aqueous 2-butoxyethanol,' H , for aqueous H2S0,,2 and HR for constant ionic-strength solutions of HC104 in aqueous d i ~x a n . ~ Good agreement between calculated and experimental values of H , for iodic acid have been obtained4 by using Wyatt's proton hydration model.' H-Scales for aqueous alkali hydroxides have come under criticism.6 The observed trend in basicity for solutions of equal concentration :is thought to be only apparent, when due allowance is made for ion-pair formation,6 attention being drawn to the danger of correlating reaction rates with uncorrected H -values.The protonation in acetonitrile of water, alcohols, and diethyl ether has been ~t u d i e d . ~The protonation of a number of aromatic and a,P-unsaturated aldehydes, ketones, and carboxylic acids in sulphuric acid has been shown to follow the H A acidity function, rather than H,.8 The basicity of some sulphoxides in aqueous sulphuric acid has been determined by measurements of chemical shifts.' An n.m.r. study of the protonation equilibria of carbamic acid esters reveals similar protonation behaviour to that of amides and indicates pK, values intermediate between structurally similar amides and esters. l o A detailed study of the protonation of azulene-1-carboxylic acid over a wide pH range has eliminated some of the previous inconsistencies." At low pH it decarboxylates slowly. Above 5 . 0 -~ acid, however, the conjugate acid (I) is formed : '

Mechanism of nucleophilic substitution of N-methyl-4-substituted pyridinium salts

“…Several attempts were made to measure the rate of reaction of 4-methoxypyridine (7) with hydroxide ion (13) R. C. Williams and J. W. Taylor, J. Chem. Educ., 47, 129 (1970).…”