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Permanganate oxidations and the carboxyl group MacPhee, John Anthony

Abstract

In order to study the possible effects on permanganate oxidation reactions of a carboxyl sustituent near the reacting centre, three systems were investigated. The first of these systems consisted of the 2-and 4-benzhydrolcarboxylic acids. It was found that the 2-compound reacted more slowly than the 4-compound. The activation parameters at pH 5.45 show that the rate difference is accounted for entirely in terms of the difference in entropy of activation between the two compounds. Solubility difficulties prevented a study of the reaction below about pH 5. The kinetic results in more basic solution indicate that the pK corresponding to alcoholic OH ionization is greater for the 2-isomer than the 4-isomer. This behaviour is readily explained by examining the pK’s of several model compounds. The second system consisted of phthalaldehydic acid and terephthalaldehydic acid. The behaviour expected for aldehydes is observed except for phthalaldehydic acid at pH 1.55. The kinetic results for phthalaldehydic acid suggest rather strongly that it exists as 3-hydroxy- phthalide in acid solution (pH 1.55) and as the free aldehyde above pH 6. This result is shown to be in accord with previous work dealing with the diverse chemical behaviour of phthalaldehydic acid. The third system was the 2-carboxy cyclohexanol system. The pH-rate profiles of a number of isomeric 2-carboxy cyclohexanols were investigated as well as-that of the parent compound, cyclohexanol. It was found that cis-2-hydroxy-cyclohexanecarboxylic acid (XIV) showed a bell-shape rate maximum in its pH-rate profile around pH 6, while trans-2-hydroxycyclohexanecarboxylic acid did not. The parent compound, cyclohexanol, does not show such an effect - the pH-rate profile is almost flat in the region pH 4 to 8. The pH-rate profiles of cis-5-t-butyl-cis-2-hydroxycyclohexanecarboxylic acid (X), cis-5-t-butyl-trans-2- hydroxycyclohexanecarboxylic acid (XI), and trans-5-t-butyl-cis-2-hydroxycyclohexanecarboxylic acid (XII) were also investigated. A mechanism is proposed which involves two opposed dissociation equilibria involving hydrogen ions. A comparison of the reactivity of XIV in protium oxide with that in deuterium oxide gives evidence consistent with the mechanism. The conformational aspects of the reaction (elucidated by means of the conformationally biased compounds -X, XI and XII) were found also to be consistent with the proposed mechanism. It was possible to make a decision between the two kinetically indistinguishable forms of the proposed mechanism on the basis of the assembled data

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