Oxidation of nine primary aliphatic alcohols b y pyridinium hydrobromide perbromide (PH PB) in aqueous acetic acid leads to the formation of the corresponding aldehydes. The reaction is first order with respect to PHPB. Michaelis-Menten-type kinetics are observed with respect to the alcohol. The oxidation of [I ,I -2H2]ethanol (MeCD,OH) exhibits a substantial kinetic isotopic effect. The effect of solvent composition indicates that the transition state is more polar than the reactants. The reaction is susceptible t o both polar and steric effects of the substituents. A mechanism involving transfer of a hydride ion from the alcohol t o the oxidant has been proposed.Pyridinium hydrobromide perbromide (PHPB) has been extensively used in synthetic organic chemistry as a brominating reagent and as an oxidant.'72 There are not many reports on the mechanistic aspects of reactions of PHPB.3.4 Gnanadoss and Vijayalaxmi4 reported the kinetics of the oxidation of some primary aliphatic alcohols and attempted a correlation of reactivity with Charton's steric parameter, V.5 However, they did not determine the order of reaction with respect to the alcohol and, in its absence, the correlation analysis is of doubtful significance. We have been interested in the study of kinetics and mechanism of oxidations by oxidants containing halogens. In this article, we report the kinetics of oxidation of nine aliphatic primary alcohols in aqueous acetic acid. Attempts have been made to correlate structure and reactivity in the reaction. Mechanistic aspects are discussed.Paper 2/044 1 7 J

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Kinetics of oxidation of five vicinal, four non-vicinal diols, and one of their monoethers by pyridinium hydrobromide perbromide (PHPB) have been studied. The vicinal diols yield products arising out of the glycol bond fission while the other diols yield hydroxyca/'bonyl compounds. The reaction is first order with respect to PHPB. Michaelis-Menten type kinetics are observed wih respect to the diol. There is no effect of added pyridinium bromide on the reaction. The oxidation of [1, 1,2, 2-2H,]ethanediol shows the absence of a primary kinetic isotope effect. The values of solvent isotope effect, k(HzO)/k(D20), at 313 K, for the oxidation of ethanediol, propane-l, 3-dioi and 3-methoxybutan-l-ol are 4.71, 2-17 and 2.23 respectively. A mechanism involving glycol-bond fission has been proposed for the oxidation of the vicinal diols. The other diois are oxidised by a hydride-transfer mechanism as they are monohydric alcohols.

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