The kinetics and mechanism of the oxidation of benzyl alcohol to benzaldehyde by bromine

Abstract

The kinetics of the oxidation of benzyl alcohol to benzaldehyde by bromine have been measured in several acetic acid–water solvents of varying proportions and with added buffer salts. The rate of oxidation is influenced by polar substituents such that the reaction constant for benzyl alcohol and nine monosubstituted derivatives has the value ρ=–2·29 at 25 °C with correlation coefficient r= 0·9935. Whereas the rate of oxidation of benzyl alcohol is linearly related to solvent composition, and is undetectable in anhydrous acetic acid, reaction constants in 25–95%(v/v) acetic acid–water mixtures are invariant. Activation enthalpies and entropies for 11 benzyl alcohols are linearly related with correlation coefficient r= 0·9768. Benzyl alcohol is oxidised by bromine at a rate 4·1 times faster than αα-dideuteriobenzyl alcohol at 25 °C. The results are discussed in terms of a mechanism involving rate-determining removal of hydride ion from C(1)–H with the possibility of a synchronous removal of hydroxylic proton.