Kinetics of oxidation of secondary alcohols by chloramine T

Abstract

The kinetics of oxidation by chloramine T(CAT) of secondary alcohols with propan-2-ol as reference compound have been studied in aqueous acidic and aqueous acetic acid media. In the presence of strong mineral acid in aqueous medium the rate law is found to be –d[CAT]/dt=k[CAT][Alcohol][H⁺].² The rate law, a low kinetic isotope effect, and the effect of solvent polarity on the rate are consistent with a mechanism involving a rate determining interaction of either the protonated chloramine T (CATH₂⁺) or H₂OCl⁺ with the secondary alcohol giving alkyl hypochlorite followed by a fast decomposition to ketone. The observed order of 1.5 in [HCl] is interpreted by a simultaneous oxidation by molecular chlorine or HOCl and CATH₂⁺. At low percentages of acetic acid (10% and 20% v/v) the reaction exhibits a second-order dependence on CAT and first-order in alcohol. Dichloramine T (DCT) formed in a disproportionation step is possibly the active oxidant under these conditions. At 50% HOAc(v/v)a fractional order dependence on [Alcohol] and first-order in [CAT] are observed. Further increase in percentage of acetic acid increases the order in alcohol and a limiting value of 1 is reached in 90% HOAc(v/v). These results are indicative of the formation of a complex, the stability of which is solvent dependent. The concentration dependence on alcohol varies in buffered acetic acid in the presence of chloride and the rate law is rationalised by a concurrent oxidation by DCT and molecular chlorine.