The mechanism of oxidation of α-glycols by periodic acid. Part IX. Propane-1,2-diol: kinetics of formation of the intermediate

Abstract

The kinetics of oxidation of propane-1,2-diol by periodate have been studied by the stopped-flow technique: rate data has been obtained for the formation of the cyclic ester intermediate, and its decomposition to the reaction products. The rate constants k_f and k_b, for the formation of the cyclic ester and its hydrolysis back to reactants respectively, have been evaluated at 25° in the range pH 0–13, both in buffered and unbuffered solutions. The dpendence of k_f and k_b on pH and buffer concentration are consistent with a mechanism involving the formation of a diol–periodate monoester which undergoes ring-closure to the cyclic ester. The ring closure is general base catalysed. In unbuffered solutions of pH >4, ring-closure is almost completely rate-limiting, but relatively strong bases such as ammonia so accelerate ring-closure that formation of the monoester becomes partially rate-limiting. Although hydroxide is a powerful catalyst for ring-closure, there is no increase in k_f with increase of pH in alkaline unbuffered solution. This is attributed to the lack of reactivity towards ring-closure of all monoester species except a monoanion in the range pH 4–11; a quantitative treatment shows that hydroxide-ion catalysis is counter-balanced by the decrease in monoanion concentration beyond pH 8. At pH >11, the formation of the cyclic ester may proceed by a mechanism involving the formation of the alkoxide ion of a monoester, and its subsequent ring-closure via nucleophilic attack at iodine and displacement of hydroxide. In acid solution k_f increases to a maximum at pH 1; a quantitative treatment shows that at pH <2 the formation of the monoester becomes partially rate-limiting.

The activation energy for decomposition of the cyclic ester monoanion to reaction products is 23·7 kcal, and for the formation of the same ester species from the reactants, ΔH=–16·3 kcal mol^–1.