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 kf and kb, 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 kf and kb 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 kf 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 kf 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.