The mechanism of oxidation of α-glycols by periodic acid. Part X. The oxidation of pinacol, and a general discussion of the stability of periodate esters and their role in the mechanism of oxidation

Abstract

The kinetics of oxidation of pinacol by periodate have been studied at 0° and 25° over the range pH 0–13. The kinetics are second order except at 0° and pH >11. Under the latter conditions, with excess pinacol, the kinetics are those of consecutive first-order reactions, showing that an intermediate is being formed in appreciable concentration. Several features of the kinetics resemble closely those observed for the formation of the cyclic ester from propane-1,2-diol and periodate. It is concluded that the oxidations of pinacol and propane-1,2-diol by periodate proceed via a mechanism which is probably common to all oxidations of 1,2-diols by periodate, viz. the formation of a periodate monoester, followed by ring-closure to a cyclic ester, then decomposition of a monoanion of the cyclic ester to the reaction products. The stages of the mechanism are discussed in detail, and it is shown that the different types of pH dependence observed in the oxidation of 1,2-diols by periodate can be readily accommodated by this mechanism. The stability of the periodate esters is discussed, and it is concluded that the general order of stability of esters containing six-co-ordinated iodine is: cyclic triester > cyclic diester (5-membered ring) > cyclic diester (6-membered ring) > monoester. A comparison of the acid strengths of H₅IO₆ and cyclic periodate esters suggests that the periodiate di- and tri-anions are appreciably dehydrated in aqueous solution to HIO₅^2– and IO₅^3–.