Permanganate Oxidations. III. Kinetics and Mechanisms of the Oxidation of Furfurals in Alkaline Media

A spectrophotometric stopped-flow kinetic study of the permanganate ion oxidation of furfural (I) and six 5-substituted furfurals at pH 11.5-13.3 reveals that the reaction follows two reaction paths. The minor pathway (Scheme I) is independent of hydroxyl ion concentration, and the major mechanism (Scheme II) is dependent on the first power of hydroxide ion concentration. Both reaction pathways are first order with respect to the concentration of I and permanganate ion. A correlation of the second-order rate constants with Hammett σ meta-substituent constants has been observed for the substituents 5-Me, 5-Et, 5-n-Bu, H, 5-01, and 5-Br at 25° with ρ = +1.30 (Scheme II). At pH 13.3 (Scheme II), ΔH≠ is 10.2 kcal/mol, ΔS≠ is -22.8 eu, and ks/ko is >1.8. Oxygen-18 experiments show that the solvent is the major source of oxygen introduced into I via Scheme II. The kinetic data are consistent with the formation of the hydrate anion of I followed by a hydride anion transfer to permanganate ion in the rate-determining step for the mechanism of Scheme II. It is postulated that the mechanism of Scheme I involves a direct attack of permanganate ion on I to give the permanganate ester, which decomposes in a subsequent slow step.