Permanganate oxidations. III. Kinetics and mechanisms of the oxidation of furfurals in alkaline media

Abstract

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 roeia-substituent constants has been observed for the substituents 5-Me, 5-Et, 5-n-Bu, H, 5-C1, and 5-Br at 25°with p = +1.30 (Scheme II). At pH 13.3 (Scheme II), AH * is 10.2 kcal/mol, AS* is -22.8 eu, and &h/&d 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.

Kinetic study of 2-propanol and benzyl alcohol oxidation by alkaline hexacyanoferrate(III) catalyzed by a terpyridyl ruthenium complex

“…Electron-withdrawing groups have also been reported to accelerate the oxidation of substituted benzaldehydes [10] and furfurals [11] by alkaline permanganate. The electron-withdrawing substituent may be encouraging the reaction of 4-nitrobenzaldehyde with hydroxide to at 25ЊC for benzyl alcohols and 35ЊC for 2-propyl alcohols.…”

Enhanced Oxidation of Tetracycline by Permanganate via the Alkali-Induced Alteration of the Highest Occupied Molecular Orbital and the Electrostatic Potential

“…The concerted mechanism was recognized to be applicable for the MnO 4 – oxidation of alkanes and alkenes. − The in situ formed manganese species was explored to enhance the transformation of phenolic compounds by MnO 4 – . , However, as a very fundamental issue, the role of hydroxyl ion (OH – ) in the base-catalyzed MnO 4 – oxidation remains unclear. Only few studies have speculated that OH – could, for instance, complex with MnO 4 – or with the unsaturated bond to form highly reactive intermediates. − …”

Oxidation with permanganate in strong alkaline medium. Oxidation of pyruvic and lactic acids

“…As a result of his investigations with furfural, Freeman [20] proposed a mechanism for aldehydes in alkaline medium similar to that of alcohols, i.e., based on H--abstraction from the hydrate form. In the case of ketones, the reactive species are enols and enolate anions, as pointed out by Wiberg [211 based on his thorough investigations with acetone.…”