The kinetics and mechanism of oxidation of hydroquinone and chlorohydroquinone in the presence of nitrous acid in aqueous acid solution

Abstract

Hydroquinone is converted quantitatively into benzoquinone by nitrous acid in air-saturated aqueous acid solution, by a process which is kinetically first order, whether or not the nitrous acid is present in excess. Chlorohydroquinone behaves similarly and substituent effects are discussed. The inorganic and organic steps in the mechanism put forward are shown by numerical integrations to account for all the observations. MOPAC calculations suggest that 4-hydroxy-6-nitrosocyclohexa-2,4-dienone is the first-formed intermediate. Homolysis of this to give the 4-hydroxyphenoxyl radical and nitric oxide is rate determining in the organic sequence and takes place by uncatalysed and acid catalysed pathways. Comparison with previous work on 4-methoxyphenol, where the later step of combination of the 4-methoxyphenoxyl radical with nitrogen dioxide is rate determining, permits an estimate of the relative rate-constants for reaction of the substituted phenoxyl radical with NO and NO₂.