Dediazoniation of p-hydroxybenzenediazonium ion in a neutral aqueous medium.

The dediazoniation of p-hydroxybenzenediazonium ion (PDQ) in a neutral aqueous medium has been studied under controlled experimental conditions to prevent photochemical and/or heterolytic side-reactions. Oxygen increased the dediazoniation rate of PDQ and caused the appearance of quinone and hydroquinone. An accumulation of quinone deriving from the reduction of PDQ by hydroquinone was also observed. In ESR analyses with different spin traps, the most stable signal was identified as belonging to the adduct of the p-hydroxyphenyl radical, even in the presence of dimethylsulfoxide or ethanol. A general scheme is proposed including three pathways for the homolytic dediazoniation of PDQ. Pathway 1 represents dediazoniation induced by a hydroxyl ion, a slow process at neutral pH and an even slower one with deaerated samples; a favored quinoid structure is put forward to explain these results. In pathway 2, the formation of a semiquinone radical via the reaction of an aryl radical with oxygen is put forward to justify the increase in the dediazoniation rate in the presence of oxygen. In pathway 3, hydroquinone, produced by semiquinone dismutation, may act as a reducing agent.