The oxidation of purpurogallin by oxygen and hydrogen peroxide

Abstract

The oxidation of purpurogallin in alkaline solution by oxygen and hydrogen peroxide is described. It is shown that initial attack on the purpurogallin anion is by oxygen (not hydrogen peroxide), producing a radical ion. This species is oxidised to, and exists in equilibrium with, purpurogalloquinone, which may undergo three separate reactions, two with hydrogen peroxide and one with solvent.

Hydrogen peroxide attacks the quinone in two ways; (i) to produce an intermediate thought to be a peroxy-purpurogallin, which decomposes to tropolone-αβ-anhydride and oxalic acid, this reaction predominates at room temperature and low alkali concentration and (ii) to produce α-carboxy-β-carboxymethyl tropolone in a reaction favoured by high temperature and alkali concentration. The solvent attacks the quinone to give an intermediate thought to be a hydroxy-purpurogallin, which is oxidised to tropolone-αβ-anhydride and oxalic acid.

Detailed paths for these reactions are proposed and discussed.