A kinetic study of the air oxidation of pyrogallol and purpurogallin

Pyrogallol (1,2.3‐benzentriol, 1 ) reacts with dioxygen in weakly alkaline solutions to form purpurogallin (2,3,4,6‐tetrahydroxy‐5H‐benzocycloheptene‐5‐one, 2 ) which then reacts to form purpurogalloquinone ( 3 ) which then decays to a product absorbing at 440 nm. The formation of 3 requires 1.5 mol O 2 per mol 1 , and 1.0 mol per mol 2 . No O 2 is consumed during the decay of 3 , and this reaction, being insensitive to O 2 concentrations and the presence of both superoxide dismutase (SOD) and catalase, does not appear to be an oxidation. The first‐order rate of decay of 3 decreases with increasing pH. 1 and 3 react at pH 9.0 with a second‐order rate constant of ca. 100 M −1 s −1 . SOD inhibits the oxidation of 1 and 2 . Slight and variable apparent inhibition of the oxidation of 2 and 3 by SOD may be due to trace impurities in 2 , but not contamination by 1 . The peak concentration of 3 is attained more rapidly during the oxidation of 1 than during the oxidation of 2 . A kinetic scheme based on parallel oxidation of 1 by dioxygen and superoxide, and it which the semiquinone of 1 oxidizes 2 , is partially successful in simulating the observed kinetic behavior.