A kinetic study of the air oxidation of pyrogallol and purpurogallin

✍ Scribed by Henry I. Abrash; Dorcas Shih; Woodrow Elias; Farshad Malekmehr

• Publisher: John Wiley and Sons
• Year: 1989
• Tongue: English
• Weight: 565 KB
• Volume: 21
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550210609

✦ Synopsis

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 O2 per mol 1, and 1.0 mol per mol2. No O2 is consumed during the decay of 3, and this reaction, being insensitive to O2 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-' 5-l. SOD inhibits the oxidation of 1 to 2. Slight and variable apparent inhibition of the oxidation of 2 to 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.