Kinetics and mechanism of the oxidation of 3,5-di-t-butyl-o-benzoquinone with hydrogen peroxide in aqueous methanol solution

Abstract

The kinetics of the reaction between 3,5-di-t-butyl-o-benzoquinone (DTBQ)(1) and hydrogen peroxide have been investigated in 50% aqueous methanol in the pH range 8–10 and an ionic strength of 0.1M using spectrophotometric techniques at temperatures between 19.5 and 36.2 °C. The rate law –d [DTBQ]/dt=k[DTBQ][O₂H^–], with k(25 °C) 22.0 ± 0.9 dm³ mol^–1 s^–1 describes the kinetic data. The activation parameters at 25 °C are ΔH^‡ 76 ± 4 kJ mol^–1 and ΔS^‡ 34 ± 1 J K^–1 mol^–1. The reaction is subject to specific base catalysis and the rate-determining step is presumably the nucleophilic attack of O₂H^– on the C(1) carbonyl group of DTBQ. The hydroperoxide intermediate so formed breaks down in two ways: (i) to 3,5-di-t-butylmuconic acid anhydride (4), which was detected by g.l.c.–m.s., and (ii) to 6-hydroxy-3,5-di-t-butyl-o-benzoquinone (8) which in the reaction with further hydrogen peroxide leads to 5-hydroxy-3,5-di-t-butylmuconic acid anhydride (9). All the products formed in the reaction and identified by g.l.c.–m.s. can be derived from the muconic acid anhydride derivatives (4) and (9).