The photo-EC′ mechanism: reduction of t-butyl bromide by excited-state tetrachlorobenzoquinone radical anion

Abstract

The mediated reduction of t-butyl bromide by the photochemically excited radical anion of tetrachlorobenzoquinone (TCBQ) is studied in acetonitrile solution using the channel electrode. The reduction of TCBQ in the dark is a reversible one-electron process which forms the corresponding stable radical anion. Upon photoexcitation of the radical anion in the presence of t-butyl bromide, photocurrents are observed and the electrode process is shown to have the characteristics of an EC' mechanism: TCBQ ⇌ TCBQ^˙–, TCBQ^˙– [graphic omitted] TCBQ^˙–*, TCBQ^˙–*+ Bu^tBr → TCBQ + Bu^tBr^˙–, Bu^tBr^˙–→ products. The value of the effective rate constant (rate =k_eff[TCBQ^˙–][Bu^tBr]) for the electron transfer between the radical anion and t-butyl bromide has been evaluated from analysis of the limiting current/flow rate behaviour at the channel electrode. k_eff was found to be 1315 ± 50 dm³ mol^–1 s^–1, at a light intensity of 0.120 W cm^–2.