A mechanistic study of the electro-oxidation of bromide in acetonitrile and the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at platinum electrodes

The oxidation of bromide has been investigated by linear sweep and cyclic voltammetry at platinum electrodes in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, ([C(4)mim][NTf2]), and the conventional aprotic solvent. acetonitrile, (MeCN). Similar voltammetry was observed in both solvents, despite their viscosities differing by more than an order of magnitude. DigiSim(R) was employed to simulate the voltammetric response. The mechanism is believed to involve the direct oxidation of bromide to bromine in a heterogeneous step, followed by a homogenous reaction to form the tribromide anion: 2Br(-) --> Br-2 + 2e(-) Br-2 + Br- reversible arrow(kb)(kr) Br-3(-) K-eq At higher potentials, the tribromide anion dissociates to bromine, Br-2, and bromide, Br-, which is immediately oxidised, leading to the emergence of a second anodic wave. Irreversible electrode kinetics were inferred from Tafel analysis and removal of the first electron is believed to be the rate-determining step in the formation of bromine. The equilibrium constant, K-eq, where K-eq = [Br-3(-)]/([Br-2][Br-]) over bar was found to be 3 x 10(3) and 9 x 10(6) M-1 in the ionic liquid and in acetonitrile, respectively. An equilibrium constant, K-complex for the complexation of 1-ethyl-3-methylimidazolium bromide in acetonitrile was found to be 35.1 M-1. (C) 2004 Elsevier B.V. All rights reserved.