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, ([C4mim][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® 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-→Br2+2e-Br2+Br-⇌kbkfBr3-KeqAthigher potentials, the tribromide anion dissociates to bromine, Br2, 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, Keq, whereKeq=[Br3-][Br2][Br-]was found to be 3 × 103 and 9 × 106 M−1 in the ionic liquid and in acetonitrile, respectively. An equilibrium constant, Kcomplex for the complexation of 1-ethyl-3-methylimidazolium bromide in acetonitrile was found to be 35.1 M−1.