Kinetics of the oxidation of quinol by aquovanadium(V) ions in perchlorate media

Abstract

Oscilloscope traces obtained by the stopped-flow technique show that the rapid oxidation of quinol QH₂ by V^V in perchlorate media is an inner-sphere reaction, and the rate is of the first order in [V^V] under all conditions but of first order in [QH₂] at low acidities and of less than first order in [QH₂] at high acidities. The rate increases with increasing acidity, and this is explained by the decomposition of two complexes, VO₂QH₂⁺_aq and VO(OH) QH₂²⁺_aq related by a protonation equilibrium. The increase in rate with acidity arises from the protonation of VO₂QH₂⁺_aq increasing the total concentration of intermediate complexes. Values for the equilibrium constants and rate constants are calculated, and energies and entropies of activation are compared with those for the oxidation of quinol by other cations.