Oxidations by the reagent O-2-H2O2-vanadium derivative-pyrazine-2-carboxylic acid. Part 13. Kinetics and mechanism of the benzene hydroxylation

It has been concluded on the basis of the kinetic study of benzene hydroxylation by the O-2-H2O2- nBu(4)NVO(3)-PCA reagent in acetonitrile at various temperatures that the oxidation is induced by the attack of hydroxyl radical on the benzene molecule. The rate-limiting step of the reaction is the monomolecular decomposition of the complex containing one coordinated PCA molecule as well as one hydrogen peroxide molecule: V-V (PCA) (H2O2) --> V-IV (PCA) + HOO. + H+. The V(IV) species thus formed reacts further in a non-limiting stage with the second H2O2 molecule to generate the hydroxyl radical: V-IV (PCA) + H2O2 --> V-V (PCA) + HO. + HO-. The effective activation energy is 19 +/- 3 kcal mol(-1).