Metal complexes as antioxidants. IV. Reaction of cupric dialkyldithiophosphates and dialkyldithiocarbamates with alkyl hydroperoxides

Abstract

The initial reaction of cupric dialkyldithiophosphates and dialkyldithiocarbamates with tert-butyl hydroperoxide and α-cumyl hydroperoxide is a free radical chain process. Initiation is achieved by a redox reaction between the complex and the hydroperoxide to give alkoxy and alkylperoxy radicals. The alkoxy radicals then abstract a hydrogen from excess hydroperoxide to give alkylperoxy radicals. The cupric complexes are converted to copper sulphate by reaction with peroxy radicals while the hydroperoxide is reduced to alcohol. About 5 mol of hydroperoxide are decomposed by each mole of complex. The decomposition of tert-butyl hydroperoxide then stops whereas complete destruction of α-cumene hydroperoxide occurs by a heterogeneous ionic reaction.The kinetics of the initial reaction are second-order for both complexes. The dithiophosphate reaction is first-order in each reactant while the dithiocarbamate reaction is zero-order in the complex concentration and second-order in the hydroperoxide concentration. Simple kinetics, however, only hold for the initial rates of complex disappearance. Total dithiophosphate decomposition exhibits three stages, an initial fast reaction followed by an induction period and a rapid third stage. The concentration–time profile for dithiocarbamate decomposition is quite different and the overall rate of reaction in some instances increases as the complex concentration decreases.