Metal complexes as antioxidants. V. Inhibition of hydrocarbon autoxidation by cupric complexes of dialkyldithiophosphoric and dialkyldithiocarbamic acids

Abstract

Liquid-phase reactions of alkylperoxy radicals with cupric complexes of dialkyldithiophosphoric and dialkyldithiocarbamic acids have been examined. These complexes have been shown to be extremely efficient peroxy radical scavenging antioxidants for hydrocarbon autoxidation. Initial rates of oxygen absorption by typical hydrocarbons were extremely slow implying inhibition rate constants > 106 M−1 s−1. The initial rate of disappearance of the dithiocarbamate was equal to the rate of chain initiation (Ri) whereas the rate of disappearance of the dithiophosphate was twice as fast as Ri. In both cases the rate of complex disappearance slowed down as the complex was consumed. Autoxidation of styrene commenced as soon as the complex disappeared while cumene did not absorb oxygen for a considerable length of time after complex destruction. Cumylperoxy radicals were converted to α-cumyl alcohol, α-methylstyrene, and acetophenone by reaction with these complexes and the copper ions were eventually precipitated as copper sulphate. In the case of the dithiocarbamate three intermediate cupric complexes were detectee by epr spectroscopy.