Development of a competitive technique for the comparison of strong oxidation inhibitors

Abstract

The mechanism of the inhibited chain oxidation of cumene has been carefully investigated in previous studies. A competitive technique for determining relative efficiencies of strong inhibitors was to be tried in this study. Such a study might extend and verify work of a similar nature on weaker inhibitors. However the relative efficiencies determined were not constant. They varied because of possible side reactions of N,N'-dipheny1-p-phenylenediamine, the inhibitor chosen as a standard. For the same reason variation of the initial concentration of the standard inhibitor caused slight unexpected deviations in the results. Hydroquinones were found to reduce N,N'-diphenyl-p-quinonimine, the oxidation product of the standard inhibitor, back to the inhibitor while stopping chains at the same time. As in previous studies, the strength of an inhibitor was found to increase with increasing ortho steric hindrance as well as with increasing electron density in the aromatic ring. Initiator concentration was found to be first order with respect to termination rate as would be expected in a chain reaction in which an inhibitor molecule stops two radical chains. The oxidation product formed from a phenolic inhibitor was found not to affect the standard inhibitor.