LIMITING RATES OF HYDROCARBON AUTOXIDATIONS

A kinetic analysis of hydrocarbon autoxklation initiated by thermal or catalyzed hydroperoxide decomposition and involving the rate-determining steps (2) ROO· + RH ⇅ ROOH + R· (kp) and (3) 2ROO· ⇅ inert products (kt) predicts a maximum rate – d[RH]/dt = nkp2[RH]2/2fkt, where n/f is determined by the stoichiometry and details of the initiation process. Under these conditions chains are near unity and much RH is consumed by alkoxy- or other nonperoxide radical attack. Application of the analysis to cooxidations, involving either added cooxidants or intermediate oxidation products, shows that great increases in rate of oxidation of an unreactive component, which would not be observed in long-chain reactions, are possible under limiting rate conditions. While literature data indicate that some metal-catalyzed autoxidations reach these limiting rate conditions, others are even faster and follow kinetics which do not agree with this analysis. Under these conditions, not only must (2) be unimportant in the consumption of RH, but additional propagation steps, e.g. peroxy radical-metal ion reactions, must be taken into account. © 1969, American Chemical Society. All rights reserved.