ABSOLUTE RATE CONSTANTS FOR HYDROCARBON AUTOXIDATION: IV. TETRALIN, CYCLOHEXENE, DIPHENYLMETHANE, ETHYLBENZENE, AND ALLYLBENZENE

Abstract

Absolute rate constants have been measured for the autoxidation of five hydrocarbons under a variety of conditions. The propagation (k p ) and termination (k t ) rate constants at 30 °C (in l mole −1 s −1 ) are: tetralin in chlorobenzene 6.3 and 3.8 × 10 6 respectively, cyclohexene in chlorobenzene 6.1 and 2.8 × 10 6 , diphenylmethane 4.8 and 8.0 × 10 7 , ethylbenzene 0.11 and 2.0 × 10 7 , and allylbenzene 10 and 2.2 × 10 8 . Measurements on tetralin, α-methylstyrene, and allylbenzene in different solvents indicate that the effect of solvents on oxidation rates is mainly connected with changes in the rate of termination rather than propagation. Experiments with α,α-d 2 -diphenylmethane gave isotope effects k H /k D ~5.1 for k p and ~1.4 for k t . The rate constant for hydrogen atom abstraction from 2,6-di-t-butyl-4-methylphenol by peroxy radicals decreases in the order expected if steric effects are important, i.e., primary peroxy > secondary peroxy > tertiary peroxy radical.The co-oxidation method of estimating chain termination constants is criticized on the grounds that it can only be used to distinguish the fairly large changes in k t commonly encountered between hydrocarbons giving tertiary peroxy radicals and those giving secondary or primary radicals.The effect of hydrocarbon structure on bimolecular chain termination rate constants is reviewed. There is a gradation in k t from ~2 × 10 8 ] mole −1 s −1 for primary peroxy radicals, through the range 8 × 10 7 to 1 × 10 6 for secondary radicals, to the range from 3 × 10 5 to 3 × 10 2 for tertiary peroxy radicals.