ペルオキシラジカルによるクメンからの水素引抜き反応における極性効果

In order to study the polar effect in hydrogen atom abstraction from alkylbenzenes by peroxy radical, oxidation of meta and para substituted cumenes has been performed under atmospheric pressure of oxygen at 70°C with some azobisisobutyronitrile added as a radical initiator. The rate of chain initiation was measured from the induction period induced by the addition of a radical scavenger, 2, 6-di-tert-butyl-4-methylphenol. Electron supplying substituents increased the rate of oxidation of cumenes (Table 1), in agreement with the results reported previously by Russell and Williamson.7) Two methods were used to determine the rate constants for hydrogen atom abstraction by tetralylperoxy radical from substituted cumenes. One of the methods is the hydroperoxide method⁴⁾, where cumenes were oxidized in the presence of tetralyl hydroperoxide. The results of kinetic analysis confirmed that, substantially, all the chain carrying species changed from cumylperoxy radical to tetralylperoxy radical by addition of 0.1M tetralyl hydroperoxide in the oxidation of cumene (Figs. 1, 2, 3), The other method is the cooxidation method²³⁾, in which oxidation of mixtures of cumene and tetralin was carried out. This cooxidation method is quite useful for this system, since the plot of the rate of cooxidation as a function of composition gives a minimum. The cross propagation rate constants determined by these two methods agreed quite well (Table 1). They could be correlated with the Hammett equation using Brown-Okamoto's σ⁺ constants, and constant ρ was obtained as -0.61 (Fig. 4). This value was somewhat larger than that obtained by Howard, Ingold and Symonds²⁾ in hydrogen atom abstraction from substituted cumenes by cumylperoxy radical (ρ=-0.29), but it was about the same as ρ=-0.56 by tert-butylperoxy radical observed by Howard and Chenier³⁾. These data suggest that peroxy radicals are electrophilic and electron donating substituents increase the rate of propagation in the autoxidation of alkylbenzenes. A brief discussion on the methods of obtaining cross propagation rate constants has been presented.