Kinetics and Regioselectivity of the Autoxidation of Alkylaromatic Hydrocarbons

Conclusions concerning the reactivities of alkylaromatic hydrocarbons in autoxidations are possible neither on the basis of the oxidizabilities nor on the basis of the chain propagation constants kp. In order to compare different alkylaromatic hydrocarbons, their rate constants with a definite peroxy radical, for example with the cumylperoxy radical, must be determined. Another possibility is the evaluation of relative rate constants by competitive oxidations of binary mixtures of hydrocarbons. These constants are not related to a definite peroxy radical. Nevertheless, these relative rate constants are real measures of the reactivities. If one also determines the composition of the reaction mixtures, which is especially convenient after LiAlH4 reduction, then it is possible to evaluate relative reactivities of the single C-H bonds, for example in relation to the tertiary C-H bond of cumene. This was done in our studies, and a great number of single bond reactivities is given in this survey. It is therefore possible to evaluate the activating effect of phenyl groups on different types of α-C-H bonds, and steric effects are also perceptible. In o-substituted isopropyl aromatics, a desactivation by steric hindrance of mesomerism takes place.