Absolute Rate Constants for Hydrocarbon Autoxidation. XXI. Activation Energies for Propagation and the Correlation of Propagation Rate Constants with Carbon–Hydrogen Bond Strengths

Abstract

The rate constants, k_p, for the abstraction of a hydrogen atom from a wide variety of organic substrates at 30° have been correlated with the strengths of the C—H bond that are broken, D[R—H] (in units of kcal/mol). Two empirical relations have been derived, one for tertiary peroxy radicals,[Formula: see text]and one for secondary peroxy radicals,[Formula: see text]These equations hold extremely well for hydrocarbons and reasonably well for substrates containing heteroatoms. The activation energies and pre-exponential factors for the oxidation of some hydrocarbons by t-butylperoxy radicals have been measured and the empirical relation[Formula: see text]derived. If D[t-ROO—H] is taken as 88 kcal/mol, this yields the Polanyi equation,[Formula: see text]where ΔH is the heat of the hydrogen abstraction reaction. These empirical equations permit the calculation of the rate constants for chain propagation in the autoxidation of many organic substrates at 30° and also at other temperatures.