Separation of polar and steric effects on absolute rate constants and arrhenius parameters for the reaction of tert‐butyl radicals with alkenes

Absolute rate constants and their temperature dependencies were measured for the reaction of tert‐butyl radicals with 24 substituted ethenes and several other compounds in 2‐propanol solution by time‐resolved electron spin resonance. At 300 K the rate constants cover the range from 60 M −1 s −1 (1,2‐dimethylene) over 16,500 M −1 s −1 (vinyl‐chloride) to 460,000 M −1 s −1 (2‐vinylpyridine). For the mono‐ and 1,1‐disubstituted ethenes log k 300 increases and the activation energy decreases with increasing electron affinity of the olefins. The frequency factors are in the range log A /M −1 s −1 = 7.5 ± 1.0 as typical for addition reactions, with minor exceptions. Electron affinity (polar) and steric effects on reactivity are separated for the addition of tert‐butyl to chloro‐ and methyl‐substituted ethylenes. A comparison with rate data for methyl, ethyl, 2‐propyl, and other radicals indicates both polar and steric effects on radical substitution.