Absolute Rate Constants for the Addition of Cyanomethyl (·CH2CN) and (tert‐Butoxy)carbonylmethyl (·CH2CO2C(CH3)3) radicals to alkenes in solution

Abstract

AbstractAbsolute rate constants and their temperature dependence were determined by time‐resolved electron spin resonance for the addition of the radicals ·CH2CN and ·CH2CO2C(CH3)3 to a variety of mono‐ and 1,1‐disubstituted and to selected 1,2‐ and trisubstituted alkenes in acetonitrile solution. To alkenes CH2CXY, ·CH2CN adds at the unsubstituted C‐atom with rate constants ranging from 3.3·103 M−1S−1 (ethene) to 2.4·106 M−1S−1 (1,1‐diphenylethene) at 278 K, and the frequency factors are in the narrow range of log (A/M−1S−1) = 8.7 ± 0.3. ·CH2CO2C(CH3)3 shows a very similar reactivity with rate constants at 296 K ranging from 1.1·104 M−1S−1 (ethene) to 107 M−1S−1 (1,1‐diphenylethene) and frequency factors log (A/M−1S−1) = 8.4 ± 0.1. For both radicals, the rate constants and the activation energies for addition to CH2CXY correlate well with the overall reaction enthalpy. In contrast to the expectation of an electro‐ or ambiphilic behavior, polar alkene‐substituent effects are not clearly expressed, but some deviations from the enthalpy correlations point to a weak electrophilicity of the radicals. The rate constants for the addition to 1,2‐ and to trisubstituted alkenes reveal additional steric substituent effects. Self‐termination rate data for the title radicals and spectral properties of their adducts to the alkenes are also given.