ABSOLUTE RATE CONSTANTS FOR THE ADDITION OF CYANOMETHYL (CENTER-DOT-CH2CN) AND (TERT-BUTOXY)CARBONYLMETHYL (CENTER-DOT-CH2CO2C(CH3)(3)) RADICALS TO ALKENES IN SOLUTION

Absolute 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 3.3.10(3) M(-1)s(-1) (ethene) to 2.4.10(6) M(-1)s(-1) (1,1-diphenylethene) at 278 K, and the frequency factors are in the narrow range of log (A/M(-1)s(-1)) = 8.7 +/- 0.3. CH2CO2C(CH3)(3) shows a very similar reactivity with rate constants at 296 K ranging from 1.1.10(4) M(-1)s(-1) (ethene) to 10(7) M(-1)s(-1) (1,1-diphenylethene) and frequency factors log (A/M(-1)s(-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.