Absolute rate constants for the addition of hydroxymethyl radicals to alkenes in methanol solution

Absolute rate constants and their temperature dependencies were determined for the addition of hydroxymethyl radicals (CH 2 OH) to 20 mono‐ or 1,1‐disubstituted alkenes (CH 2 = CXY) in methanol by time‐resolved electron spin resonance spectroscopy. With the alkene substituents the rate constants at 298 K ( k 298 ) vary from 180 M −1 s −1 (ethyl vinylether) to 2.1 middot; 10 6 M −1 s −1 (acrolein). The frequency factors obey log A /M −1 s −1 = 8.1 ± 0.1, whereas the activation energies ( E a ) range from 11.6 kJ/mol (methacrylonitrile) to 35.7 kJ/mol (ethyl vinylether). As shown by good correlations with the alkene electron affinities (EA), log k 298 /M −1 s −1 = 5.57 + 1.53 · EA/eV (R 2 = 0.820) and E a = 15.86 − 7.38 · EA/eV (R 2 = 0.773), hydroxymethyl is a nucleophilic radical, and its addition rates are strongly influenced by polar effects. No apparent correlation was found between E a or log k 298 with the overall reaction enthalpy.