Absolute Rate Constants for the Addition of Hydroxymethyl Radicals to Alkenes in Methanol Solution

Article abstract of DOI:10.1002/kin.550270207

Absolute rate constants and their temperature dependencies were determined for the addition of hydroxymethyl radicals (^.CH2OH) to 20 mono- or 1,1-disubstituted alkenes (CH2 = CXY) in methanol by time-resolved electron spin resonance spectroscopy.With the alkene substituents the rate constants at 298 K (k₂₉₈) vary from 180 M^-1s^-1 (ethyl vinylether) to 2.1*10⁶ M^-1s^-1 (acrolein).The frequency factors obey log A/M^-1s^-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 k₂₉₈/M^-1s^-1 = 5.57 + 1.53 * EA/eV (R² = 0.820) and E_a = 15.86 - 7.38 * EA/eV (R² = 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 k₂₉₈ with the overall reaction enthalpy.