Some observations on the kinetics and thermochemistry of the reactions of HO₂ radicals with aldehydes and ketones

The rapid, gas phase equilibrium addition of HO 2 radicals to CH 2 O to form the peroxy radical HOCH 2 OO • is in agreement with the known thermochemistry of these species. The recent study of the similar addition of HO 2 • to ketones shows no significant reaction, which is again in agreement with known thermochemistry. All these reactions are notable for significant dipole attraction between the reactants ranging from 3 to 7 kcal/mol. The thermochemistry shows that the hydroperoxyl alkoxy species, the primary possible adduct, is not favored by the free energy change for direct addition. This and the observed kinetics favor a concerted addition, H‐atom transfer, as the transition state for the reactions. Kinetic estimates for forward and reverse reactions are in good agreement with observations. A thermochemical examination of the step‐wise addition of HO 2 • to the carbonyl shows that the reaction proceeds through a concerted, cyclic transition state involving simultaneous H‐transfer, 3 + 2 cyclo‐addition of HO 2 • to the carbonyl group.