Solvent effect on the decarbonylation of acyl radicals

Abstract

The rate constant k_CO of decarbonylation of phenylacetyl and 2-hydroxy-2-methylpropanoyl radicals, generated by photolysis of dibenzyl ketone and 2,4-dihydroxy-2,4-dimethyl-3-pentanone, was measured in a number of solvents over a wide temperature range. The pre-exponential factors A and activation energies E_a were found for all solvents. The rate constant of phenylacetyl decarbonylation decreases with the increase of the solvent relative permittivity ε, but increases in protic solvents. The results of quantum chemistry calculations confirm the mutual cancellation of the contributions of specific and nonspecific solvations to the activation energy of the decarbonylation reaction in alcohols. For the 2-hydroxy-2-methylpropanoyl radical, the solvent effect on the decarbonylation rate constant is very small.