Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals

Five aryl-substituted phenacetyl radicals (X = p-MeO, p-Me, H, p-Cl, p-CF3) were generated by laser photolysis of the corresponding dibenzyl ketones in n-hexane and acetonitrile. The decarbonylation reaction was monitored through the rise in time-resolved absorption of the benzyl radical chromophore at 317 nm. The decarbonylation rate constants were obtained by a numerical integration procedure, where second-order radical reactions were explicitly taken into account. Values of (2–3) × 106 s−1 in acetonitrile and (6–10) × 106 s−1 in n-hexane revealed a large solvent effect for all derivatives (by a factor of ∼3). The electronic substituent effect indicates that both electron-withdrawing and electron-donating para substituents accelerate the decarbonylation slightly. The rate constants followed the order MeO > Me, Cl, CF3 > H. The substituent effects are interpreted in terms of the ability of the para substituent to stabilize the benzyl radical resulting from decarbonylation. Evidence for a polar effect was not obtained. The underlying reasons for the observed solvent effect are discussed.