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 i n n-hexane and acetonitrile. The decarbonylation reaction was monitored thr ough the rise in time-resolved absorption of the benzyl radical chromophore at 317nm, 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) x 10(6) s(-1) in acetonitrile and (6-1 0) x 10(6) s(-1) in n-hexane revealed a large solvent effect for all deriva tives (by a factor of similar to3). The electronic substituent effect indic ates that both electron-withdrawing and electron-donating para substituents accelerate the decarbonylation slightly, The rate constants followed the o rder MeO > Me, Cl, CF3 > H. The substituent effects are interpreted in term s of the ability of the para substituent to stabilize the benzyl radical re sulting from decarbonylation. Evidence for a polar effect was not obtained. The underlying reasons for the observed solvent effect are discussed. Copy right (C) 2000 John Wiley & Sons. Ltd.