Acyl/aroylperoxyl radicals: a comparative study of the reactivity of peroxyl radicals resulting from the α-cleavage of ketones
Employing 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS2−) as a selective radical probe, nanosecond laser flash photolysis (LFP) and competitive kinetic methods have been used to investigate the reactions of acyl/aroylperoxyl radicals derived from the addition of oxygen to the α-cleavage products of various ketones in methanol. The selectivity of ABTS2− for acyl/aroylperoxyl radicals (as opposed to alkylperoxyl radicals), under the experimental conditions used, is supported by the oxygen concentration dependence of the ABTS˙− radical ion absorption amplitudes for acyl radicals that undergo decarbonylation on sub-microsecond timescales. The oxygen concentration dependence of the ABTS˙− transient absorption reflects the competition between decarbonylation and oxygen addition and therefore this data also provides information on decarbonylation and oxygen addition rates. Further evidence in support of the selectivity of ABTS2− for acyl/aroylperoxyl radicals is the agreement between results obtained from using different ketone precursors for specific acyl/aroylperoxyl radicals. The order of reactivity of the peroxyl radicals towards ABTS2− is found to be aroylperoxyl > acylperoxyl ≫ alkylperoxyl. The use of ABTS2− as a monitor for acyl/aroylperoxyl radicals has been exploited by using competitive kinetics to obtain rate constants for the reactions of various acyl/aroylperoxyl radicals with vitamin E and trolox.