Reactions of Triplet Decafluorobenzophenone with Alkenes. A Laser Flash Photolysis Study

Abstract

Rate constants for the reactions of triplet decafluorobenzophenone (3DFB) with 30 alkenes have been measured, with values in the range 107−109 L mol-1 s-1. The rate constant increases upon substituting electron-donating groups on the double bond, whereas it decreases upon substituting electron-withdrawing groups. For cyclohexene and 2-butene, the rate constant increases with increasing methyl substitution at the double bond, suggesting that steric effects play a minor role in the reaction. Further, there is a linear correlation between log kr (kr = the second-order rate constant of the reaction of 3DFB and alkene) and the ionization potential of the alkene. The extent of hydrogen abstraction was determined by measuring the yield of the ketyl radical. Quantum yields for ketyl radical formation, ΦK, increase with the ionization potential and the number of activated hydrogens, nH, on the alkene. The absolute values of ΦK and a linear correlation between ΦK/nH and ionization potential suggest that hydrogen abstraction is preceded by complex formation.