REACTIONS OF TRIPLET DECAFLUOROBENZOPHENONE WITH ALKENES - A LASER FLASH-PHOTOLYSIS STUDY

Rate constants for the reactions of triplet decafluorobenzophenone ((D FB)-D-3) with 30 alkenes have been measured, with values in the range 10(7)-10(9) L mol(-1) s(-1). The rate constant increases upon substitu ting 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 substitutio n at the double bond, suggesting that steric effects play a minor role in the reaction. Further, there is a linear correlation between log k (r) (k(r) = the second-order rate constant of the reaction of (DFB)-D- 3 and alkene) and the ionization potential of the alkene. The extent o f hydrogen abstraction was determined by measuring the yield of the ke tyl radical. Quantum yields for ketyl radical formation, Phi(K), incre ase with the ionization potential and the number of activated hydrogen s, nH, on the alkene. The absolute values of Phi(K) and a linear corre lation between Phi(K)/nH and ionization potential suggest that hydroge n abstraction is preceded by complex formation.