SOLVENT EFFECTS ON THE COMPETITIVE BETA-SCISSION AND HYDROGEN-ATOM ABSTRACTION REACTIONS OF THE CUMYLOXYL RADICAL - RESOLUTION OF A LONG-STANDING PROBLEM

Laser flash photolysis (LFP) techniques with detection in the infrared and in UV-visible regions of the spectrum have been used in combination with detailed product studies to assess solvent effects on the hydrogen abstraction and beta-scission reactions of cumyloxyl radicals. The variation in the ratio of the products of these two competing processes in solvents with different polarities is due to the solvent's influence on beta-scission. The rate constants for beta-scission at 30-degrees-C, k(beta)CumO, were (2.6(3) +/- 0.2(4)) x 10(5), (3.7(5) +/- 0.5(3)) X 10(5), (5.5(4) +/- 2.0(0)) X 10(5), (5.8(4) +/- 1.0(6)) X 10(5), (6.3(3) +/- 0.43) X 10(5), and (19.(6) +/- 3.(4)) X 10(5) s-1 in CCl4, C6H6, C6H5Cl, (CH3)3COH, CH3CN, and CH3COOH, respectively. The rate constants for hydrogen abstraction from cyclohexane were essentially identical in these six solvents, viz., k(a)CumO = (1.2(4) +/- 0.1(2)) X 10(6) M-1 s-1. There is a reasonably good linear correlation between log (k(beta)CumO/s-1) and certain cybotactic solvent parameters indicating that solvent effects on k(beta)CumO are due to a localized interaction between the transition state for beta-scission and adjacent solvent.