Solvent effects on the competitive β-scission and hydrogen atom abstraction reactions of the cumyloxyl radical. resolution of a long-standing problem

Article abstract of DOI:10.1021/ja00055a015

Laser flash photolysis (LPF) 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 β-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 β-scission. The rate constants for β-scission at 30°C, k_β^CumO, were (2.6₃ ± 0.2₄) × 10⁵, and (19_.6 ± 3_.4 × 10⁵s^-1 in CCl4, C₆H₆, C₆H₅Cl, (CH₃)₃COH, CH₃CN, and CH₃COOH, respectively. The rate constant for hydrogen abstraction from cyclohexane were essentially identical in these six solvents, viz., k_a^CumO = (1.2₄ ± 0.1₂) × 10⁶ M^-1 s^-1. There is a reasonably good linear correlation between log (k_β^CumO/s^-1) and certain cybotactic solvent parameters indicating that solvent effect on k_β^CumO are due to a localized interaction between the transition state for β-scission and adjacent solvent.