Entropic control of photochemical reactivity. The transition state for γ-hydrogen abstraction of alkyl phenyl ketones

The photochemical reactivity of several alkyl phenyl ketones, having similar n,π* triplet energies and γ C-H bond strengths but differing in conformational mobility, has been investigated. Measurements of Arrhenius activation parameters show that the rate enhancements in conformationally restricted molecules are entirely entropic in origin. Rate factors of 5-8 for each internal rotation which must be frozen in the transition state are observed. Primary and secondary kinetic deuterium isotope effects have been measured for photochemical γ-hydrogen abstraction. The results are consistent with an unsymmetrical transition state with less than 50% γ C-H bond breaking. The transition state model for γ-hydrogen abstraction is in accord with experimental results and recent theoretical predictions.