Major vs. minor electronic substituent effects. Influence on the kinetics of radical anion reactions

The rate constants for dimerization of radical anions formed by reduction of a series of p-substituted ethyl cinnamates have been measured. The substituents range from strongly electron donating to strongly electron withdrawing and constitute a major electronic pertubation of the electrophore. The influence of the substituents on the rate constant for dimerization is compared with the effects obtained for a similar series but with remote substituents. The differences can be rationalized by different effects on distribution of charge and unpaired electron density in the radical anions.