DEGREE OF ELECTRON-TRANSFER IN SOME NUCLEOPHILIC REACTIONS DETERMINED BY ACTIVATION PARAMETERS

Activation parameters have been measured from the temperature dependence of rate constants for a number of reactions between, on the one hand, electron donors such as aromatic anion radicals and nucleophiles such as the anions of 4-methoxy-carbonyl-1,4-dihydro-1-methylpyridinium and 4-benzoyl-1,4-dihydro-1-methyl-pyridinium, and, on the other hand, different kinds of electrophile such as benzyl halides and vicinal dichlorides. For the sterically very hindered substrates the same activation enthalpy and entropy is found in the nucleophilic substitutions and reductive eliminations as in the electron transfer (ET) reactions with the anion radicals indicating that ET is the rate-determining step in these cases. When the electron-donating ability of the anion or the steric hindrance of the substrate is diminished the values of the activation parameters become smaller reflecting increasing stabilization of the transition state and thus a change of mechanism towards the more polar reactions (S(N)2 and E2). Even in the ET reactions between aromatic anion radicals and the less sterically hindered benzyl halides a certain inner-sphere character seems to be present. These conclusions are in accordance with those previously drawn from kinetic measurements. The results are compared with activation parameters for similar reactions reported in the literature.