ON THE OCCURRENCE OF ELECTRON-TRANSFER IN ALIPHATIC NUCLEOPHILIC-SUBSTITUTION

Three types of evidence, the activation entropy and enthalpy based on temperature measurements, stereochemical results based on preparative electrolysis, and k(SUB)/k(ET) values based on kinetic measurements give the picture that sterically very hindered alkyl halides react with aromatic anion radicals and certain anions in an outer-sphere electron transfer, whereas a certain stabilization of the transition state is found when the steric hindrance is diminished. The activation entropies and activation enthalpies for the reactions between aromatic anion radicals or the anion of 1,4-dihydro-4-methoxycarbonyl-1-methylpyridine (1-) and alkyl halides have been calculated from the temperature dependence of the rate constants. A relatively small negative activation entropy and relatively high activation enthalpy is found for the reactions with sterically hindered alkyl halides, whereas reactions with less hindered alkyl halides have more negative activation entropies and lower activation enthalpies. These properties change gradually through the series as the steric hindrance for an S(N)2 reaction changes. The change in activation parameters in a series occurs sooner for 1-than for the aromatic anion radicals.