DEGREE OF ELECTRON-TRANSFER IN SOME NUCLEOPHILIC REACTIONS DETERMINEDBY ACTIVATION PARAMETERS

Activation parameters have been measured from the temperature dependen ce of rate constants for a number of reactions between, on the one han d, electron donors such as aromatic anion radicals and nucleophiles su ch 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 di chlorides. For the sterically very hindered substrates the same activa tion enthalpy and entropy is found in the nucleophilic substitutions a nd reductive eliminations as in the electron transfer (ET) reactions w ith the anion radicals indicating that ET is the rate-determining step in these cases. When the electron-donating ability of the anion or th e steric hindrance of the substrate is diminished the values of the ac tivation parameters become smaller reflecting increasing stabilization of the transition state and thus a change of mechanism towards the mo re polar reactions (S(N)2 and E2). Even in the ET reactions between ar omatic anion radicals and the less sterically hindered benzyl halides a certain inner-sphere character seems to be present. These conclusion s are in accordance with those previously drawn from kinetic measureme nts. The results are compared with activation parameters for similar r eactions reported in the literature.