Correlation of solvent effects on rates of solvolysis and SN2 reactions

Abstract

The effect of solvent transfer on rates of some classical sN1 solvolyses and SN2 reactions depends on the anion solvating properties of the solvents, as measured either by AG.(Cl-) or by the changes in solvent acceptor number [A(AN)]. For transfer through high dielectric (c > 20) solvents other than water, the transfer free energies of activation show the approximate linear free-energy relationships, AG*tr = nAGtr(C1-) = -n’A(AN). These very simple relationships are extreme cases of the more precise expressions, AG*tr(SN1) = pAGt,(K+) + nAGt,(Cl-) - AGt,(RX) = -p’A(DN) - n’h(AN) -AGtr(RX) and AG*‘,,(S$2) = -nAGtr(C1-) - AG,,(RX) = n’A(AN) - AGtARX). These expressions apply to a greater range of reactions and correlate rates in water and formamide as well as other less structured solvents. Even for “limiting” solvolyses of tertbutyl chloride and p-methoxyneophyl tOSylate, cation solvation plays a significant role in sN1 but not sN2 reactions, if transfer is through a sufficiently wide range of low dielectric as well as high dielectric solvents. The usefulness of donor and acceptor numbers, as well as free energies of transfer of K+ and C1- as measures of cation and anion solvating power of solvents, is demonstrated.