Substitution at saturated carbon. Part VIII. Solvent effects on the free energy of trimethylamine, the nitrobenzyl chlorides, and the trimethylamine–nitrobenzyl chloride transition states
Standard free energies of transfer from ethyl benzoate to 17 other solvents of trimethylamine, p-nitrobenzyl chloride, and the trimethylamine–p-nitrobenzyl chloride transition state have been calculated (at 298 K). The transition state is stabilised by polar solvents and is destabilised by non-polar solvents; dipolar aprotic solvents are much more effective in stabilising the transition state than are the aliphatic alcohols. Similar calculations have been carried out for the substitution of o-, m- and p-nitrobenzyl chlorides by trimethylamine for 6 solvents (at 303 K). A comparison of solvent effects on the free energy of the trimethylamine–p-nitrobenzyl chloride transition state, the t-butyl chloride solvolysis transition state, and the Et4N+l– ion pair indicates that the solvolysis transition state markedly resembles an ion pair in behaviour, but that the transition state for substitution of p-nitrobenzyl chloride by trimethylamine is very much less polar.