Substitution at saturated carbon. Part XIX. The effect of alcohols and water on the free energy of solutes and on the free energy of transition states in SN and SE reactions

Abstract

Standard free energies of transfer from methanol to other alcohols and to water are reported for nearly 40 solutes, ranging from hydrocarbons to amino-acids. These transfer free energies from methanol to alcohols are negative for nonpolar inert solutes, but quite positive for amino-acids and ion pairs; from the assembled data it is predicted that linear free energy relationships between ΔG^‡ values for substitution reactions in alcohols will in general be approximate only. By combination of initial-state effects with values of ΔG^‡ previously determined, free energies of transfer from 20 different substitution reactions involving electrically neutral reactants. The various values of ΔG^o_t(Tr) are by no means linearly related, but by comparison with values for polar species such as amino-acids and ion pairs it is shown that the polarity of transition states increases in the order [R₄Pb–I₂]^‡ < [R₃N–RI]^‡⩽[R₄Sn–I₂]^‡ < [R₄Sn–HgX₂]^‡ < [Bu^tX]^‡⩽[phCHMeCl]^‡⩽[ph₂CHCl]^‡. The α-amino-acids are shown to be suitable model solutes for highly polar transition states, especially when any differences in molar volume between solute and transition states, especially when any differences in molar volume between solute and transition are taken into account.