Substitution at saturated carbon. Part XX. The effect of 39 solvents on the free energy of Et3N, EtI, and the Et3N–EtI transition state. Comparison with solvent effects on the equilibria Et3N + EtI ⇌ Et4N+I–and Et3N + EtI ⇌ Et4N++ I–

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Abstract
Limiting values of the Raoult's law activity coefficient of triethylamine in five solvents and of ethyl iodide in 32 solvents have been determined. When combined with previous data, these results yield values of the standard free energy of transfer of the Et3N–EtI transition state from dimethylformamide to 31 other aprotic solvents and to 7 hydroxylic solvents (including water). It is shown that although solvent effects on ΔG‡ for the reaction of Et3N with EtI are often the result of mainly transition-state effects, there are numerous solvents in which initial-state effects are large. Combination of the present data with previous data on the ion pair Et4N+I– and the pair of ions Et4N++ I– enables the solvent effect on values of ΔG°t for the title equilibria to be calculated. Comparison of values of ΔG°t(Tr) with ΔG°t(Et4N+I–) and ΔG°t(Et4N++ I–), and comparison of δΔG‡ with δΔG° values for the equilibria shows conclusively that the Et3N–EtI transition state lies between the reactants and the ion pair, rather closer to the reactants than to the latter. The pair of ions is far away from the transition state on the reaction coordinate.

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