Substitution at saturated carbon. Part XIV. Solvent effects on the free energies of ions, ion-pairs, non-electrolytes, and transition states in some SN and SE reactions

Abstract

Standard free energies of transfer from methanol to various other solvents are reported for dissociated species R₄N⁺+ X^–(20–30 solvents), ion pairs R₄NX (20–30 solvents), t-butyl chloride (20 solvents), t-butyl bromide (10 solvents), tetramethyltin (15 solvents), tetraethyltin (15 solvents), and tetraethyl-lead (10 solvents). A dissection on solvent influences on ΔG‡ into initial-state and transition-state contributions has been accomplished for the decomposition of t-butyl chloride (18 solvents), the decomposition of t-butyl bromide (10 solvents), the Menschutkin reaction of NN-dimethylaniline with methyl iodide (3 solvents), and the iododemetallation of tetra-alkyl-leads (8 solvents).

A comparison of solvent effects on the free energies of ion pairs with solvent effects on the free energies of transition states suggests that transition states carry the following units of charge separation: [Bu^tCl]‡ 0·70 (polar solvents), [graphic omitted] 0·5 (non-polar solvents); [Bu^tBr]‡ 0·68 (polar solvents); [p-NO₂·C₆H₄·CH₂Cl–Me₃N]‡ 0·35. Charge separation in the [R₄Pb–l₂]‡ transition states is probably rather higher than in the transition state of the Menschutkin reaction.