Solvent effects on solvolysis of alkyl halides

Abstract

The separation of solvent effects on reaction rates into initial-state and transition-state contributions will be discussed with reference to the solvolysis of alkyl halides and to the Menschutkin reaction of triethylamine with ethyl iodide. In both of these cases, gas-chromatographic head-space analysis has been used to determine the effect of solvents on the free energy of the reactants, and solution calorimetry to determine the corresponding effect on the enthalpy (and hence the entropy) of the reactants. Combination of these measurements with the activation parameters ΔG, ΔH, and ΔS enables the solvent effect on the free energy, enthalpy and entropy of the transition states to be obtained. It is shown that initial-state solvent effects on the various reactants are usually not very large, except when water or aqueousorganic mixtures are used as solvents, unless there are specific solutesolute interactions, for example in the triethylaminechloroform system. Determinations have also been made of solvent effects on the standard free energies, enthalpies and entropies of a number of compounds that might be useful as models for transition states. In terms of free energy, ion-pairs (R 4N +X −) and α-aminoacids are shown to be suitable models for highly polar but electrically neutral transition states, especially if corrections are made for the ‘size’ or ‘cavity’ effect on introducing a solute into a solvent. It is more difficult to select highly polar model compounds for which enthalpy and entropy data can be obtained. It is shown that pairs of dissociated ions (R 4N + + X −) are usually not very suitable, but that α-aminoacids can be used, again provided that size or cavity corrections are made. It is suggested that measurements and calculations on model compounds and transition states enable deductions to be made as to the polarity of the transition states and their position along the reaction coordinate.