The effect of pressure on the rates of some substitution reactions of alkyl halides

Measurements have been made of the influence of hydrostatic pressure on the rates of three kinds of bimolecular substitution reactions of alkyl halides in solution. They are reactions with (a) halide ions in acetone solution (Finkelstein reactions), (b) silver nitrate in aqueous ethanol and (c) mercuric nitrate in aqueous dioxan. ��� The Finkelstein reactions were unsymmetrical with respect to the halogen atoms, for instance I- + PrCl → PrI+Cl-. They were all accelerated by an increase of pressure, in contrast to the sym- metrical exchange I*- + PrI → PrI* + I-, which is known to be retarded by an increase of pressure. This fact supports Gonikberg's view of the role of partial desolvation in the symmetrical reactions. Reactions involving LiCl as the source of halide ions were accelerated much more than those involving KI, simply because an increase of pressure enhances the dissociation of unreactive LiCl ion pairs. ��� Reactions of the types (6) and (c) were also accelerated by an increase of pressure, a fact which argues against a suggestion that the attacking species might be ion pairs.