Solvent effects

Abstract

When a reaction takes place in solution, the solvent itself frequently does not enter into the stoichiometric equation, but is present in equal quantities before and after the reaction has taken place. Under these circumstances any effect which the solvent exerts on the reaction velocity may be described as a catalytic effect. The best quantitative measure of the catalytic effect of the solvent is obtained by comparing the reaction velocity in solution with the reaction velocity in the gas phase; however, this is frequently difficult or impossible, in which case we can only obtain the relative catalytic effects by measuring velocities in different solvents. One such effect of the solvent has already been dealt with in connection with acid-base catalysis (cf. p. 239), where it was shown that the so-called “spontaneous” reaction can be attributed to acidic or basic catalysis by the solvent molecules. This type of solvent catalysis involves the transfer of a proton to or from the solvent molecule, which is thus involved in a specific chemical reaction, and the catalytic effect of a given solvent can be related fairly closely to its acidic or basic properties. (Thus the velocity of the “spontaneous” decomposition of nitramide in different solvents decreases in the series water, isoamyl alcohol, cresol, and is zero in solvents like chloroform which have no basic properties.) In most cases, however, the effect of the solvent cannot be related to any specific chemical participation in the reaction, and may be regarded as essentially physical in nature. In this respect it may be compared with the primary salt effects already dealt with (p. 197), though the general problem of solvent action is much more complex and cannot be related to any single property of the medium. In spite of the large volume of theoretical and experimental work which has been done on the subject it cannot be said that even the outlines of a satisfactory theoretical treatment have yet been agreed upon. This section will therefore con sist chiefly of a brief review of the experimental material, followed by a critical comparison of the various attempts at a theoretical interpretation.