Nature of Salt Effects and Mechanism of Covalent Bond Heterolysis

Abstract

Data on the influence of neutral salts on the rates of unimolecular heterolyses of organic substrates, obtained mainly by the verdazyl method, are summarized here. It is assumed that heterolysis takes place with consecutive formation of four ion pairs: contact (CIP), cavity-separated (CSIP), one solvent moleculeseparated (SIP) and solvent-separated (SSIP). [Formula: see text] In the limiting step, the CIP interacts with a solvent cavity and the CSIP is formed, which converts quickly into the SIP and subsequently to the SSIP, which also quickly gives the reaction products. In the transition state, bonds between the molecules solvating the CIP are broken. In the absence of salt, the return from external ion pairs does not have much importance. The verdazyl indicator quickly and quantitatively reacts with the SSIP. The normal salt effect takes place due to the action of salt on the covalent substrate, which catalyses CIP formation. The special salt effect is caused by the association of the salt with the CIP, which prolongs the lifetime of the intermediate and increases the probability of its contact with a solvent cavity. The negative special salt effect is caused by association of the salt with the SIP or SSIP, which prolongs the lifetime of the intermediates and increases the probability of their contact with a solvent cavity to return to the covalent substrate. When the salt reacts with the SIP, the salt effect does not depend on the concentration and nature of verdazyl, but such a dependence takes place when the salt reacts with the SSIP. The site of the action of the salt is determined by the Hard and Soft Acids and Bases (HSAB) principle.