Cross-interaction constants as a measure of the transition-state structure. Part 1. The degree of bond formation in nucleophilic substitution reactions

Abstract

The kinetics of seven reaction series in methanol involving halides as leaving groups and aromatic amines as nucleophiles are reported. Analyses of various Hammett ρ values, especially the cross-interaction constants ρ_xy between substituents X in the nucleophile and Y in the substrate, are used to characterize the transition-state structure for the nucleophilic substitution reactions. The degree of bond formation in the transition state as measured by |ρ_xy| indicated that the reactions can be classified into two groups according to the magnitude of the cross-interaction constants; the fluoride series gave larger |ρ_XY| values indicating a greater degree of bond formation, under the same reactants condition, compared with other series with relatively good leaving groups, e.g. Cl^–, Br^–, and ^–OSO₂C₆H₅. When, however, a non-conjugative intervening group such as CO or CH₂ is present between the reaction centre and the benzene ring of a reactant, the cross-interaction constant is reduced by a factor of ca. 2, demonstrating the fall-off of the susceptibility of the reaction centres to substituent changes.