Effect of the leaving group solvation on solvolytic behavior of benzhydryl derivatives

摘要

An effect of the leaving group (LG) solvation on reactivity of benzhydryl derivatives in S(N)1 reactions has been investigated by using X,Y-substituted benzhydryl phenyl carbonates, methyl carbonates, 3,5-dinitrobenzoates (DNB), and the corresponding benzhydryl chlorides as reference compounds. Reaction constants (s(f)) derived from LFER equation log k (25 degrees C) = s(f)(N-f + E-f) indicate that s(f) parameters of carbonates and DNBs decrease as the fraction of the water in a given solvent/water mixture increases, while those of chlorides remain unchanged. This phenomenon is due to less important solvation and less charge separation in the TS. Effects of the solvents on the reaction rates were analyzed by Grunwald-Winstein correlations using various solvent-ionizing power scales. The m values obtained for carbonates and DNBs are considerably smaller than the m values for chlorides. Also, the solvolysis rate constants of substrates that have stronger electrofuges are less influenced by solvent (lower m) than those with weaker electrofuges. Values of m parameters obtained for a given substrate in a given binary solvent system correlate well with the electrofugality of the generated benzhydrylium ion. Abscissa at which m = 0 represents the extrapolated critical electrofugality E-f(crit) of the substrates whose solvolysis rates should not depend on the water fraction in the aqueous/organic solvent mixtures. Similar values for the critical electrofugality have also been obtained from extrapolated logk versus E-f plots.