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

Abstract

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 LFERindicate 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 ¼ 0 represents the extrapolated critical electrofugality E crit f 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.

Solvolytic Behavior of Aliphatic Carboxylates

“…It has been shown earlier that for some leaving groups the slope parameter s f decreases as the polarity of solvent increases, similarly to the trend shown in Figure 1 8,10. If it is borne in mind that the log k versus E f correlation lines are obtained with substrates that have the same leaving group but different electrofuges, this phenomenon shows that in some cases substrates with more reactive electrofuges are less sensitive to the solvent change than substrates with less reactive electrofuges.…”

“…The ratio between the solvolysis rate constants of, for example, 4 ‐fluoroacetate (X = OMe, Y = H, E f = –2.09) in 60 % aqueous ethanol and in 90 % aqueous ethanol is 8.0, whereas that of the substrate 1 ‐fluoroacetate (X = Y = OMe; E f = 0) in the same solvents is only 4.9. The convergence of the log k versus E f plots towards higher electrofugalities has been explained in terms of diminished demand for solvation in transition states for substrates (i.e., electrofuges) in which the positive charge is more delocalized, as in, for example, dianisylmethyl derivatives 8,10. Consequently, Grunwald–Winstein plots for solvolysis of series of carboxylates with the same leaving group and different benzhydrylium electrofuges yield electrofuge‐dependent m values 8.…”

Effect of the Leaving Group and Solvent Combination on the LFER Reaction Constants

“…Thus, polarity of solvent has a bigger impact on the reactivity for the substrates that produce less stabilized benzhydrylium ions than for those that produce more stabilized benzhydrylium ions. This consideration is in accord with experimental findings [ 42 , 43 ]. For example, the ratio between the solvolysis rate constants of benzhydryl phenyl carbonate in which X = Y = CH 3 in Scheme 1 ( E f = −3.44) in 60% aqueous ethanol and 90% aqueous ethanol is 11.8, while that of the substrate in which X = MeO and Y = CH 3 ( E f = −1.32) in the same solvents is only 5.7.…”

“…The log k vs. E f correlation lines for phenyl carbonates obtained in ethanol-water binary solvents are presented on Figure 2 . In our further investigations the same phenomena have been observed for 3,5-dinitrobenzoate (DNB) [ 43 ] ( Figure 2 ) and 2,4-dinitrophenolate (DNP) [ 28 ]. Preliminary results obtained with fluorinated benzoates and fluorinated phenolates also show that the same phenomenon occurs [ 41 ].…”

Solvolytic Reactivity of 2,4‐Dinitrophenolates

“…The nucleofugality parameters in various solvents of some frequently used halides, carboxylates, [1,2] and carbon-volyze through a late transition state (TS) in which the negative charge delocalization causes considerably diminished solvation. Because of the late TS, the nucleofuge-specific slope parameters, s f , are relatively high, i.e., the log k vs. E f plots are steeper than for most of the previously investigated leaving groups.…”