Sulfur nucleophile equilibrium constant

For the p-meLhoxybenzyl cation the equilibrium constant for reaction with the sulfur nucleophile is more favorable than that for the chloride ion by a factor of 107. As already discussed on p. 73 (cf. Table 6) this is a normal reflection of the greater carbon basicity of sulfur than chlorine. However in the case of the quinone methide the relative magnitudes of the equilibrium constants is reversed, with Me2s/ ci = 0.008. Toteva and Richard attribute this to the unfavorable steric and electrostatic interactions between the CF3 groups of the quinone methide adduct and the positively charged sulfonium ion. 

The significance of these results for differences in reactivities of nucleophiles is that, despite the unfavorable relative equilibrium constants, Me2S is more reactive toward the quinone methide than chloride ion by a factor of nearly 3000. This mismatch of rate and equilibrium effects is summarized in Scheme 35. It must imply (a) that there is a relatively long partial bond between sulfur and carbon in the transition state so that the unfavorable steric and electrostatic effects are not developed and (b) that the favorable carbon-sulfur bonding interaction is well developed despite the long bonding distance. 

There have been several studies reporting equilibrium and rate measurements for reactions of TNB with thiolate ions. Values of the equilibrium constants measure the thermodynamic affinity of the nucleophile for the carbon center, a quantity usually known as the carbon basicity. This is distinct from nucleophilicity, which reflects the rate constant for nucleophilic attack. The results show that in comparison with oxygen bases, the polarizable sulfur bases have higher carbon basicities and nucleophilicities than expected from the values of the parent thiols [84, 161,162]. 

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