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Kinetic studies on the reactions of Y-phenyl chloroformates with X-pyridines in acetonitrile are carried out at 25.0 °C. Both the Hammett and Brönsted plots are linear with enhanced substituent constants, σp -, and basicities, pK a -, for strong para π-acceptor X-substituents, p-CN and p-CH3CO. This indicates that the electron-rich formate (O−C−O) moiety overlaps with the pyridine ring π-system enabling, through conjugation with the para π-acceptors, the rate-limiting formation of a tetrahedral intermediate. The difference in the aminolysis mechanism between methyl, II, and phenyl chloroformate, III, is attributed to the much stronger electron-donating polarizability effect of C6H5 than of CH3. The proposed mechanism is supported by a relatively small βX (≅0.3) and by the lower ΔH ⧧ (6.7 kcal mol-1) and ΔS ⧧ (−44 eu) values for a stronger donor Y (Y = p-CH3O) coupled with a stronger para π-acceptor (X = p-CN) in pyridine.

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Kinetic studies of the pyridinolysis (XC(5)H(4)N) of aryl dithioacetates (CH(3)C(=S)SC(6)H(4)Z) are carried out in acetonitrile at 60.0 degrees C. A biphasic Brönsted plot is obtained with a change in slope from a large value (beta(X) congruent with 0.9) to a small value (beta(X) congruent with 0.4) at pK(a) degrees = 5.2, which is attributed to a change in the rate-limiting step from breakdown to formation of a zwitterionic tetrahedral intermediate, T(+/-), in the reaction path as the basicity of the pyridine nucleophile increases. A clear-cut change in the cross-interaction constants rho(XZ) from a large positive value (rho(XZ) = +1.34) to a small negative value (rho(XZ) = -0.15) supports the mechanistic change proposed.