Kinetics and mechanism of the pyridinolysis of phenacyl bromides in acetonitrile

Kinetic studies of the reactions of substituted phenacyl bromides (YC6H4COCH2Br) with pyridines (XC5H4N) are carried out in acetonitrile at 45.0 °C. A biphasic Bronsted plot is obtained with a change in slope from a large (βX ≅ 0.65−0.80) to a small (βX ≅ 0.36−0.40) value at pKa° = 3.2−3.6, which can be attributed to a change in the rate-determining step from breakdown to formation of a tetrahedral intermediate in the reaction path as the basicity of the pyridine nucleophile increases. This mechanism is supported by the faster rates with pyridines than with anilines and the change of cross-interaction constant ρXY from a large positive (ρXY = +1.4) to a small positive (ρXY ≅ +0.1) value. The large magnitude of Hammett ρX (= −5.5 to −6.9) values for the pyridines with electron-withdrawing substituents and positive deviations of the π-acceptors, p-CH3CO and p-CN, are quite similar to those for the pyridinium ion formation equilibria. The activation parameters are also in line with the proposed mechanism.