Kinetics and mechanism of the pyridinolysis of benzenesulfonyl chlorides in methanol

The kinetics of reactions between Y-benzenesulfonyl chlorides and X-pyridin es in methanol at 35.0 degrees C were investigated. Strong para pi-acceptor s, X = p-CN and p-COCH3, give good linear fits to Bronsted-type plots with low beta(X) (=0.32-0.45) but show positive deviations from linear Hammett p lots (rho(X) = -1.98 to -2.79). Excellent linear Hammett plots are obtained with substituent variations in the substrate with rho(Y) = 0.6-1.0 which a re similar to those for the anilinolysis. These results support an S(N)2 me chanism for the pyridinolysis of benzenesulfonyl chlorides. The cross-inter action constant rho(XY) is a small negative value (-0.48). Much smaller mag nitudes of beta(X), rho(X) and rho(XY) than the corresponding values for th e anilinolysis suggest that the transition state is formed at a relatively earlier position along the reaction coordinate than for anilinolysis. Copyr ight (C) 1999 John Wiley & Sons, Ltd.