Abstract

The reactions of 4-methylphenyl and 4-chlorophenyf 4-nitrophenyl carbonates (1 and 2, respectively), phenyl, 4-methylphenyl, 4-chlorophenyl, and 4-nitrophenyl 2,4-dinitrophenyl carbonates (3, 4, S, and 6, respectively), and bis(2,4-dinitrophenyl) carbonate (7) with a series of pyridines are studied kinetically at 25.0°C in 44 wt% ethanol-water and an ionic strength of 0.2 M (KCl). The reactions are followed spectrophotometrically and under excess amine pseudo-first-order rate coefficients (k obs) are found. For all these reactions, plots of k obs versus free amine concentration at constant pH are linear, the slope (k N) being independent of pH. The Bronsted-type plots (log k N vs. pK a of the conjugate acids of the pyridines) are all biphasic (linear portions at high and low pK a and a curvature in between). These plots are in accordance with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T ±), and a change in the rate-determining step from formation of T ± to its breakdown to products, as the pyridine basicity decreases. Also studied are the effects of the leaving, non-leaving, and electrophilic groups of the substrate, and of the amine nature, on the pK a 0 value (value at the center of curvature of the Brønsted-type plots). Copyright © 2008 John Wiley & Sons, Ltd.