Dual-parameter correlations on rate of an aromatic nucleophilic substitution reaction in aqueous solutions of methanol, ethanol, and propan-2-ol

Reaction kinetics of 1-chloro-2,4-dinitrobenzene with piperidine was studie d spectrophotometricaliy in aqueous solutions of methanol, ethanol, and pro pan-2-ol at 25 degreesC. The reaction in these solutions is not catalyzed b y piperidine. The plots of second-order rate constants of the reaction vs. mole fraction of water show maxima in the all-aqueous solutions. Single-par ameter correlations of log k(2) vs. pi* (dipolarity/polarizability), alpha (hydrogen-bond donor acidity), and E-T(N) (normalized polarity parameter) a re very poor in the all solutions (For example, in aqueous solutions of eth anol, regression coefficients are 0.814, 0.113, and 0.486, respectively). D ual-parameter correlations of log k(2) vs. pi* and alpha in all cases repre sent significant improvement with regard to the single-parameter models tin aqueous solutions of ethanol: n = 11, r = 0.980, and s = 0.034). Dipolarit y/polarizability and hydrogen-bond donor acidity (HBD) of media have opposi te effects on the reaction rate. The activated complex leading to the zwitt erionic intermediate is expected to be favored by increasing the solvent di polarity/polarizability parameter. Increasing the hydrogen-bond donor acidi ty of solvent stabilizes piperidine and hence the reaction rate decreases. A dual-parameter equation of log k(2) vs. pi* and alpha was obtained in the all-aqueous solutions (n = 31, r = 0.956, s = 0.055) in which nf and a, ha ve approximately equal and opposite effects on the reaction rate. (C) 2001 John Wiley & Sons, Inc.