Effect of aqueous solutions of various alcohols on kinetics of aromatic nucleophilic substitution reaction of 1-fluoro-2,4-dinitrobenzene with piperidine

Reaction kinetics of 1-fluoro-2, 4-dinitrobenzene with piperidine has been studied spectrophotometrically in aqueous solutions of methanol, ethanol, propan-2-ol and 2-methylpropan-2-ol at 10degreesC. The reaction in the aqueous solutions is not catalyzed by nucleophile(piperidine). Second-order rate constants of the reaction decrease with increasing mole fraction of water and E-T(N)(norinalized polarity parameter). Therefore, polarity of the reactants of the reaction is higher than that of the activated complex. Single-parameter correlations of log k(A) vs E-T(N) give satisfactory results (regression coefficients in T aqueous solutions of methanol, ethanol, propan-2-ol and 2-methylpropan-2-ol are 0.988, 0.994, 0.995 and 0.996, respectively). Single-parameter correlations of log k(A) vs pi*(dipolarity/polarizability) and alpha (hydrogen-bond donor acidity) do not give acceptable results( fore example, in the all aqueous solutions regression coefficients are 0.893 and 0.789, respectively), hence pi* and alpha are not individually major factors in determining the reaction rate. Dual-parameter correlations of log k(A) vs pi* and alpha in the all solutions represent significant improvement in statistical factors with regard to the single parameter models. Dipolarity/polarizability and hydrogen-bond donor acidity (HBD) of media have parallel and approximately equal effects on the reaction rate. Increasing of hydrogen-bond donor acidity of media stabilizes piperidine via hydrogen bonding interactions (one of the reactants) and hence the reaction rate decreases. A dual-parameter equation of logk(A) vs pi* and alpha is obtained in the all aqueous solutions (n = 41, r = 0.990, s.e = 0.070, F-2.38 = 968.10) in which pi* and alpha have parallel and approximately equal effects on the reaction rate.