Abstract
Abstract
The reaction kinetics of the dehydration step of the condensation reaction between phenylhydrazine and benzaldehyde was studied spectrophotometrically in aqueous solutions of ethanol, propan‐2‐ol and 2‐methylpropan‐2‐ol at pH 11.5 at 25 °C. The apparent second‐order rate constants k2app of the reaction increase with mole fraction of water in all the aqueous solutions. Single‐parameter correlations of log k2app versus π* (dipolarity/polarizability), α (hydrogen‐bond donor acidity) and $E_{\rm T}^{\rm N}$ (normalized polarity parameter) were obtained in all the aqueous solutions. In all cases, the correlations versus $E_{\rm T}^{\rm N}$ and π* are acceptable, but correlations versus α are poor (e.g. in aqueous solutions of ethanol the correlation coefficients are 0.964, 0.967, and 0.751 respectively). The results of dual‐parameter correlations of log k2app versus π* and α in all cases represent improvements with regard to the single‐parameter models (in aqueous solutions of ethanol r = 0.995; s.e. = 0.038; n = 13). The apparent second‐order rate constants of the reaction increase with α, π* and $E_{\rm T}^{\rm N}$. Increasing the hydrogen‐bond donor acidity of the solvent stabilizes the activated complex of the reaction via hydrogen‐bond formation with the intermediate. A dual‐parameter equation of log k2app versus π* and α was obtained for the combined data set of aqueous solutions (n = 35, r = 0.989, s.e. = 0.062, F2,32 = 719.21), in which π* has the major effect on the reaction rate relative to α. Copyright © 2001 John Wiley & Sons, Ltd.