Dual-parameter correlations on rate of an aromatic nucleophilic substitution reaction in aqueous solutions of methanol, ethanol, and propan-2-ol
✍ Scribed by M. R. Gholami; A. Habibi Yangjeh
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 102 KB
- Volume
- 33
- Category
- Article
- ISSN
- 0538-8066
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✦ Synopsis
Reaction kinetics of 1-chloro-2,4-dinitrobenzene with piperidine was studied spectrophotometrically in aqueous solutions of methanol, ethanol, and propan-2-ol at 25ЊC. The reaction in these solutions is not catalyzed by piperidine. The plots of second-order rate constants of the reaction vs. mole fraction of water show maxima in the all-aqueous solutions. Single-parameter correlations of log k 2 vs. * (dipolarity/polarizability), ␣ (hydrogen-bond donor acidity), and E T N (normalized polarity parameter) are very poor in the all solutions (for example, in aqueous solutions of ethanol, regression coefficients are 0.814, 0.113, and 0.486, respectively). Dual-parameter correlations of log k 2 vs. * and ␣ in all cases represent significant improvement with regard to the single-parameter models (in aqueous solutions of ethanol:
and Dipolarity/polarizability and hydrogen-bond donor n ϭ 11, r ϭ 0.980, s ϭ 0.034). acidity (HBD) of media have opposite effects on the reaction rate. The activated complex leading to the zwitterionic intermediate is expected to be favored by increasing the solvent dipolarity/polarizability parameter. Increasing the hydrogen-bond donor acidity of solvent stabilizes piperidine and hence the reaction rate decreases. A dual-parameter equation of log k 2 vs. * and ␣ was obtained in the all-aqueous solutions in which (n ϭ 31, r ϭ 0.956, s ϭ 0.055) * and ␣ have approximately equal and opposite effects on the reaction rate.