The Reaction of 2-Chloroquinoxaline with Piperidine in DMSO–H2O and DMF–H2O Mixtures: Kinetics and Solvent Effects

The rate constant of the reaction of 2-chloroquinoxaline with piperidine was measured spectrophotometrically using different aqueous solutions containing DMSO or DMF. Whatever the experimental conditions used, this reaction follows pseudo first order kinetics and is not amine catalyzed. Furthermore, the second order rate constant, kA, increases with increasing percentage of DMSO in the solution, in contrast to DMF. The kA values were then correlated with solvent parameters α, β, π*, ETN\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$E_{\text{T}}^{\text{N}}$$\end{document} and Y. Plots of log10kA against the reciprocal of the dielectric constant at 25 °C were found to be nonlinear in DMSO, while a linear relationship with a negative slope was found in the case of DMF. This difference between the solvents is presumably due to different solvation pathways between their initial and transition states. Thus, activation parameters ΔH#, ΔS# and ΔG# were evaluated and discussed to support this hypothesis. Finally, DFT calculations were performed, using the B3LYP functional and 6-311G(d,p) basis set, to determine optimum molecular geometry. IR, NMR spectra for both reactant and product and were then compared with experimental values.