Solvent effects on aromatic nucleophilic substitutions. Part 3. The kinetics of the reaction of 2,4-dinitrochlorobenzene with piperidine in aprotic solvents

Abstract

The kinetics of the reaction between 2,4-dinitrochlorobenzene and piperidine were studied in nitromethane, NN-dimethylformamide, ethyl acetate, dioxane, and 1,1,1-trichloroethane at 15,25, and 40 °C. This reaction was chosen as the simplest example of aromatic nucleophilic substitution. Different correlations of the second-order rate coefficients k_A with several solvent parameters were examined for this and earlier work combining 13 aprotic solvents in all with a range of 43 units in the dielectric constants (ε) in the series. It was found that the E_T(30) parameter is the most useful available measure of the solvent influence in the present reaction if hydrogen-bond donor (HBD) solvents are excluded; the deviation from linearity of these solvents is explained as arising from the highly exposed strong negative charge on Dimroth's betaine which gives a high E_T value to HBD solvents. Conversely, the correlation is remarkably successful for the hydrogen-bond acceptor (HBA) solvents since the positive charge in the present reaction is buried similarly to that in the model solute, and the amine hydrogen of the zwitterionic intermediate, ZH, is strongly involved in an intramolecular hydrogen bond. Aliphatic, aromatic, and polychlorinated aliphatic solvents are all well correlated.