Solvent effects on aromatic nucleophilic substitution reactions .7. Determination of the empirical polarity parameter E-T(30) for dipolar hydrogen bond acceptor-co-solvent (chloroform or dichloromethane) mixtures. Kinetics of the reactions of halonitrobenzenes with aliphatic amines

Empirical solvent polarity parameters E-T(30) were determined by UV/VIS spectroscopy, using Dimroth-Reichardt's betaine dye, as a function of composition, for several binary solvent mixtures [i.e. polar hydrogen bond acceptor (PHBA) solvents + chloroform or dichloromethane]. Each solvent system was analyzed according to its deviations from additivity due to preferential solvation of the chemical probe and also from complicated intermolecular interactions of the mixed solvents. The E-T(30) parameter of many of these mixtures has presented synergism, The synergetic effects were more significant for those binary solvent systems in which chloroform is the co-solvent. These results were related to the solvent effects on some aromatic nucleophilic substitution reactions. The kinetics of the reactions between 1-halo-2,4-dinitrobenzenes and primary or secondary aliphatic amines were studied in three solvent systems (PHBA + chloroform) where the synergism for the E-T(30) polarity parameter is the rule. In all the aminodehalogenation reactions discussed the formation of the intermediate is the rate-determining step. The kinetic data show a tendency to decrease with decrease in the overall solvation capability of the binary mixture. In general, the reaction rates presented a gradual decrease in the PHBA solvent-rich zone and a large decrease at high co-solvent concentrations. The E-T(30) values corresponding to binary dipolar hydrogen bond acceptor-hydrogen bond donor mixtures may be not generally valid for interpreting solvation effects on the reactions under consideration. (C) 1997 John Wiley & Sons, Ltd.