The use of chemical probes for the characterization of chemical properties is explored for aprotic binary solvent mixtures. The solvatochromic indicators N,N-diethyl-4-nitroaniline, 4-nitroanisole, 4-nitroaniline and 4-nitrophenol were used to characterize binary solvent mixtures of a polar aprotic hydrogen-bond acceptor solvent (ethyl acetate, acetonitrile and dimethyl sulfoxide) with a polychlorinated hydrogen-bond donor solvent (chloroform or dichloromethane). The solvent parameters p*, a and b of the binary mixtures were calculated from the solvatochromic shifts of the indicators. In each case the degree of convergence for a solvent property values obtained from different probes was analyzed. Data obtained by using the non-polar solvatochromic indicator b-carotene are additionally presented. The behavior of the solvent systems was analyzed according to their deviation from ideality due to preferential solvation of the solutes and the complicated intermolecular interactions of the two components of the solvent mixture. The validity of the concept of an intrinsic absolute property of a solvent mixture and whether such a property can be defined by means of chemical probes is discussed. Theoretical equations (preferential solvation models) were used to compute the solvatochromic data. The results were analyzed and related to the solvent effects on some aromatic nucleophilic substitution reactions, comparing the application of single-and multiparametric treatments of solvent effects.

Empirical solvent polarity parameters ET(30) were determined by UV–visible spectrophotometry using Dimroth– Reichardt's betaine dye, as a function of composition, for (aprotic + aprotic) and (aprotic + protic) binary solvent mixtures. For (aprotic + aprotic) solvent systems the cosolvent was toluene, and the other solvents used were selected with different structural characteristics and an extensive range of polarity: chloroform, 1,4‐dioxane, ethyl acetate, tetrahydrofuran, acetone, nitromethane and N,N‐dimethylformamide. For (aprotic + protic) solvent systems, the protic cosolvent used was methanol, and the aprotic solvents selected were toluene, chloroform, 1,1,1‐trichloroethane, tetrahydrofuran, acetone, N,N‐dimethylformamide, and dimethyl sulphoxide. Each system was analysed according to its deviations from additivity due to selective solvation of the betaine. A preliminary application of these empirical solvent polarity parameters was related to the solvent effects in a simple example of a nucleophilic aromatic substitution reaction.

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.

The kinetics of the reactions of 1‐fluoro‐2,4‐dinitrobenzene with morpholine and piperidine were studied at 25 °C in several binary solvent mixtures of the polar aprotic hydrogen bond acceptor solvent + chloroform or dichloromethane type, which, in some cases, exhibit synergism for the ET(30) solvent polarity parameter. In each case, the kinetic response model was analyzed as a function both of the amine concentration and of the solvent mixture composition. The kinetic data show that for some reaction systems under certain experimental conditions, the reactions are faster in binary solvent mixtures than in the corresponding pure solvents, manifesting a special kinetic synergetic behavior. This kinetic effect was attributed to a complex combination of factors related to the variations of the influence of base catalysis with the solvent composition, and to the consequence of specific solvent effects. The kinetic results were related to the ET(30) solvent polarity parameter, analyzing the general validity of the chemical probe for interpreting the solvent effects in the aromatic nucleophilic substitution reactions explored. The applicability of the preferential solvation model allowed the special behavior observed to be attributed to the influence of preferential solvation phenomena. Copyright © 1999 John Wiley & Sons, Ltd.

This paper presents a comparative study of the solvent effects on the kinetics of the reactions between 1‐fluoro‐2,6‐dinitrobenzene and hexahydro‐1H‐azepine in ethyl acetate + chloroform or acetonitrile and acetonitrile + chloroform binary solvent mixtures. The purpose was to asses the contribution of each molecular‐microscopic solvent property to the overall solvation effect of the reaction critical state. The influence of the dipolarity/polarizability, hydrogen‐bond donor acidity and hydrogen‐bond acceptor basicity of the solvent mixtures on the reaction rate was analyzed by the application of the multiparametric approach of Kamlet, Abboud and Taft. The evaluation of the correlation coefficients shows that the solvation effects are dominated by the non‐specific interactions. Moreover, the incidence of the hydrogen‐bond acceptor solvent ability is more important than that corresponding to the hydrogen‐bond donor character, except for the reactions that manifest kinetic retarding effects. The influence of the solvation effects on the base catalysis was analyzed, describing the response patterns of the partial rate coefficients to the mixture composition and also to each solvent property. The application of a preferential solvation model to the kinetic results yields information on the preferential solvation of the reaction critical state by any of the pure solvents, or by the hydrogen‐bonded complexed media. Copyright © 2002 John Wiley & Sons, Ltd.