Solvent effects on aromatic nucleophilic substitution reactions. Part 9. Special kinetic synergistic behavior in binary solvent mixtures

The kinetics of the reactions of 1-fluoro-2,4-dinitrobenzene with morpholin e and piperidine were studied at 25 degrees C in several binary solvent mix tures of the polar aprotic hydrogen bond acceptor solvent + chloroform or d ichloromethane type, which, in some cases, exhibit synergism for the E-T(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 m ixture composition. The kinetic data show that for some reaction systems un der certain experimental conditions, the reactions are faster in binary sol vent mixtures than in the corresponding pure solvents, manifesting a specia l kinetic synergetic behavior. This kinetic effect was attributed to a comp lex combination of factors related to the variations of the influence of ba se catalysis with the solvent composition, and to the consequence of specif ic solvent effects. The kinetic results were related to the E-T(30) solvent polarity parameter, analyzing the general validity of the chemical probe f or interpreting the solvent effects in the aromatic nucleophilic substituti on reactions explored. The applicability of the preferential solvation mode l allowed the special behavior observed to be attributed to the influence o f preferential solvation phenomena. Copyright (C) 1999 John Wiley & Sons, L td.