Kinetic Studies on the Nucleophilic Substitution Reaction of 4-X-Substituted-2,6-dinitrochlorobenzene with Pyridines in MeOH-MeCN Mixtures

Abstract

The reaction rates of 4-X-2,6-dinitrochlorobenzenes (X = NO2, CN, CF3) With Y-substituted pyridines (Y = 3-OCH3, H, 3-CH3, 4-CH3) in methanol-acetonitrile mixtures were measured by conductometry at 25 degrees C. It was observed that the rate constant increased in the order of X = 4-NO2 > 4-CN > 4-CF3 and the rate constant also increased in the order of Y = 4-CH3 > 3-CH3 > H > 3-OCH3. When the solvent composition was varied, the rate constant increased in order of MeCN > 50% MeOH > MeOH. The electrophilic catalysis by methanol may be ascribed to the formation of hydrogen bonds between alcoholic hydrogen and nitrogen of pyridines in ground state. Based on the transition parameters, rho(S), rho(N), beta(Y), rho(XY) and solvent effects, the reaction seems to proceed via SNAr-Ad.E mechanism. We also estimated the isokinetic solvent mixtures (rho(XY) = 0) based oil cross-interaction constants, where the substituent effects of the substrate and nucleophile are compensated.