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

The reaction rates of 4-X-2,6-dinitrochlorobenzenes (X=NO 2 , CN, CF 3 ) with Y-substituted pyridines (Y=3-OCH 3 , H, 3-CH 3 , 4-CH 3 ) in methanol-acetonitrile mixtures were measured by conductometry at 25 °C. It was observed that the rate constant increased in the order of X = 4-NO 2 > 4-CN > 4-CF3 and the rate constant also increased in the order of Y= 4-CH 3 > 3-CH3 > H > 3-OCH 3 . 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, ρ S , ρ N , β Y , ρ XY and solvent effects, the reaction seems to proceed via S N Ar-Ad.E mechanism. We also estimated the isokinetic solvent mixtures (ρ XY = 0) based on cross-interaction constants, where the substituent effects of the substrate and nucleophile are compensated.