852-855Second order rate constants have been de ermined for the reaction of 2,4-dinitrophenyl acetate with 3- and 4-substituted pyridines and 4'-substituted 4’-styrylpyridines in acetonitrile-water (50%, v/v) mixture at 25°, 30°, 35° and 40°C. Activation parameters have been evaluated. Electron-releasing substituents increase the rate while electron-withdraw in substituents retard it. In both the reaction series linear free energy relationship is found to exist between the logarithms of rate constants and σ constants with ρ = -3.39 ± 0.38 (r = 0.989; s = 0.163; n= 12 for 3- and 4-substituted pyridines and ρ = -0.390 ± 0.080 (r = 0.984; s = 0.028; n = 8) for 4'-substituted 4-styrylpyridines at 30°C. The ratio of effectiveness of transmission of substituent effect in styryl system relative to pyridine system in this reaction is 0.115. The Bronsted plot obtained is linear and βN is found to be 0.692 ± 0.056 (r = 0.990; s= 0.146; n = 16) indicating the extensive bond formation between pyridine and the reaction centre in the transition state
