Kinetics of alkaline hydrolysis of p-substituted benzylidenemalononitriles in 50% aqueous acetonitrile: substituent effects and quantification of the electrophilic reactivity

摘要

The rate constants of the reaction of p-X-substituted benzylidenemalononitriles 1a-h with hydroxide ion weremeasured in 50% water-50%acetonitrile at 20 °C. The experimental kinetic data reveal that the points pertaining to electron donating substituted compounds (X=Me, OMe and NMe_2) exhibit negative deviations from the Hammett plot. However, the Yukawa-Tsuno plot for the same rate constants resulted in a good straight line with an excellent correlation coefficient (r~2 = 0.9916) and an r value of 1.15. Possible ground-state stabilization through resonance interactions has been suggested to explain the origin of the nonlinear Hammett plot. On the basis of the relationship between E and σ_p~+, the electrophilicity parameter E of some benzylidenemalononitriles 1c and 1e-h has been evaluated. More importantly, the three compounds 1f (E =-7.90), 1g (E =-7.80) and 1h (E =-7.55) exhibit high electrophilicities that compare well with that of 4,6-dinitrobenzoselenadiazole (E =-7.40), a compound which has a general behaviour representative for the superelectrophilic dimension.We have shown that the second-order rate constants calculated from Mayr's approach for the reaction of 1a-h with hydroxide ion do not agree with the available experimental data. On the other hand, a good linear correlation between log k_(exp) and log k_(calc) has been observed and discussed.