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

Abstract

The rate constants of the reaction of p-X-substituted benzylidenemalononitriles 1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h with hydroxide ion were measured 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 NMe2) 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 (r2 = 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 σp+, the electrophilicity parameter E of some benzylidenemalononitriles 1c and 1e, 1f, 1g, 1h 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, 1b, 1c, 1d, 1e, 1f, 1g, 1h with hydroxide ion do not agree with the available experimental data. On the other hand, a good linear correlation between log kexp and log kcalc has been observed and discussed. © 2014 John Wiley & Sons, Ltd.