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

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 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 , 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 k exp and log k calc has been observed and discussed. Copyright © 2014 John Wiley & Sons, Ltd.