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-h 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-h has been evaluated. More importantly, the three compounds 1f (E = À7.90), 1g (E = À7.80) and 1h (E = À7.55) exhibit high electrophilicities that compare well with that of 4,6-dinitrobenzoselenadiazole (E = À7.40), 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.

Reactions of N1-Methyl-4-nitro-2,1,3-benzoselenadiazolium Tetrafluoroborate with Aliphatic and Cyclic Amines in Acetonitrile: Kinetic and Structure-Reactivity Correlations

“…A similar situation was, indeed, recently observed by Dharhi and co‐workers in the case of reactions of the of p‐X‐substituted benzylidenemalononitriles (X = NO 2 , CN, Cl, H, Me, OMe, and N(Me) 2 ) with hydroxide ions in 50% water–50% acetonitrile at 20°C . In this case, the correlation log k exp versus log k calcd is given by Eq.…”

“…As was explained in a previous paper , it has been suggested that the predicted rate constants for this type of reactions could be described by Eq. , corresponding to the introduction of an additional term C in the equation of Mayr (Eq.…”

The ambident electrophilic behavior of 5‐nitro‐3‐X‐thiophenes in σ‐complexation processes

“…Breugst and co‐workers have shown that Equation predicts the rate constants for reactions of amide anions with Michael acceptors in dimethyl sulfoxide solution within a factor of 2–21. In a recent paper, we have shown that the experimental rate constants of the reactions of hydroxide ions with various p ‐X‐substituted benzylidene malononitriles in 50% water–50% acetonitrile at 20°C are 10–23 orders of magnitude higher than those calculated from Equation . It is difficult to explain the causes of these deviations.…”

“…However, it is important to note that the linear free energy relationship (1) usually predicts rate constants of polar organic reactions within a factor of 10–100 . a, …”

“…In a recent paper, we have shown that the experimental rate constants of the reactions of hydroxide ions with various p-X-substituted benzylidene malononitriles in 50% water-50% acetonitrile at 20 • C are 10-23 orders of magnitude higher than those calculated from Equation 1. 69 It is difficult to explain the causes of these deviations. However, it is important to note that the linear free energy relationship (1) usually predicts rate constants of polar organic reactions within a factor of 10-100.…”