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.

The kinetics of the reactions of benzylidenecyanoacetates 1a-d (X = H, Me, OMe, and NMe 2 ) with nitroalkyl anions 2a-c have been studied in aqueous solution at 20°C. The second-order rate constants are used to evaluate the electrophilicity parameter E of these series of Michael acceptors 1a-d according to the linear free enthalpy relationship log k (20°C) = s N (E + N). The measured E values were found to cover a domain of reactivity, ranging from −10.07 for the most reactive electrophile 1a (X = H) to −14.04 for the less electrophile 1d (X = NMe 2 ). Mayr's approach was found to correctly predict the rate constants for the reactions of these series of olefins 1a-d with the hydroxide ion in water and 50% water-50% acetonitrile at 20°C. Analysis of the kinetic measurements using the Brönsted relationship shows that β nuc values remain remarkably constant for changes in the nature of the substituent X. A notable finding of this work is perhaps provided by the observed large changes in the electrophilicity parameter E on going from benzylidenecyanoacetates 1 to their analogues benzylidenemalonates 3 ( E ß 9.06-10.48), whereas the replacement of second CO 2 Et group by the CN group in 1 to give benzylidenemalononitriles 4 has little effect on electrophilic reactivity, i.e., E ß 0.65-0.95. AZAIEZ, DHAHRI, AND BOUBAKERThis abnormal pattern in the E values has been attributed to the resonance interaction and salvation effects. On the other hand, the effect of benzylidenecyanoacetate substituents on the electrophilic reactivity was examined quantitatively on the basis of the electrophilicity parameter E, leading to linear correlation of E with Hammett-Brown substituent constants (σ + p ). More importantly, the four electrophiles have comparable log k o values, which are located at a relatively low level, i.e., log k o ࣘ 4-5, in the intrinsic reactivity scale.

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A kinetic study is reported for the reactions of 2-methoxy-3-nitropyridine 1a and 2methoxy-5-nitropyridine 1b with three secondary amines 2a-c (morpholine, piperidine, and pyrrolidine) in aqueous solution at 20 • C. The Brønsted-type plots are linear with nuc = 0.52 and 0.55 for pyridines 1a and 1b, respectively, indicating that the reaction proceeds through a S N Ar mechanism in which the first step is the ratedetermining step. Additional theoretical calculations using the DFT/B3LYP method confirm that the C-2 carbon being the most electrophilic center for the both pyridines 1a and 1b. The second-order rate constants have been used to evaluate the electrophilicity parameters E of 1a and 1b according to the linear free energy relationship log k (20 • C) = s N (N + E). The E parameters thus derived are compared with the electrophilic reactivities of a large variety of anisoles. The validity of these E values has been satisfactorily verified by comparison of calculated and experimental second-order rate constants for the reactions of pyridines 1a and 1b with anion of ethyl benzylacetate. K E Y W O R D S density functional theory, electrophilicity parameter (E), equation of Mayr, kinetics, S N Ar, substituted pyridine Int J Chem Kinet. 2019;51:249-257.