Nucleophilic Addition to Olefins. 9. Kinetics of the Reaction of Benzylidenemalononitrili with Malononitrile Anion

Article abstract of DOI:10.1021/jo00177a019

The rates of the reversible addition of - to benzylidenemalononitrile, to form -, have been measured in water and in 50percent Me2SO - 50percent water.In water at 20 deg C, k₁ = 2.30*1E5 M^-1 s^-1, k_-1 = 5.95 s^-1, and K₁ = 3.86*1E4 M^-1.In 50percent Me2SO - 50percent water at 20 deg C, k₁ = 9.50*1E5 M^-1 s^-1, k_-1 = 6.52 s^-1, and K₁ = 1.45*1E5 M^-1.The pK_a of the adduct is about 5 pK units lower than that of malononitrile.K₁ for the addition of - is much higher than K₁ for piperidine addition, despite the slightly lower proton basicity of -.This reflects the common observation that the carbon basicity of carbanions is higher than that of amines.The intrinsic rate contante for nucleophilic attack by - appears to be somewhat smaller than for amine attack, just as the intrinsic rate constant for protonation of - is lower than for the protonation of amines.This is not unexpected, since structural factors should affect the coordination of bases with Lewis acids in a similar way as coordination with the proton.The change from water to 50percent Me2SO - 50percent water has the effect of increasing k₁ by a slighthly larger factor (4.13) than K₁ (3.75).This implies that the intrinsic rate constant is higher in the less aqueous solvent, which is consistent with the notion that solvent reorganization contributes to the intrinsic barrier of the reaction.