Nucleophilic Addition to Olefins. 11. Kinetics of the Reversible Hydrolysis of Benzylidenemalononitrile in Water

Article abstract of DOI:10.1021/ja00334a058

The hydrolysis of benzylidenemalononitrile (BMN) to form benzaldehyde and malononitrile (or its anion) is significantly reversible, with an equilibrium constant K_h=/=3.36x10^-3 M in water at 25 deg C.It involves the following steps (see Scheme III): (1) nucleophilic attack by OH^- or by water to form the adduct T_OH(1-); (2) carbon protonation of T_OH(1-) to form T_OH0; (3) oxygen deprotonation of T_OH0 to form T_O(1-); and (4) collapse of the tetrahedral intermediate, T_O(1-), to form benzaldehyde and malononitrile anion, which is in rapid equilibrium with malononitrile.There is also a direct, water-catalyzed breakdown of T_OH0 to products.The pH-rate profile for the hydrolysis reaction has four phases, with the following rate-limiting steps: at pH>8, OH^- attack on BMN; at pH 5-8, water attack on BMN which is subject to non-enforced general base catalysis; at pH 1-5, collapse of T_O(1-); at pH<1, collapse of T_OH0.Rate and equilibrium constants for all steps are calculated or estimated and compared with the same parameters in the hydrolysis of a β-nitrostyrene and benzylidene Meldrum acid.They provide new evidence that the nucleofugality of carbanions is relatively high in carbonyl-forming but low in olefin-forming eliminations.They also confirm previous observations according to which the intrinsic barriers in carbanion-forming reactions are much higher for nitro-stabilized than for cyano-stabilized carbanions.