Kinetics of the Reactions of Phenyl-Substituted Benzylidenemalonodialdehydes with Water and Hydroxide Ion

Autor: Claude F. Bernasconi, James J. Claus, Francis X. Flores, Dalimil Dvorak
Rok vydání: 1994
Předmět:
Zdroj: The Journal of Organic Chemistry. 59:4917-4924
ISSN: 1520-6904
0022-3263
DOI: 10.1021/jo00096a039
Popis: A kinetic study of the reactions of substituted benzylidenemalonodialdehydes, ZC 6 H 4 CH=C(CHO) 2 with Z=p-NMe 2 , p-OMe, m-OMe, p-Cl, p-Br, m-NO 2 , and p-NO 2 , with OH - and water in aqueous solution is reported. The reactions lead reversibly to the formation of adducts, ZC 6 H 4 CH(OH)C-(CHO) 2 , although the kinetics are complex because of the formation of the enol form, ZC 6 H 4 CH-(OH)C(CHO)=CHOH, and OH - addition to one of the carbonyl groups. Analysis of the pH-dependence of the rates allows a determination of the rate and equilibrium constants for water and OH - addition as well as, in some cases, of the pKa of the enol form and the equilibrium constant of OH - addition to a carbonyl group. Furthermore, at low pH, where the p-NMe 2 derivative is protonated, k H2O 1 and K H2O 1 for this protonated form are also obtained. The rate and equilibrium constants correlate better with σ + than standard Hammett σ-values, indicating that a substantial amount of resonance stabilization is lost in converting the olefin to the adduct. BrOnsted-type plots of log k H2O 1 and log OH 1 vs log K H2O 1 show that the points for p-NMe 2 and p-OMe deviate negatively from the line defined by the others substituents. The observed deviations imply a reduced intrinsic rate constant (k o ) for these compounds, due to a transition state in which the loss of resonance is ahead of bond formation. This behavior is reminiscent of that in the reactions of substituted benzylidene Meldrum's acids and benzylidenemalononitriles with nucleophiles, but opposite to that found with substituted β-nitrostyrenes and other nitro-activated olefins
Databáze: OpenAIRE
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