Kinetics and Mechanism of Benzaldehyde Girard T Hydrazone Formation

Article abstract of DOI:10.1021/jo00004a016

In aqueous solution, Girard T hydrazone formation from para-substituted benzaldehydes does not proceed to completion under the condition used.The corresponding equilibrium constants were determined and employed to calculate the rate of the reaction at completion.For all the reactions studied, the pH-rate profiles, extrapolated to zero buffer concentration, show one break at pH 4-5, characteristic of a change in the rate-determining step from carbinolamine dehydration to carbinolamine formation upon going from pH 7 to pH 1.The formation of the carbinolamine is subject to catalysis by hydronium ion and by carboxylic acids present in the buffers used to maintain pH.Broensted α values for this catalysis varied from 0.19 to 0.37.Rate constants for the cyanoacetic, chloroacetic, and formic acid catalyzed formation of carbinolamine from the para-substituted benzaldehydes correlate with ?⁺ substituent constants, giving a value of ρ⁺ equal to 0.70.Rate constants for the hydronium ion catalyzed formation of the carbinolamine from the same benzaldehydes are, however, insensitive to the substituent effect.These observations lead to the conclusion that the reactions occur by different routes as a function of the catalyst.We suggest a stepwise preassociation mechanism for the reaction catalyzed by carboxylic acids and a concerted preassociation mechanism for that catalyzed by hydronium ion.