Studies on Girard hydrazones. Part III. Further kinetic studies on the hydrolysis of Girard hydrazones

Abstract

The hydrolysis of Girard T and D hydrazones of several aldehydes and ketones has been studied kinetically in aqueous solution (containing 2% ethanol) by a polarographic method. In the hydrolysis of Girard hydrazones derived from aliphatic carbonyl compounds, a change in the rate-determining step from the rate-determining decomposition of the intermediate amino-alcohol in the highly acid region to a rate-determining attack of water (or hydroxide ion) on the substrate in slightly acidic, neutral, and alkaline regions, is demonstrated, which is similar to the hydrolysis of other azomethines. Four lines of evidence, however, strongly suggest that the pH-independent hydrolysis observed in the neutral region is the rate-determining attack of water on the neutral substrate in contrast to the hydrolysis of other azomethines. In the case of an aromatic Girard hydrazone, only rate-determining decomposition of intermediate is observed. In the decomposition of the intermediate amino-alcohol to the substrate, intramolecular general acid catalysis of an eight-membered ring is suggested in the hydrolysis of Girard D hydrazone that has a dissociable proton on its terminal nitrogen atom. The kinetic results do not indicate any essential difference between the mechanisms of hydrolysis of ketone and aldehyde Girard hydrazones which is responsible for the so-called selective hydrolysis of ketone hydrazone. The two types of pH–rate profile (inflexion type and bell-type) are discussed.