Kinetics and Mechanism for Oxime Formation from 4-Pyridinecarboxaldehyde N-Oxide

Abstract

Oxime formation from 4-pyridine carboxaldehyde N-oxide is shown to occur with rate-determining carbinolamine dehydration under acidic and neutral conditions. First, saturation effects observed at pH~ 7 are strongly suggestive of carbinolamine accumulation, requiring that dehydration of the intermediate be the rate-determining step. Second, this reaction occurs exclusively with general acid catalysis in the pH range investigated; the value of the Brønsted exponent, α=0.66, is suggestive of rate-determining carbinolamine dehydration, not amine attack. Third, no evidence was found for the usual pH-independent reaction typical of addition of weakly basic amines to the carbonyl function. Finally the break observed in the pH-rate plot at pH near 0.5 can be interpreted in terms of protolytic equilibrium of the substrate.