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.
The evidence establishes that oxime formation from benzoylformic acid in the pH range from 0.25 to 5.5 occurs with acid-catalyzed dehydration of carbinolamines derived from the acid and its anion. The pH-rate profile shows in order of decreasing pH in two regions: (1) In the pH range from 5.5 to ∼2.2 the second-order rate constants are linearly dependent on the concentration of hydronium ion and (2) from pH ∼2.2 to 0.25 the rate constants deviate slightly from the line of slope = −1. This slight deviation is a consequence of the very similar values of limiting rate constants of the two forms of the substrate. Electrostatic interactions between charged carbinolamines and hydronium ions are analyzed. C 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: [554][555][556][557][558] 2008
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