Oxime formation from pyridine-2-, -3-, and -4-carboxaldehydes occurs with Tate-limiting carbinolamine dehydration under both acidic and neutral conditions. Carbinolamine dehydration of pyridine-2carboxaldehyde occurs via a transition state bearing a single positive charge, unlike the corresponding reaction for formyl-l-methylpyridinium ion and the reaction under acidic conditions for pyridine-3and -4-carboxaldehydes.Although certain equilibrium constants2-3 456and rate constants3 for addition of amines to pyridinecarboxaldehydes and to structurally related aldehydes including

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Data obtained from the reaction of oxime formation from benzoylformic anion lead to the suggestion that the spontaneous dehydration observed after pH ∼ 5.0, proceeds through a transition state in which an intramolecular general base catalysis is exerted by the carboxylate group. Copyright © 2007 John Wiley & Sons, Ltd.

Second‐order rate constants for substituted benzaldehyde oxime formation increase linearly with the activity of hydrated protons over the pH range ca 2–7. Under these conditions, first‐order rate constants show saturation behavior with increasing hydroxylamine concentration, establishing carbinolamine dehydration as the rate‐determining step. Equilibrium constants for the formation of the neutral carbinolamine are correlated by the σ+ substituent constants; ρ = 1.26. Under more acidic conditions, second‐order rate constants increase less rapidly than the activity of hydrated protons, indicative of a transition to pH‐independent carbinolamine formation, presumably the uncatalyzed addition of amine to aldehyde. The corresponding value of ρ for this process is 1.21. This value, together with that for the equilibrium constants (see above), suggests that C—N bond formation is nearly complete in the transition state. The rate constants for acid‐catalyzed carbinolamine dehydration are correlated by the σ substituent constants; ρ = −0.85. Copyright © 1999 John Wiley & Sons, Ltd.

Rate and equilibrium constants for methyl pyruvate oxime formation were determined as a function of pH over the range 0-7 in aqueous solution at 30°C and ionic strength 0.5 by spectrophotometric methods. The reaction occurs with rate-determining carbinolamine dehydration over the entire range of pH investigated. Carbinolamine dehydration is not susceptible to detectable general acid-base catalysis by a carboxylic acid buffer or hydroxylamine/hydroxylammonium ion buffer. Specific acid catalysis for carbinolamine formation is dominant at pH values below 5. Above that value, a pH-independent, water-catalyzed reaction becomes apparent. The pH-independent carbinolamine dehydration is unusually important with this substrate. a Standard deviation of K exp add was about 5%. b In most cases the data agree within 3%. † Pocker et al. report several values of k f over a temperature range of 0-25.6°C, pH 4.6 and m = 0.1. A simple Arrhenius plot from their data allowed us to obtain k f = 0.15 s À1 at 30°C.