Kinetics and mechanism for oxime formation from methyl pyruvate

Abstract

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.

A DFT-based mechanistic study on the formation of oximes

“…However, in the acid‐catalyzed model system, the rate‐determining step corresponds to the internal proton transfer step, Step 2. This observation, pH dependence of the rate of oxime/hydrazone formation, is in good agreement with the reported kinetic studies …”

“…Based on our calculations, Step 2 is the rate‐limiting step and requires a substantial activation energy. Kinetic studies for the formation of oximes under acidic conditions reported that the dehydration (decomposition) of carbinolamine was observed to be the rate‐limiting step in the pH range of 0 to 6 . The calculated value of activation barrier for this step, 31.36 kcal/mol, seems incompatible with the experimental observations now that oximes can be formed with and without catalyst from alkyl or aryl reactants within hours .…”

“…The proposed mechanism for the formation of oximes relies mostly on the early studies done by Jencks . Later on, other studies have supported this mechanism. Accordingly, the formation of oximes proceeds through a multistep mechanism.…”

“…In the following steps, an intramolecular proton transfer process occurs and the carbinolamine intermediate decomposes into an oxime and a water molecule. The rate of oxime formation shows a bell‐shaped pH dependence, namely, at low pH values, carbinolamine formation is found to be rate‐limiting step, whereas at high pH values, carbinolamine decomposition becomes rate‐limiting step . Imine formation reactions are reversible, and the equilibrium favors free carbonyl when primary amines were used as the nucleophiles.…”

Liquid–liquid synthesis of oximes from carbonyl compounds: formation under neutral conditions and degradation at acidic hydrometallurgical process conditions

“…The kinetics of the oximation reaction has been studied in the past for various aldehydes and ketones . The reaction rate tends to vary widely among different carbonyl reagents because steric hindrance and electronic effects play an important role in the reactivity of carbonyl compounds . Another important factor is the reaction medium, as was observed while developing analytical methods for quantification of aldehydes and ketones in the 1960s…”