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Formaldehyde reacts with substituted nitrosobenzenes giving the corresponding N-phenylhydroxamic acids. A mechanism involving three sequential steps in this reaction is proposed. The first step is the nucleophilic attack of the nitroso group on the carbonyi group which leads to the formation of the unstable tetrahedral zwitterionic intermediate. This step is followed by the proton transfer to the zwitterionic intermediate to form more stable nitrosocarbinolic cation intermediate, which in the subsequent step undergoes the rate-controlling elimination of proton from the C-atom ofnitrosocarbinolic group, leading to the final product, hydroxamic acid. The first and the second step appear to be reversible. The experimental evidence obtained, which is the basis for such a description of the investigated reaction, includes: a ) the order of reactivity of substituted nitrosobenzenes, as demonstrated by the plot of log kobs vs. u Hammett parameters with slope of -1.74; b ) the observation of a general-acid catalysis; c) the observation of the inverse solvent deuterium isotope effect of ca. 1.8 in the reaction; d ) the observation of kinetic primary deuterium isotope effect of ca. 8 related to the 'water' reaction in the reactions of formaldehyde with substituted nitrosobenzenes; e ) the observation of general-base catalysis in the reaction;f) the observation of the kinetic primary deuterium isotope effect of ca. 2.1 for the acetate-ion-catalyzed reaction.
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