Spontaneous dehydration mechanism of aromatic aldehyde reactions with hydroxyl and non‐hydroxyl amines

The plot of rate constants vs. pH for the dehydration step of the reaction between furfural and 5‐nitrofurfural with hydroxylamine, N ‐methylhydroxylamine, and O ‐methylhydroxylamine, shows two regions corresponding to the oxonium ion‐catalyzed and spontaneous dehydration. The oxonium ion‐catalyzed dehydration region of the reaction of furfural with the above mentioned hydroxylamines exhibits general acid catalysis with excellent Brønsted correlation (Brønsted coefficients: 0.76 ( r = 0.986), 0.68 ( r = 0.987), and 0.67 ( r = 0.993) respectively). However, the rate constants of the spontaneous dehydration of these hydroxylamines, where water is considered the general acid catalyst, exhibit a large positive deviation from the Brønsted line. This fact was not observed in the reaction of non‐hydroxyl amines with different aromatic aldehydes by other authors, thus supporting that the spontaneous dehydration steps for these reactions proceed by intramolecular catalysis. The mechanism of intramolecular catalysis might be stepwise. First, a zwitterionic intermediate is formed. It can then evolve in the second step by loss of water, or follow a concerted pathway, with the transference of a proton through a five‐membered ring (general intramolecular acid catalysis). In the case of non‐hydroxyl amines, data suggested the possibility of a mechanism of intramolecular proton transfer through one or two water molecules, from the nitrogen of the amine to the leaving hydroxide ion. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 685–692, 2002