Homogeneously catalyzed formaldehyde condensation to carbohydrates: III. Concentration instabilities, nature of the catalyst, and mechanisms

Abstract

The formose reaction, the homogeneously catalyzed condensation of formal dehyde to sugars, proceeds simultaneously with Cannizzaro and cross-Cannizzaro reactions. Reaction studies in a continuous stirred tank reactor have shown that rate instabilities are exhibited. There are temperature instabilities as well as concentration instabilities in calcium hydroxide catalyst, formaldehyde reactant, and hydroxyl ion. It is postulated that Ca(OH) + is the actual catalytic species for the formose system. A unifying mechanism is developed that uses observed rate phenomena to explain why almost any base, regardless of valence, is a catalyst for the formose and Cannizzaro reactions of formaldehyde. The mechanism postulates that reactions proceed from a common intermediate complexed species, and the selectivity for each reaction depends on the nature of the catalyst forming the carbohydrate complex. The catalytic mechanism explains the Lobry de Bruyn-van Eckenstein aldose-ketose rearrangements and mutarotations of sugars that also proceed in the system.