Homogeneously catalyzed condensation of formaldehyde to carbohydrates: VII. An overall formose reaction model

Abstract

Carbohydrates may be naturally present in ppm quantities in paraformaldehyde and these are believed to be the cause of autocatalysis in the Ca(OH) 2-catalyzed formose reaction of paraformaldehyde solutions. Sublimed paraformaldehyde was not transformed at 40 °C, 1.75 M HCHO, 0.197 M Ca(OH) 2 to sugars by formose reaction, only to methanol and formate by Cannizzaro reaction. A minute trace of added glycolaldehyde—3 ppm—was sufficient to initiate autocatalysis. The autocatalytic process does not require that formaldehyde react with itself for initiation. The overall formose system was described by five reactions: (1) aldol condensation of formaldehyde with sugars; (2) aldol condensation; (3) dealdolization of sugars; (4) Cannizzaro reaction; and (5) saccharinic acid formation. Crossed Cannizzaro reaction was neglected. Simple power law kinetics resulted in a formose reaction model that described the autocatalytic progress of formaldehyde conversion to sugars in time. The relative rates for the reactions at 40 °C, 3 ppm glycolaldehyde, vary with conversion because of the different processes. At 49% conversion, the relative rates are (1) 59, (2) 1.0, (3) 7.3, (4) 2.5, (5) 0.28.