Reactions of Carbohydrates with Strong Alkali

Two Dutch chemists, Lobry de Bruyn and Alberda van Eckenstein, collaborated in the study of the effects of alkali on carbohydrates. The reaction with alkali produces epimerization of aldoses and ketoses and aldose-ketose isomerization [2]. At pH values of 11-13 and 20°C, alkali catalyzes the transformation of D-glucose into D-fructose and o-mannose. The transformation most probably takes place by the formation of two enediols, although the enolic forms of the sugars have never 

There are several enzyme-catalyzed reactions that involve aldo-keto isomer-izations of both phosphorylated and nonphosphorylated carbohydrates. The following illustrate some of the important enzyme-catalyzed aldose-ketose isomer-izations  

The commercial enzyme (glucose isomerase) is used to convert high-glucose com symps into high-fructose com symps that are commonly used as a sweetening agent in soft drinks. This enzyme is actually a xylose isomerase that can catalyze the isomerizations of both D-glucose and D-xylose. 

As with mutarotation, acid will catalyze aldo-keto isomerization and epimerization reactions, but it is not as effective as alkali. In fact, carbohydrates usually have maximum stability in slightly acid conditions of pH 3-4. 

When the alkaline conditions are stronger (pH 14) and when the temperature is increased, the keto-enol isomerizations will take place all along the carbohydrate chain to give enediols between carbons 2 and 3, 3 and 4, 4 and 5, and 5 and 6 for hexoses, the consequent keto groups at C-3, C-4, and C-5, aldehyde at C-6, and the isomerization of the hydroxyl groups at C-3, C-4, and C-5, resulting in all of the possible aldohexoses and ketohexoses [3]. These reactions are illustrated in Fig. 3.4. 

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