Monosaccharides Tollens’ reagents

In the second place, oxidation by Fehling s or Tollens reagent cannot be used for the preparation of aldonic acids (monocarboxylic acids) from aldoses. Both Fehling s and Tollens reagents are alkaline reagents, and the treatment of sugars with alkali can cause extensive isomerization and even decomposition of the chain. Alkali exerts this effect, in part at least, by establishing an equilibrium between the monosaccharide and an enediol structure. 

A number of oxidizing agents are used to identify functional groups of carbohydrates, in elucidating their structures, and for syntheses. The most important are (1) Benedicts or Tollens reagents, (2) bromine water, (3) nitric acid, and (4) periodic acid. Each of these reagents produces a different and usually specific effect when it is allowed to react with a monosaccharide. We shall now examine what these effects are. 

The chemical properties of monosaccharides are further complicated by the fact that they can exhibit tautomerism in aqueous basic solutions (Figure 1.15). This means that after a short time a basic aqueous solution of a monosaccharide will also contain a mixture of monosaccharides that will exhibit their characteristic chemical properties. For example, a solution of fructose will produce a silver mirror when treated with an ammoniacal solution of silver nitrate (Tol-len s reagent). This is because under basic conditions fructose undergoes tautomerism to glucose, whose structure contains an aldehyde group, which reduces Tollen s reagent to metallic silver. 

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