Oxidation of Monosaccharide Carbohydrates

Aldoses containalcohol and aldehyde functional groups, which can beoxidized to carboxylic acids. Depending on the oxidizing agent, we can either selectively oxidize the aldehyde or involve the primary alcohol as well. Ifboth groups are oxidized to a carboxylic acid, analdaric acidis formed, while the selective oxidation of thealdehyde results in analdonic acid:

Using a mild oxidizing agent allows distinguishing aldoses and ketoses since the aldehyde, being more reactive, is converted to a carboxylic acid, while the ketone and the alcohols present in both are not. Br₂is an ideal candidate for this, as itsred color disappears upon reduction to Br⁻ when reacted with the aldehyde. Although the mechanisms are different, the color loss behind the test is much like what we learn in thebromination reaction of alkenes.

If the monosaccharide is, on the other hand, a ketose, the red color will persist, indicating no reaction with Br₂.

One thing to keep in mind, though, is theisomerization between aldoses and ketosesthat occurs in basic conditions. If the ketose is isomerized to an aldose, it will give a positive signal in the Br₂ test.To prevent this, the reaction with Br₂ is usually carried out at slightly acidic conditions (pH = 6).

There are different ways ofoxidizing both aldoses and ketoses performed underbasic conditions to promote the isomerization:

The most common examples are theTollens’ reagent (Ag⁺ NH₃, HO⁻),Fehling’s reagent (Cu²⁺ in aqueous sodium tartrate), andBenedict’s reagent (Cu²⁺ in aqueous sodium citrate).

The indicator in Tollens reagentis the by-productAg, which coats the surface of the flask, forming amirror:

TheFehling’sand Benedict’stests rely on the formation ofCu₂O as a red precipitate when the Cu²⁺ salt is reduced by the aldose or ketose present in the solution:

A common way of oxidizingboth the aldehyde and 1^o alcohol groups to carboxylic acids (aldaric acid) is the use of nitric acid (HNO₃), which is astronger oxidizing agent. This reaction, however, is not a test but rather a method for converting the functional groups:

One feature ofaldaric acids is that they have two identical groups (COOH) on the terminal carbons, and therefore, it is always a good idea to check whether it is ameso compound. For example, D-Glucose is chiral whileD-glucaric acid is a meso compound, as can be seen from the perpendicular plane of symmetry on theFischer projection. Remember, meso compounds are not optically active and therefore are achiral.

Reducing andNonreducing Sugars

Recall fromgeneral chemistry that anyredox reaction involves an oxidation and reduction, so if one species is oxidized, then the other is reduced.

Now, in all the reactions above, the carbohydrates were oxidized and therefore, the oxidizing agents were reduced. So, the oxidizing agents arereduced by the carbohydrates, and because of this, any carbohydrate thatgives a positive test with Tollens, Benedict’s, or Fehling’s reagent is called areducing sugar.Consequently, carbohydrates that donot react with these oxidizing reagents are classified asnonreducing sugars.

You may wonder what some examples of non-reducing sugars are, then. To have a general answer to this question, first, remember thatcyclic forms of sugarsdo not have the aldehyde functional group, but rather they are in thehemiacetal oracetal form in the case ofglycosides.

Remember also thatcyclic hemiacetals are always in equilibrium with the open-chain form and therefore, they can also be oxidized to aldonic or aldaric acids – they arereducing sugars: