How is fructose a reducing sugar?

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Hint: Fructose or fruit- sugar is a monosaccharide. It is a ketonic simple sugar, that is the sugar compound that contains a keto group. It is a dietary monosaccharide.

Complete answer:
A reducing sugar is a sugar compound which on a reaction reduces another compound and gets oxidized. The basic characteristic of a reducing sugar is that it should be an open chain with a free aldehyde or a ketone group. Monosaccharides that contain the aldehyde group are called aldose and those that contain ketone groups are called ketose. The keto group present in the structure of fructose makes it a reducing sugar by its potential to get oxidized through several intermediate tautomeric shifts into an aldehyde and reducing the other compound it reacts with.

Fructose reduces various compounds such as Tollen’s reagent and Benedict’s solution and itself gets oxidized. All the monosaccharides, both aldose and ketose are reducing in nature. Fructose functions as an alternative energy source to the body in the absence of glucose by functioning as an intermediate for cellular respiration.

Like tollens reagent, an oxidizing agent is basic in nature therefore, the ketonic group gets isomerized to the aldehyde group and then can be oxidized to the acid group. So we can say that reducing sugar are those which can reduce reagents like tollen's reagent or Benedict solution. So fructose is reducing sugar.

Note: A disaccharide is a sugar molecule formed by the combination of two monosaccharide units. Sucrose is an example of a disaccharide, which is formed from the combination of two monosaccharides glucose and fructose. Disaccharides can either be reducing or non- reducing. In a disaccharide which is reducing in nature, the carbonyl carbon is not free, or it is bound, there by hindering the reaction. Whereas in a reducing disaccharide there is a free hemiacetal group containing a carbonyl carbon is present that facilitates the reduction reaction.