Question

Which one given below is a non-reducing sugar?
A. Maltose
B. Lactose
C. Glucose
D. Sucrose

Answer 1

Sugar which does not have any aldehyde or ketone group attached to their carbon chain are called as non-reducing sugar. They do not even get reduced by Tollen’s reagent.

Complete answer:
A non-reducing sugar is a carbohydrate that is not oxidized by a weak oxidizing agent (an oxidizing agent that oxidizes aldehydes but not alcohols, such as the Tollen’s reagent) in basic aqueous solution. These sugars have acetal in place of hemiacetal. An acetal consists of two O-R groups, one -R and -H group respectively. A sugar without a hemiacetal is non-reducing because it does not behave as a reducing agent toward oxidizing metal salts. Disaccharides like lactose and Maltose being a reducing sugar has one of their two anomeric carbons involved in the glycosidic bond, while the other is free to undergo mutarotation at its hemiacetal anomeric center via an open-chain structure containing an aldehyde group attached to it. Similarly, Glucose has a free anomeric carbon at $C-1$ position making it a reducing sugar.
Whereas sucrose is a non-reducing sugar because the two monosaccharide units are held together by a glycosidic linkage between $C-1$ of $\alpha-glucose$ and $C-2$ of $\beta-fructose$. Since the reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a non-reducing sugar.
Therefore in the above question Sucrose is a non-reducing sugar.

**Hence the correct option is option (D).

Additional Information:**
Benedict’s test is used for identifying if the given sugar is reducing or non-reducing in nature. When Benedict’s solution is added to the given sugar, if there is no change in the colour, then it is non-reducing sugar. If there is a change in the colour of the sugar, it is reducing sugar.

Note :
All monosaccharide are reducing sugars because they either have an aldehyde group or can tautomerize in solution to form an aldehyde group. Many disaccharides are also reducing sugar, as one of the two units may have an open-chain form with an aldehyde group.