Question

Non-reducing sugar have
(a) free ${ C }{ H }{ O }$ group and bound ${ C }{ O }$ group
(b) free${ C }{ O }$ group and bound${ C }{ H }{ O }$ group
(c) both${ C }{ O }$ and ${ C }{ H }{ O }$ free group
(d) neither free${ C }{ O }$ nor free ${ C }{ H }{ O }$ group

Answer 1

A non-reducing sugar is a carbohydrate that is not oxidized in an aqueous solution by a weak oxidizing agent. It does not have any hemiacetal functional groups. Therefore they cannot act as a reducing agent.

Complete answer:
The saccharides that do not reduce Fehling’s solution, Benedict’s solution, and the Tollen’s reagent are called non-reducing sugars. They neither contain a free ${ C }{ H }{ O }$ group (aldehydes) nor ${ C }{ O }$ group (ketones) on the carbon adjacent to the ${ C }{ O }$ group. The cyclic form of non-reducing sugars can not be opened into an open chain that has a free carbonyl group.
So, the correct answer is, “neither free ${ C }{ O }$ group nor free ${ C }{ H }{ O }$ group”.

Additional Information:
- All polysaccharides are non-reducing sugars like starch, cellulose
- Sucrose or disaccharides are also a non-reducing sugar because the anomeric carbon of both the monosaccharides is involved in glycoside or acetal form on.

Note:
- The disaccharides may be reducing or non reducing sugar depending upon the post on-of linkage between the two monosaccharide units.
- If the glycosidic linkage involves the carbonyl group of both the monosaccharide units, the disaccharide would be non-reducing sugar(Eg. Sucrose).
- If one of the carbonyl groups in any one of the monosaccharides is free, the disaccharide would be a reducing sugar (Eg. Maltose, Lactose).