Which one of the following sugars will not reduce Tollen’s reagent?

Correct option is D

To determine which sugar will not reduce Tollen's reagent, we need to analyze whether the sugar is a reducing or non-reducing sugar.

• Reducing sugars: These sugars have a free anomeric carbon (with a free aldehyde or ketone group) that can reduce Tollen’s reagent (Ammoniacal Silver Nitrate, AgNO₃/NH₃), leading to the formation of a silver mirror.
• Non-reducing sugars: These sugars do not have a free anomeric carbon and cannot reduce Tollen's reagent. Typically, disaccharides like sucrose fall into this category because their glycosidic bond involves the anomeric carbons of both monosaccharide units.

Step 1: Identifying the Structures

Looking at the structures:

• Option (a): Appears to be a disaccharide with a free anomeric carbon.
• Option (b): Another disaccharide, possibly lactose or maltose, which are reducing sugars.
• Option (c): Seems to be a trisaccharide, which could be maltotriose.
• Option (d): Appears to be sucrose, which is a non-reducing sugar.

Step 2: Understanding Sucrose

Sucrose is a non-reducing sugar because:

• It is composed of glucose and fructose linked by a glycosidic bond between their anomeric carbons (α-1,2 linkage).
• This bond locks both anomeric carbons, preventing them from opening into their aldehyde or ketone forms.
• Since there is no free anomeric carbon, it cannot reduce Tollen’s reagent.

Final Answer:

The sugar that will not reduce Tollen's reagent is sucrose in option (d).

Information Booster

1. Reducing sugars contain a free aldehyde (-CHO) or ketone (-C=O) functional group that can be oxidized.
2. Tollen’s test uses ammoniacal silver nitrate (AgNO₃ + NH₃) to detect reducing sugars. A silver mirror forms if the sugar is reducing.
3. Common reducing sugars: Glucose, fructose, galactose, maltose, lactose.
4. Sucrose is a non-reducing sugar because it has an α-1,2 glycosidic bond, locking both anomeric carbons.
5. Disaccharides like maltose and lactose are reducing sugars because one anomeric carbon remains free.
6. Polysaccharides (starch, cellulose) do not react with Tollen’s reagent because their glycosidic bonds prevent oxidation.
7. Barfoed’s test is another test for reducing sugars, distinguishing monosaccharides from disaccharides.