The structure of sugars plays a crucial role in determining whether they are reducing or non-reducing. The key difference lies in whether the sugar has a free aldehyde or ketone group (which makes it a reducing sugar) or whether the carbonyl group is involved in a glycosidic bond, making it non-reducing.
Reducing Sugar Structure
A reducing sugar has a free aldehyde (-CHO) or ketone group (-C=O) that is not involved in any bond with another molecule. This free group is what allows the sugar to reduce other substances, such as metal ions in chemical tests like the Benedict’s or Fehling’s tests.
Note: Osazone test is used to identify the reducing sugars.
Monosaccharides (Reducing Sugars)
- Glucose has a free aldehyde group at the C1 carbon when in its open-chain form.
- Fructose has a free ketone group at the C2 carbon in its open-chain form.
In solution, many monosaccharides are in a cyclic form, but they still contain a free anomeric carbon which allows them to act as reducing agents.
Example: Glucose
- In its open-chain form, glucose has a free aldehyde group at the anomeric carbon (C1).
- In the cyclic form, this aldehyde group can still be “free” and capable of reducing other substances.
Disaccharides (Reducing Sugars)
- A disaccharide like maltose has one free anomeric carbon (the C1 carbon of glucose) and the other anomeric carbon is involved in a glycosidic bond. The free anomeric carbon allows maltose to act as a reducing sugar.
Example: Maltose (as a reducing sugar)
In maltose, the glucose units are linked by a 1→4 glycosidic bond. The glucose on the right side has a free aldehyde group (C1), making it a reducing sugar.
Non-Reducing Sugar Structure
A non-reducing sugar does not have a free aldehyde or ketone group because both anomeric carbons are involved in a glycosidic bond. This structural feature prevents these sugars from reducing other substances.
Monosaccharides (Non-Reducing Sugars)
Monosaccharides themselves are typically reducing sugars, but once they form disaccharides or larger sugars and are involved in glycosidic bonding, their reducing properties may be lost if the carbonyl group is involved in the bond.
Disaccharides (Non-Reducing Sugars)
Sucrose is a typical example of a non-reducing sugar. The glucose and fructose units in sucrose are linked via their anomeric carbons (C1 of glucose and C2 of fructose).
This glycosidic bond locks both carbonyl groups, making them unavailable for oxidation and preventing sucrose from acting as a reducing sugar.
Example: Sucrose
In sucrose, both glucose and fructose are connected through their anomeric carbons which means there is no free aldehyde or ketone group. This prevents sucrose from acting as a reducing sugar.
Structural difference be reducing sugar and non reducing sugar table
| Feature | Reducing Sugar | Non-Reducing Sugar |
|---|---|---|
| Functional Group | Free aldehyde (-CHO) or ketone (-C=O) group available | No free aldehyde or ketone group due to glycosidic bonds |
| Monosaccharides | Typically have a free aldehyde or ketone group | Monosaccharides are reducing, but become non-reducing in certain disaccharides |
| Disaccharides | One anomeric carbon has a free aldehyde or ketone | Both anomeric carbons are involved in the glycosidic bond |
| Examples | Glucose, Maltose, Fructose | Sucrose, Trehalose, Raffinose |
Conclusion
The fundamental structural difference between reducing and non-reducing sugars lies in the availability of a free carbonyl group. Reducing sugars possess a free aldehyde or ketone group, allowing them to act as reducing agents in chemical reactions.
In contrast, non-reducing sugars have both anomeric carbons involved in glycosidic bonds, which locks the carbonyl group, making them unable to participate in reduction reactions.
1 thought on “Reducing sugar vs Non reducing sugar structure”