Simple Introduction of Classification of Carbohydrates based upon their reducing properties.

1. Classification of carbohydrates
based on their reducing properties.
Janani Palpandi, II BT, Department of Biotechnology, MEPCO Schlenk Engineering College

2. What is reduction?
• Reduction is a chemical reaction that involves the gaining of electrons by
one of the atoms involved in the reaction. The term refers to the element that
accepts electrons, as the oxidation state of the element that gains electrons
is lowered.
• Sugars are also proved to possess reducing property.

3. Classification
• Sugars can also be classified based on their reducing properties.
• Reducing sugars
• Non Reducing sugars

4. Comparision
Reducing sugars
• have access to their open chain
form.
• basically sugars with an aldehyde
group (in their open form) or a
hemiacetal group (in their ring
form)at the anomeric carbon that is
ready to oxidize.
• allow for chain formation and
elongation.
• Eg: Maltose, glucose, fructose
Non Reducing sugar
• do not have access to free
aldehyde or ketone such as
glycosides.
• Since the anomeric carbon is fixed
in a glycosidic linkage, the sugar
chain cannot form or be elongated.
• can also be identified by an α-
glycosidic bond in which an α
anomeric carbon is involved in the
linkage.
• Eg: Sucrose, trehalose

5. A pictorial
representation
of reducing
and non
reducing
sugar.

6. Why some sugars are reducing?
• A characteristic of a reducing sugars is the β-glycosidic bond, where the β
anomeric carbon is involved in the linkage.
• Examples of reducing sugars are glucose, maltose, and lactose.
• Reducing sugars can form aldehyde or ketone groups under basic conditions
as the carboxyl group is reduced to carbonyl group of aldehyde or ketone
group and occurs when the anomeric carbon is not bonded to hemiacetal or
hemiketal hydroxyl group.

7. What about ketoses?
• It has only been mentioned that aldoses can form reducing sugars.
• Mechanistically, when ketoses isomerize to their ring form, acetals are
formed.
• So no, ketoses do not form reducing sugars.
• However, ketoses can tautomerize to aldoses, where a hemiacetal can then
be formed upon ring closure.

8. Quantitative Analysis for reducing sugars
• The presence of reducing sugars can be tested using Fehling's solution, a
solution of Cu2+ ions that readily oxidizes free aldehydes or ketones.
• A similar test can be done with Benedict's reagent, which also contains the
critical Cu2+ ion.