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Examples of Reducing and Non Reducing Sugars

Carbohydrates or sugar molecules are most important biological molecules which mainly act as energy coins in living bodies. They are hydrolysis to form simple monomer units with release of a large amount of energy. 
They are polymeric forms of monomer units which are known as monosaccharide. These units are bonded through glycosidic linkage with the elimination of water molecule. 

The monosaccharide units are poly-hydroxy carbonyl compounds which involve in condensation reactions to form di-saccharide units such as maltose, sucrose etc. The carbonyl group of monosaccharide unit may or may not involve in glycosidic linkage to form di-saccharide units. 

The carbonyl carbon atom of monosaccharide units is known as anomeric carbon atom. 
If the carbonyl carbon atom or anomeric carbon atom involves in glycosidic linkage, it will not involve in any other chemical reaction of carbonyl group such as with oxidising agent.
 
On the basis of the properties of monosaccharide molecules with oxidising reagents monosaccharide can be classified in two types; reducing sugar and non-reducing sugar molecules.

A non-reducing sugar cannot oxidize by a weak oxidizing agent such as Tollen’s reagent, Fehling solution or Benedict solution. 
In general an oxidising agent oxidizes the carbonyl group of Aldehydes but cannot oxidise an alcohol. In case of non-reducing sugars; the carbonyl group involves in the glycosidic linkage between di-saccharide molecules. 

Sucrose is an example of non-reducing sugar which cannot produce any compound containing an –CHO group in basic aqueous medium. It proves that it neither contains any hemi-acetal group nor a hemi-ketal group. 
That is the reason it is quite stable in water. All sugar molecules exist as an equilibrium mixture of open chain and cyclic ring structure. 
The open-chain form contains the carbonyl group (>C=O) while in a closed cyclic ring structure, this carbonyl group is bonded with the hydroxy group. 

Reducing sugar molecules can reduced the oxidising agents due to the presence of free >C=O group in the molecule. Common mono-saccharides are reducing in nature such as glucose. These monosaccharide units can form glycosidic linkage to form di-saccharide units. 

Maltose and lactose are reducing sugar molecules. 
Let’s discuss these oxidising agents and their reactions with sugar molecules. 
  • Benedict's Solution is composed of copper sulphate with alkaline citrate. It is a deep blue colour solution which turns to brick red precipitate with reducing sugar. 
  • The red precipitate shows the presence of Cu2O. The reducing sugar is oxidised to carboxylic acid. 
  • The reaction involves the conversion of Cu2+ to Cu+ ion. 
  • Fehling's Solution is composed of copper sulphate with alkaline tartrate and has deep blue colour. It also forms brick red colour precipitate of Cu2O due to reduction of Cu2+ to Cu+ ion. 
  • Tollen's Reagent is solution of silver nitrate with aqueous ammonia. 
  • It is a colourless solution which forms silver mirror with reducing sugar. 
  • The aldehyde group (-CHO) reacts with Cu2+ ion in alkaline medium to form red precipitate of Cu2O. 

The reaction can be written as given below;
                                           RCHO + 2Cu2+ + OH- --> RCOO- + Cu2O(s) + 3 H2O
Similarly the reaction of Tollen’s reagent with aldose sugar results the formation of gray colour precipitate of silver metal with ammonia and water. 
RCHO + 2Ag (NH3)2+ + 3OH- --> RCOO- +2Ag(s) +4NH3(g) +2H2O