3.2.3.1 Reducing action of sugars in alkaline solution

3.2.3.1 Reducing action of sugars in alkaline solution

All the sugars that contain the free sugar group undergo enolization and various other changes when placed in alkaline solution. The enediol forms of the sugars are highly reactive and are easily oxidized by oxygen and other oxidizing agents. These sugars in alkaline solution are very powerful reducing agents and the sugars are oxidized to complex mixtures of acids.

This reducing action of sugars in alkaline solution is utilized for both the qualitative and quantitative estimation of sugars. When a reducing sugar is heated with the alkaline copper reagents, the following reaction occurs:

        Sugar + alkali →  Dienol   

                                      Fig. 3.2.3.1 Reaction of glucose with alkali

 

     Copper complex of

tartarate or citrate (Cu ++) 

 

 

Cu2+ + -OH- + mixture of sugar acids

        Δ ↓           Δ       

   Cu(OH)2      →     Cu2O( red)

 

The Cu2+ ions take electrons from the enediols and oxidize them to sugar acids and are in turn reduced to cuprous ions, Cu+. The cuprous ions combine with hydroxyl ions to the yellow cuprous hydroxide, which upon heating is converted to red cuprous oxide, Cu2O.

The appearance of a yellow-to-red precipitate indicates reduction, and the quantity of sugar present can be roughly estimated from the amount of colour. 

In quantitative determination, the amount of copper reduced is obtained by iodometric titration or colorimetric methods by treatment with reagents, and the amount of sugar is calculated.

 reaction
Last modified: Monday, 9 April 2012, 5:50 AM