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Both glucose and fructose are reducing sugars but sucrose is non-reducing in nature. Why?


Glucose and fructose are reducing sugars as they reduce Fehling's solution and Tollen's reagent. In aqueous solution these saccharides containing hemiacetal or hemiketals exist in equilibrium with a small amount of open chain aIdehyde or alpha-hydroxy ketonic form. When treated with the above said reagents the open chain form is oxidised. As the open chain form is consumed with some reagent, the equilibrium gets disturbed and more of open chain aldehyde or alpha-hydroxy ketone is formed and thus reduction of these reagents keeps on going. On the other hand sucrose is a non reducing sugar because it does not reduce these reagents. This is due to the reason that the reducing groups present in the constituting monosaccharide units are not free and are involved in the formation of glycosidic linkages. 

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