Question

Write short notes on reducing and non-reducing sugars.

Answer 1

Reducing Sugar

• Reducing sugar is one that possesses a free aldehyde or ketonic group.
• It can reduce Fehling and Tollens reagent.
• Fehling A is Copper sulfate $\left({\mathrm{CuSO}}_4\right)$and Fehling B is an aqueous solution of potassium sodium tartrate $\left({\mathrm{KNaC}}_4{\mathrm{H}}_4{\mathrm{O}}_6·4{\mathrm{H}}_2\mathrm{O}\right)$.
• Tollens's reagent is the ammoniacal solution of silver nitrate $\left(\right[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2{]}^{+})$
• Reducing sugar can act as a reducing agent.
• It can be further reduced to alcohol.
• For example maltose and lactose.

Non reducing sugar

• In a basic aqueous solution, a non-reducing sugar is not reduced by a weak oxidising agent (one that oxidises aldehydes but not alcohols, such as Tollen's reagent).
• There is no free aldehyde or ketonic group.
• Non-reducing sugars do not have an $-\mathrm{OH}$ group attached to the anomeric carbon so they cannot reduce other compounds.
• E.g. sucrose and Raffinose
• The structure of sucrose is as follows:-