Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups - monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as D(+)-glucose. It was assigned the open structure on the basis of its reactions with HI, \(\mathrm{NH_2OH}\), \(\mathrm{Br_2}\) water, \((\mathrm{CH_3CO})_2\mathrm{O}\) and nitric acid.
Despite having the \(-\mathrm{CHO}\) group, glucose does not give Schiff's test and hydrogen sulphite addition product with \(\mathrm{NaHSO_3}\). It was found that glucose forms a six-membered ring in which one of the \(-\mathrm{OH}\) groups add to the \(-\mathrm{CHO}\) group and form a cyclic hemiacetal structure.
What type of carbohydrates are called reducing sugars ?
Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups - monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as D(+)-glucose. It was assigned the open structure on the basis of its reactions with HI, \(\mathrm{NH_2OH}\), \(\mathrm{Br_2}\) water, \((\mathrm{CH_3CO})_2\mathrm{O}\) and nitric acid.
Despite having the \(-\mathrm{CHO}\) group, glucose does not give Schiff's test and hydrogen sulphite addition product with \(\mathrm{NaHSO_3}\). It was found that glucose forms a six-membered ring in which one of the \(-\mathrm{OH}\) groups add to the \(-\mathrm{CHO}\) group and form a cyclic hemiacetal structure.
What type of carbohydrates are called reducing sugars ?
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Solution and Explanation
Some sugars can reduce mild oxidising agents. Such sugars are called reducing sugars.
The test reagents are Fehling's solution and Tollens' reagent.
Step 2: State the definition.
Reducing sugars are carbohydrates that reduce Fehling's solution and Tollens' reagent.
They can do this because they have a free aldehyde group, a free ketone group or a free hemiacetal -OH group. In solution the ring opens and the free -CHO group is formed again.
Examples are glucose, fructose, maltose and lactose.
Step 3: Compare with non-reducing sugars.
In sucrose the glycosidic bond joins C-1 of glucose and C-2 of fructose. Both reducing groups are used up in this bond.
So sucrose has no free -CHO or hemiacetal group and does not reduce Fehling's or Tollens' reagent.
Final Answer:
Carbohydrates with a free aldehydic or ketonic group (or a free hemiacetal group) that reduce Fehling's solution and Tollens' reagent are called reducing sugars.
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Top CBSE CLASS XII Glucose Questions
- How do you explain the following ?
(a) Presence of an aldehyde group in glucose
(b) Presence of a primary alcoholic group in glucose - How do you explain the following ? (a) Presence of a carbonyl group in glucose (b) Presence of five --OH groups in glucose
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