CBSE 2026 · Region 4 · Set 1 · Q30 · 4 marks
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 $\displaystyle \mathrm{HI}, \mathrm{NH}_{2} \mathrm{OH}, \mathrm{Br}_{2}$ water, $\displaystyle \left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}$ and nitric acid. Despite having the - CHO group, glucose does not give Schiff's test and hydrogen sulphite addition product with $\displaystyle \mathrm{NaHSO}_{3}$. It was found that glucose forms a six-membered ring in which one of the - OH groups add to the - CHO group and form a cyclic hemiacetal structure. Answer the following questions :(a)How do you explain the following ?(i)Presence of a carbonyl group in glucose(ii)Presence of five - OH groups in glucose which are attached to different carbon atoms.(b)What type of carbohydrates are called reducing sugars ?(c)In D(+)-glucose, what do the letter 'D' and sign '(+)' represent?(ii)Draw the six-membered ring structure of $\displaystyle \alpha-\mathrm{D}(+)-$ glucose.
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 $\displaystyle \mathrm{HI}, \mathrm{NH}_{2} \mathrm{OH}, \mathrm{Br}_{2}$ water, $\displaystyle \left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}$ and nitric acid. Despite having the - CHO group, glucose does not give Schiff's test and hydrogen sulphite addition product with $\displaystyle \mathrm{NaHSO}_{3}$. It was found that glucose forms a six-membered ring in which one of the - OH groups add to the - CHO group and form a cyclic hemiacetal structure. Answer the following questions :
(a)
How do you explain the following ?
(i)
Presence of a carbonyl group in glucose
(ii)
Presence of five - OH groups in glucose which are attached to different carbon atoms.
(b)
What type of carbohydrates are called reducing sugars ?
(c)
In D(+)-glucose, what do the letter 'D' and sign '(+)' represent?
(ii)
Draw the six-membered ring structure of $\displaystyle \alpha-\mathrm{D}(+)-$ glucose.
Marking-scheme solution
(i)
Glucose on reaction with H2N-OH forms oxime / with HCN forms cyanohydrin
(Or correct chemical equation)
(ii)
Glucose gives stable pentaacetate with (CH3CO)2O
(Or correct chemical equation)
(b)
Those which reduce Tollen’s reagent/Fehling solution.
(c)
D- stands for Configuration
(+) stands for dextrorotatory nature of the molecule.
(ii)
BiomoleculesCarbohydratesUnderstandcase_studymedium
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CBSE Class 12 Chemistry past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.