Carbohydrates — Reducing sugars, anomers, mutarotation
Biomolecules
8
JEE Qs
8%
Hard
75
min
Master the drawing of cyclic structures (Haworth projections) of common monosaccharides and clearly identify the anomeric carbon and its configuration (α vs β) to correctly assess reducing nature and mutarotation.
✅ Key Points for JEE
- 1Anomers are stereoisomers that differ in configuration at the anomeric carbon (the hemiacetal/hemiketal carbon formed during cyclization). Alpha (α) and beta (β) anomers are specific examples.
- 2Mutarotation is the spontaneous change in the optical rotation of a sugar solution due to the interconversion of α and β anomers via an open-chain aldehyde/ketone intermediate, establishing an equilibrium.
- 3Reducing sugars possess a free hemiacetal or hemiketal group (anomeric carbon not involved in a glycosidic bond), allowing them to open into an aldehyde/ketone form capable of being oxidized (e.g., by Tollen's or Fehling's reagents).
- 4Sugars with their anomeric carbon involved in a glycosidic linkage (e.g., in disaccharides like sucrose or polysaccharides) cannot open to the aldehyde/ketone form at that specific anomeric carbon, rendering that specific end non-reducing.
- 5The presence of a free hemiacetal or hemiketal functional group is the structural requirement for a sugar to exhibit reducing properties and mutarotation.
⚠️ Common Mistakes
- ✕Confusing anomers with epimers or other diastereomers; anomers differ *only* at the anomeric carbon.
- ✕Incorrectly identifying the anomeric carbon, which is the carbon derived from the carbonyl group in the open-chain form and is part of the hemiacetal/hemiketal group in the cyclic form.
- ✕Assuming all disaccharides or polysaccharides are non-reducing; their reducing nature depends on whether a free hemiacetal/hemiketal end exists.
- ✕Not understanding that mutarotation involves an open-chain intermediate, not direct interconversion between α and β cyclic forms.
📝 Practice Questions
See allQ51.Given below are two statements: Statement I: Fructose does not contain an aldehydic group but still reduces Tollen's reagent Statement II: In the presence of base, fructose undergoes rearrangement to give glucose. In the light of the above statements, choose the correct answer from the options given below (1) Both Statement I and Statement II are true (2) Both Statement I and Statement II are false (3) Statement I is true but Statement II is false (4) Statement I is true but Statement II is false
Q51.The carbohydrate "Ribose" present in DNA, is A. A pentose sugar B. present in pyranose from C. in "D" configuration D. a reducing sugar, when free E. in ∝-anomeric form Choose the correct answer from the 2025 (24 Jan Shift 1) JEE Main Previous Year Paper options given below: (1) A, D and E Only (2) A, C and D Only (3) A, B and E Only (4) B, D and E Only
Q60.Which of the following acids is a vitamin? (1) Adipic acid (2) Ascorbic acid (3) Saccharic acid (4) Aspartic acid
Q61.Identify the number of structure/s from the following which can be correlated to D-glyceraldehyde. (1) four (2) one (3) two (4) three
Q59.The α-Helix and β - Pleated sheet structures of protein are associated with its : (1) tertiary structure (2) quaternary structure (3) secondary structure (4) primary structure ∣∣∣ ∣∣∣ ∣∣∣ ∣∣ 2025 (23 Jan Shift 2) JEE Main Previous Year Paper
Q51.Given below are two statements: Statement I : D-glucose pentaacetate reacts with 2, 4-dinitrophenylhydrazine Statement II : Starch, on heating with concentrated sulfuric acid at 100∘C and 2-3 atmosphere pressure produces glucose. In the light of the above statements, choose the correct answer from the options given below 2025 (28 Jan Shift 1) JEE Main Previous Year Paper (1) Statement I is false but Statement II is true (2) Both Statement I and Statement II are false (3) Both Statement I and Statement II are true (4) Statement I is true but Statement II is false
NCERT Chapters
- Class 12 Chemistry Part II Ch 14: Biomolecules