Carbohydrates — Reducing Sugars + Anomers
Biomolecules
40
JEE Qs
10%
Hard
50
min
Master the structural features defining the anomeric carbon and free hemiacetal/hemiketal groups to correctly identify reducing sugars and anomers.
🧮 Key Formulas
✅ Key Points for JEE
- 1Reducing sugars are carbohydrates that have a free anomeric hydroxyl group (part of a hemiacetal or hemiketal) which allows them to exist in equilibrium with an open-chain aldehyde or alpha-hydroxy ketone form, capable of reducing Tollen's or Fehling's/Benedict's reagents.
- 2Anomers are a specific type of stereoisomer (diastereomers) that differ in configuration only at the anomeric carbon (C-1 for aldoses, C-2 for ketoses) in the cyclic form. They are designated as alpha (α) or beta (β).
- 3All monosaccharides (e.g., glucose, fructose, galactose) are reducing sugars because their cyclic hemiacetal/hemiketal forms are in dynamic equilibrium with their open-chain aldehyde/ketone forms.
- 4Mutarotation is the spontaneous change in the specific optical rotation of a carbohydrate solution due to the interconversion of α- and β-anomers via the open-chain form until an equilibrium mixture is attained.
- 5Disaccharides like maltose and lactose are reducing sugars as they possess a free hemiacetal unit. Sucrose is a non-reducing sugar because its anomeric carbons are involved in the glycosidic linkage, preventing ring-opening to an aldehyde/ketone.
⚠️ Common Mistakes
- ✕Confusing anomers with epimers or other stereoisomers; anomers differ only at the anomeric carbon.
- ✕Incorrectly identifying the anomeric carbon in cyclic sugar structures, especially in less common sugars or derivatives.
- ✕Assuming all sugars with a carbonyl group are reducing; the key is the presence of a *free* hemiacetal/hemiketal and the ability to interconvert with the open-chain form.
- ✕Forgetting that glycosides (acetals/ketals) are non-reducing because the anomeric -OH is involved in an ether linkage, preventing ring opening and the formation of the aldehyde/ketone form.
📝 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 Ch 14: Biomolecules