Monosaccharides are the simplest carbohydrates — they cannot be hydrolysed further into smaller sugar units. The general formula is C_n(H₂O)_n. When n = 5, you have pentoses (ribose, deoxyribose); when n = 6, hexoses (glucose, fructose). Both glucose and fructose share the molecular formula C₆H₁₂O₆ — they are functional-group isomers, not identical molecules.
The high-frequency confusion: Glucose is an aldohexose (aldehyde at C-1). Fructose is a ketohexose (ketone at C-2). NEET distractors routinely swap these classifications or present fructose as an aldose. The functional group position — not the molecular formula — determines the class.
NCERT anchor (Class 12 Chemistry (pre-2023 edition), Chapter 14, page 412): Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield these on hydrolysis. Monosaccharides are the monomeric units.
Structural distinctions to lock in:
Classification by carbon count: trioses (n=3, e.g. glyceraldehyde), tetroses (n=4), pentoses (n=5), hexoses (n=6). NEET sticks almost exclusively to pentoses and hexoses.
Watch-out: The term "reducing sugar" applies to both glucose AND fructose (fructose tautomerises in alkaline Tollens'/Fehling's conditions). Don't fall for distractors that claim fructose is non-reducing because it's a ketose.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The general formula of monosaccharides is:
Answer: C. The general formula of monosaccharides is C_n(H₂O)_n, where n typically equals 5 or 6. This is stated directly in NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 412.
Why A is wrong: A is wrong: Cₙ(H₂O)ₙ₋₁ does not fit even deoxyribose (C₅H₁₀O₄, which lacks one oxygen, not one H₂O). Monosaccharides follow Cₙ(H₂O)ₙ.
Why B is wrong: B adds an extra water molecule beyond the carbon count, which does not correspond to any standard carbohydrate class.
Why D is wrong: D doubles the carbon count relative to water, which does not match any monosaccharide or disaccharide general formula.
Glucose is classified as:
Answer: A. Glucose has an aldehyde group at C-1 and six carbon atoms, making it an aldohexose. NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 414.
Why B is wrong: B is wrong on carbon count — glucose has 6 carbons (hexose), not 5 (pentose).
Why C is wrong: C confuses glucose with fructose — fructose is the ketohexose (C=O at C-2).
Why D is wrong: D is wrong on both counts — glucose is neither a ketose nor a pentose.
Fructose differs from glucose in that fructose contains:
Answer: B. Fructose is a ketohexose with the carbonyl (keto) group at C-2, distinguishing it from glucose which has an aldehyde at C-1. NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 414.
Why A is wrong: A describes glucose, not fructose — glucose has the aldehyde at C-1.
Why C is wrong: C describes a carboxylic acid — neither glucose nor fructose contains a –COOH group in their standard forms.
Why D is wrong: D places the ketone at C-1, but in fructose the ketone is at C-2. A ketone requires carbon atoms on both sides, so it cannot be at C-1 of a straight chain.
Glucose and fructose are:
Answer: C. Glucose (aldehyde at C-1) and fructose (ketone at C-2) share molecular formula C₆H₁₂O₆ but differ in functional group type — they are functional group isomers. This requires applying the definition of isomer types to the structural difference.
Why A is wrong: A (chain isomers) would require different carbon-chain arrangements with the same functional group — both have straight 6-carbon chains.
Why B is wrong: B (position isomers) would require the same functional group at different positions — glucose has an aldehyde while fructose has a ketone, which are different functional groups.
Why D is wrong: D (enantiomers) would require non-superimposable mirror images with identical functional groups — glucose and fructose differ in functional group type, not just spatial arrangement.
The cyclic form of glucose is a:
Answer: A. Glucose cyclises to form a six-membered ring (5 carbons + 1 oxygen), called a pyranose, named after the parent heterocycle pyran. NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 414.
Why B is wrong: B describes the cyclic form of fructose — fructose forms a five-membered furanose ring, not glucose.
Why C is wrong: C describes an oxetane (4-membered ring) which is not formed by any common monosaccharide in biological systems.
Why D is wrong: D describes an oxepane (7-membered ring) which does not correspond to the cyclisation of any hexose sugar.
Which of the following statements about fructose is correct?
Answer: D. Fructose is a ketose but reduces Tollens' reagent because in the alkaline conditions of the test, it tautomerises to an aldose (enediol intermediate), which then reacts. This requires applying the concept of tautomerisation to predict test outcomes.
Why A is wrong: A is the common trap — students assume ketoses cannot reduce Tollens' reagent, but fructose tautomerises under alkaline conditions.
Why B is wrong: B gives the right conclusion but wrong reasoning — fructose does NOT inherently contain an aldehyde group; it FORMS one through tautomerisation in alkaline medium.
Why C is wrong: C is wrong — fructose IS a reducing sugar precisely because of the tautomerisation described in B.
An unknown monosaccharide has the molecular formula C₅H₁₀O₅. It is a:
Answer: D. Applying C_n(H₂O)_n: C₅H₁₀O₅ = C₅(H₂O)₅, so n = 5, making it a pentose. This requires applying the general formula to identify the carbon count.
Why A is wrong: A (triose) would have n=3, giving C₃H₆O₃ — does not match C₅H₁₀O₅.
Why B is wrong: B (tetrose) would have n=4, giving C₄H₈O₄ — does not match C₅H₁₀O₅.
Why C is wrong: C (hexose) would have n=6, giving C₆H₁₂O₆ — does not match C₅H₁₀O₅.
α-D-glucose and β-D-glucose differ in the configuration of the –OH group at:
Answer: B. When glucose cyclises into the pyranose form, C-1 becomes the anomeric carbon (new chiral centre). The α and β anomers differ only in the orientation of the –OH at this C-1 position. NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 414.
Why A is wrong: A (C-2) — the –OH at C-2 is fixed and identical in both α and β forms; it does not change during ring closure.
Why C is wrong: C (C-4) — C-4 configuration distinguishes glucose from galactose (epimers), not α from β anomers.
Why D is wrong: D (C-6) — C-6 is the –CH₂OH group outside the ring and is not involved in anomer formation.
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.
Given
A carbohydrate X has molecular formula C₆H₁₂O₆. It gives a positive Tollens' test. On treatment with HI, it gives n-hexane. Its Kiliani–Fischer synthesis yields a product with an aldehyde group.
Required
Identify X and its cyclic ring form.
Concept
Monosaccharide classification: C₆H₁₂O₆ is a hexose. Positive Tollens' test → reducing sugar. n-Hexane from HI reduction → straight-chain (no branching). Aldehyde confirmed via Kiliani–Fischer → aldohexose. The most common aldohexose is glucose. Its cyclic form is a pyranose (6-membered ring).
Formula
C_n(H₂O)_n with n = 6 → hexose. Aldehyde at C-1 → aldohexose.
Substitution
C₆(H₂O)₆ = C₆H₁₂O₆ ✓. Straight chain confirmed by HI reduction to n-hexane (all 6 carbons in one chain). Aldehyde confirmed → glucose.
Calculation
No numerical calculation required — this is a classification and identification problem.
Final answer
X is D-glucose (an aldohexose). Its cyclic form is a pyranose ring (6-membered: 5C + 1O).
Common trap
Students confuse glucose with fructose here. Both are C₆H₁₂O₆ and both give positive Tollens' test. The distinguishing evidence is the Kiliani–Fischer product retaining an aldehyde — this confirms an aldose (glucose), not a ketose (fructose). Fructose's Tollens' positive result comes from tautomerisation, not from an inherent aldehyde.
Similar NEET-style question
"Compound Y has molecular formula C₆H₁₂O₆, does NOT reduce Fehling's solution directly, and on acid hydrolysis gives equal moles of glucose and fructose. Y is: (A) Maltose (B) Sucrose (C) Lactose (D) Fructose." [Answer: B — Sucrose is a non-reducing disaccharide that hydrolyses to glucose + fructose.]
---
Monosaccharides (simplest): glucose, fructose, galactose. Disaccharides: sucrose (glucose+fructose), lactose (glucose+galactose), maltose (2×glucose). Polysaccharides: starch, cellulose, glycogen.
-- NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, p. 412Open chain (Fischer): CHO at C1; aldohexose with -OH at C2-C5, -CH₂OH at C6. D-glucose (most stable): C5 -OH on right. Cyclic: α (axial -OH at C1) and β (equatorial); equilibrium via mutarotation.
-- NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, p. 414More in Biomolecules: 4 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
2 questions from NEET 2024, 2025. Answers verified against NTA official keys.
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
Test yourself on this topic with real past-paper questions:
Practice this topic →