The trap that catches repeaters on carbohydrate classification: confusing the basis of classification. You know glucose and fructose share the molecular formula C₆H₁₂O₆ — but one is an aldohexose and the other a ketohexose. NEET distractors exploit exactly this overlap.
What carbohydrates are. NCERT Class 12 Chemistry Chapter 14 (Biomolecules) defines carbohydrates as polyhydroxy aldehydes or ketones, or compounds that yield these on hydrolysis. The general formula for monosaccharides is Cₙ(H₂O)ₙ.
Classification hierarchy:
By hydrolysis products:
By carbonyl group:
By carbon count:
Reducing vs non-reducing sugars: A sugar is reducing if it has a free anomeric carbon (hemiacetal/hemiketal) that can open to expose the carbonyl. All monosaccharides are reducing. Sucrose is non-reducing (both anomeric carbons are locked in the glycosidic bond). Maltose and lactose are reducing (one free anomeric carbon remains).
Watch-out for NEET: When a stem says "a ketohexose that is laevorotatory," the answer is fructose — but the distractor will offer glucose (an aldohexose) because both are C₆H₁₂O₆. Classify by functional group first, carbon count second.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is a non-reducing sugar?
Answer: D. Sucrose is non-reducing because both anomeric carbons (C-1 of glucose and C-2 of fructose) are involved in the glycosidic bond, leaving no free carbonyl group. NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 412.
Why A is wrong: A is wrong because maltose has a free anomeric carbon on one glucose unit, making it a reducing sugar.
Why B is wrong: B is wrong because lactose retains a free anomeric carbon on the glucose unit, making it a reducing sugar.
Why C is wrong: C is wrong because glucose is a monosaccharide with a free aldehyde group — all monosaccharides are reducing sugars.
Fructose is classified as a:
Answer: D. Fructose has a ketone group at C-2 and six carbon atoms, making it a ketohexose. NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 412.
Why A is wrong: A is wrong because fructose has six carbons (hexose, not pentose) and a ketone group (not aldehyde).
Why B is wrong: B is wrong because fructose has six carbons, not five. Ribulose would be a ketopentose.
Why C is wrong: C is wrong because fructose has a ketone group at C-2, not an aldehyde at C-1. Glucose is the aldohexose.
Hydrolysis of sucrose yields:
Answer: B. Sucrose is a disaccharide of glucose (α-1,2) fructose. Hydrolysis cleaves the glycosidic bond to yield one molecule each of glucose and fructose. NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 412.
Why A is wrong: A is wrong because glucose + glucose is the hydrolysis product of maltose, not sucrose.
Why C is wrong: C is wrong because glucose + galactose is the hydrolysis product of lactose, not sucrose.
Why D is wrong: D is wrong because no common dietary disaccharide yields two fructose units on hydrolysis.
A monosaccharide has the molecular formula C₅H₁₀O₅. It contains an aldehyde group. This sugar is best classified as:
Answer: C. Five carbons = pentose; aldehyde group = aldose. Therefore it is an aldopentose. Ribose is the common example. Classification uses functional group + carbon count per NCERT Class 12 Chemistry Chapter 14.
Why A is wrong: A is wrong because the stem specifies an aldehyde group (aldose), not a ketone group (ketose).
Why B is wrong: B is wrong because the molecule has 5 carbons (pentose), not 6 (hexose).
Why D is wrong: D is wrong on both counts — the molecule has an aldehyde (not ketone) and 5 carbons (not 6).
Which of the following disaccharides will give a positive Tollens' test?
Answer: A. Maltose is a reducing sugar (free anomeric carbon on one glucose unit) and will reduce Tollens' reagent to give a silver mirror. Sucrose has no free anomeric carbon and is non-reducing. NCERT Class 12 Chemistry Chapter 14.
Why B is wrong: B is wrong because sucrose has both anomeric carbons locked in the glycosidic bond — it cannot open-chain to expose a free carbonyl and therefore does not reduce Tollens' reagent.
Why C is wrong: C is wrong because sucrose is non-reducing and will not give a positive Tollens' test. Only maltose qualifies.
Why D is wrong: D is wrong because maltose does have a free anomeric carbon and is a confirmed reducing sugar.
Cellulose and starch are both polysaccharides of glucose. The key structural difference is:
Answer: B. Cellulose is a linear polymer of β-D-glucose units linked by β(1→4) glycosidic bonds. Starch (amylose + amylopectin) uses α(1→4) and α(1→6) linkages. NCERT Class 12 Chemistry Chapter 14.
Why A is wrong: A is wrong because it reverses the linkage types. Starch uses α-linkages; cellulose uses β-linkages.
Why C is wrong: C is wrong because both cellulose and starch are polymers of glucose (a hexose). Neither is a pentosan.
Why D is wrong: D is wrong because both cellulose and starch are composed entirely of glucose units.
Glucose and fructose have the same molecular formula (C₆H₁₂O₆) but differ in their functional group position. They are best described as:
Answer: C. Glucose (aldehyde at C-1) and fructose (ketone at C-2) have the same molecular formula but different functional groups — this is functional group isomerism. NCERT Class 12 Chemistry Chapter 14.
Why A is wrong: A is wrong because epimers differ in configuration at only one chiral centre (e.g., glucose and galactose differ at C-4). Glucose and fructose differ in functional group type, not just stereochemistry.
Why B is wrong: B is wrong because anomers differ only at the anomeric carbon (α vs β forms of the same sugar in cyclic form). Glucose and fructose differ in functional group identity.
Why D is wrong: D is wrong because geometrical isomers require restricted rotation (C=C or ring). Glucose and fructose differ in functional group type, not geometry.
A polysaccharide on complete hydrolysis yields only D-glucose. A sample is found to dissolve in hot water, giving a blue colour with iodine. The polysaccharide and its structural component are:
Answer: A. Step 1: Yields only D-glucose on hydrolysis → a glucan (starch, cellulose, or glycogen). Step 2: Blue colour with iodine → amylose (helical structure traps iodine; amylopectin gives violet-red, glycogen gives brown-red). Therefore the polysaccharide is amylose with α(1→4) linear helical chains. NCERT Class 12 Chemistry Chapter 14.
Why B is wrong: B is wrong because cellulose is insoluble in hot water and does not give a colour with iodine under standard conditions. Its β-linkages prevent helical coiling needed for iodine entrapment.
Why C is wrong: C is wrong because amylopectin gives a violet-red (not blue) colour with iodine due to its branched, shorter helical segments.
Why D is wrong: D is wrong because glycogen gives a brown-red colour with iodine due to extensive branching that shortens helical regions further than amylopectin.
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Given
• Sugar: Ribose• Need to determine: carbon-count class, functional group type, molecular formula
Required
Complete classification of ribose by (a) carbon number, (b) carbonyl type, and (c) molecular formula using Cₙ(H₂O)ₙ.
Concept
Monosaccharide classification: first identify the number of carbons (triose/tetrose/pentose/hexose), then identify the functional group (aldehyde → aldose; ketone → ketose). Apply the general formula Cₙ(H₂O)ₙ.
Formula
Cₙ(H₂O)ₙ with n = number of carbons
Substitution
Ribose has 5 carbons → n = 5
C₅(H₂O)₅ = C₅H₁₀O₅
Calculation
Molecular formula: C₅H₁₀O₅
Ribose has an aldehyde group at C-1 → aldose
Five carbons → pentose
Note: n = 5 is a counting integer (exact); it does not affect significant-figure considerations.
Final answer
Ribose is an aldopentose with molecular formula C₅H₁₀O₅.
Common trap
Confusing ribose (aldopentose, C₅H₁₀O₅) with deoxyribose (also a pentose but with formula C₅H₁₀O₄ — one fewer oxygen). The prefix "deoxy" means one –OH is replaced by –H. NEET distractors may offer C₅H₁₀O₄ for ribose.
Similar NEET-style question
"Ribulose is a monosaccharide with 5 carbons and a ketone group. Classify it and write its molecular formula." (Answer: ketopentose, C₅H₁₀O₅.)
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. 412More in Biomolecules: 4 exam traps and mistakes · 3 formulas from its other lessons.
No question in our NEET 2020–2025 set targets this topic directly.
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
swapped classes
Tempts surface-level recall.
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.
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