Key Fact
Glycosidic bond
C-O-C bond between two monosaccharides. α-1,4 in maltose/starch; β-1,4 in cellulose/lactose. Hydrolysis cleaves to monosaccharides.
-- NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, p. 417Disaccharides
8 MCQs2 revision cards9-step worked example
Source: NCERT BiomoleculesPYQ coverage: NEET 2020Official key: NTA-verifiedLast updated: 27 Sep 2026
Disaccharides, explained for NEET
Disaccharides: Sucrose, Lactose, and Maltose
Disaccharides are sugars formed when two monosaccharide units join through a glycosidic bond with the loss of one water molecule. NEET questions on disaccharides test whether you can correctly pair each disaccharide with its component monosaccharides, its glycosidic linkage type, and its reducing or non-reducing character.
The three NEET-relevant disaccharides:
Sucrose is formed from one glucose and one fructose unit linked by a C1–C2 glycosidic bond (α-1,2). Because this bond involves the anomeric carbons of both monosaccharides, sucrose has no free anomeric carbon and is a non-reducing sugar. This is the single most tested property of sucrose in NEET.
Maltose consists of two glucose units joined by an α-1,4 glycosidic bond. One anomeric carbon remains free, so maltose is a reducing sugar. Maltose is produced during starch hydrolysis.
Lactose consists of one galactose and one glucose unit joined by a β-1,4 glycosidic bond. It retains a free anomeric carbon and is a reducing sugar. Lactose is the sugar found in milk.
The high-frequency confusion: aspirants swap the monosaccharide components — placing galactose in maltose or two glucoses in lactose. A reliable mnemonic: Maltose = Malt = starch breakdown = glucose + glucose. Lactose = Lactation = milk = galactose + glucose.
Reducing vs non-reducing: the test is whether a free anomeric carbon (hemiacetal/hemiketal –OH) exists to open-chain and reduce Tollens' or Fehling's reagent. Among the three, only sucrose fails this test because both anomeric carbons are locked in the glycosidic bond.
Hydrolysis of sucrose by the enzyme invertase (or dilute acid) yields an equimolar mixture of glucose and fructose called invert sugar — the optical rotation sign flips from dextrorotatory to levorotatory, which is why the process is called inversion (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 417).
Can you answer these Disaccharides MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which of the following disaccharides is a non-reducing sugar?
Show answer and why every option is right or wrong
Answer: C. Sucrose is a non-reducing sugar because both anomeric carbons (C1 of glucose and C2 of fructose) are involved in the glycosidic bond, leaving no free anomeric –OH to reduce Tollens' or Fehling's reagent (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 417).
Why A is wrong: A is wrong because maltose has a free anomeric carbon on the second 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 D is wrong: D is wrong because cellobiose (β-1,4 linked two glucose units) retains a free anomeric carbon and is a reducing sugar.
MCQ 2Easy RecallPractice
Sucrose on hydrolysis gives:
Show answer and why every option is right or wrong
Answer: A. Sucrose is composed of one α-D-glucose and one β-D-fructose unit linked by a C1–C2 glycosidic bond. Hydrolysis cleaves this bond to yield equimolar glucose and fructose (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 417).
Why B is wrong: B is wrong because glucose + galactose is the hydrolysis product of lactose, not sucrose.
Why C is wrong: C is wrong because glucose + glucose is the hydrolysis product of maltose, not sucrose.
Why D is wrong: D is wrong because no common disaccharide yields fructose + galactose on hydrolysis.
MCQ 3Easy RecallPractice
Lactose is composed of which two monosaccharides?
Show answer and why every option is right or wrong
Answer: D. Lactose consists of one β-D-galactose and one β-D-glucose unit joined by a β-1,4 glycosidic linkage. It is the sugar present in milk (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 418).
Why A is wrong: A is wrong because glucose + glucose describes maltose (α-1,4 linkage) or cellobiose (β-1,4 linkage), not lactose.
Why B is wrong: B is wrong because glucose + fructose describes sucrose, not lactose.
Why C is wrong: C is wrong because no common disaccharide is composed of galactose + fructose.
MCQ 4CalculationPractice
Acid-catalysed hydrolysis of sucrose has an activation energy of 6.22 kJ mol⁻¹, while enzyme-catalysed hydrolysis (by the enzyme sucrase) has an activation energy of only 2.15 kJ mol⁻¹. By what percentage does the enzyme lower the activation energy, relative to the acid-catalysed value?
Show answer and why every option is right or wrong
Answer: B. The drop in activation energy is 6.22 − 2.15 = 4.07 kJ mol⁻¹. Expressed as a percentage of the acid-catalysed (reference) value: (4.07 / 6.22) × 100 ≈ 65.4%. NCERT states these exact activation-energy values for sucrose hydrolysis to illustrate how an enzyme lowers activation energy (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 425).
Why A is wrong: A (34.6%) is the fraction of the ORIGINAL (acid-catalysed) activation energy that REMAINS with the enzyme (2.15 / 6.22 × 100), not the percentage by which it was lowered (trap: reporting the remaining fraction instead of the decrease).
Why C is wrong: C (189.3%) comes from dividing the drop by the wrong (smaller, enzyme-catalysed) value instead of the acid-catalysed reference value: (4.07 / 2.15) × 100 (trap: using the new value as the denominator instead of the original value).
Why D is wrong: D (4.07%) mistakes the raw activation-energy drop in kJ mol⁻¹ (4.07) for a percentage directly, skipping the required division by the reference value (trap: treating a kJ mol⁻¹ quantity as already being a percentage).
MCQ 5Direct ApplicationPractice
Inversion of sucrose refers to:
Show answer and why every option is right or wrong
Answer: B. Sucrose is dextrorotatory (+66.5°). On hydrolysis by invertase or dilute acid, the equimolar mixture of glucose (+52.7°) and fructose (−92.0°) has a net levorotatory rotation. This sign change is called inversion, and the product mixture is invert sugar (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 417).
Why A is wrong: A is wrong because inversion of sucrose does not involve converting a D-sugar to its L-enantiomer — it refers to the sign flip in optical rotation of the hydrolysis mixture.
Why C is wrong: C is wrong because inversion is not an isomerisation reaction. Hydrolysis of sucrose produces both glucose and fructose as separate monosaccharides.
Why D is wrong: D is wrong because mutarotation is the interconversion of α and β anomers of a single sugar in solution, which is a different phenomenon from the inversion of sucrose.
MCQ 6Direct ApplicationPractice
Which of the following statements about disaccharides is CORRECT?
Show answer and why every option is right or wrong
Answer: A. Lactose is formed by a β-1,4 glycosidic bond between the C1 of β-D-galactose and the C4 of D-glucose. This is the correct structural description (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 418).
Why B is wrong: B is wrong because maltose IS a reducing sugar — only one anomeric carbon (C1 of the first glucose) is involved in the α-1,4 bond; the second glucose retains a free anomeric –OH.
Why C is wrong: C is wrong because sucrose is a non-reducing sugar and does NOT give a positive Fehling's test. Both anomeric carbons are locked in the glycosidic bond.
Why D is wrong: D is wrong because maltose on hydrolysis gives two molecules of glucose (not glucose and fructose). Glucose + fructose is the hydrolysis product of sucrose.
MCQ 7Direct ApplicationPractice
Sucrose does not reduce Tollens' reagent because:
Show answer and why every option is right or wrong
Answer: B. In sucrose, C1 of glucose and C2 of fructose are both locked in the α-1,2 glycosidic bond. Neither monosaccharide can open its ring to expose a free aldehyde or α-hydroxy ketone group, so sucrose cannot reduce Tollens' reagent (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 417).
Why A is wrong: A is wrong because sucrose has many hydroxyl groups — what it lacks is a FREE anomeric hydroxyl, which is specifically required for reducing activity.
Why C is wrong: C is wrong because free fructose IS a reducing sugar — ketoses reduce Tollens' reagent through tautomeric equilibrium with an enediol form. The issue in sucrose is that fructose's anomeric carbon is bonded, not that ketoses can't reduce.
Why D is wrong: D is wrong because the α or β configuration of the glycosidic bond does not determine reducing ability. What matters is whether a free anomeric carbon exists. The sucrose linkage involves both anomeric carbons regardless of its stereochemistry.
MCQ 8Concept TrapPractice
Consider the following disaccharides:
(i) Sucrose
(ii) Maltose
(iii) Lactose
Which of these will give a positive Benedict's test?
Show answer and why every option is right or wrong
Answer: C. Benedict's test detects reducing sugars. Maltose (free anomeric –OH on the second glucose) and lactose (free anomeric –OH on the glucose unit) are both reducing sugars and give a positive test. Sucrose is a non-reducing sugar because both anomeric carbons participate in the glycosidic bond (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 417).
Why A is wrong: A is wrong because sucrose is a non-reducing sugar and will NOT give a positive Benedict's test. Including sucrose while excluding lactose (which is reducing) is incorrect.
Why B is wrong: B is wrong because sucrose is non-reducing (negative Benedict's test), while lactose is reducing (positive). This option wrongly includes sucrose and excludes maltose.
Why D is wrong: D is wrong because sucrose does not give a positive Benedict's test. Only maltose and lactose are reducing disaccharides among the three listed.
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Disaccharides: quick recall before you leave
How do you solve a Disaccharides question? A worked example
- 1
Given
Three disaccharide solutions: sucrose, maltose, lactose. Fehling's test performed on each. Two give red precipitate; one does not.
- 2
Required
Identify which disaccharide does not react with Fehling's solution. Explain the structural basis.
- 3
Concept
Fehling's test detects reducing sugars — sugars with a free anomeric carbon that can open to expose an aldehyde (or enediol from a ketose) and reduce Cu²⁺ to Cu₂O (red precipitate). A disaccharide is non-reducing if both anomeric carbons of its monosaccharide units are locked in the glycosidic bond.
- 4
Structural analysis
| Disaccharide | Components | Glycosidic bond | Anomeric C status | Reducing? |
|---|---|---|---|---|
| Sucrose | Glucose + Fructose | α-1,2 (C1 of Glc ↔ C2 of Fru) | Both locked | No |
| Maltose | Glucose + Glucose | α-1,4 (C1 of Glc₁ → C4 of Glc₂) | C1 of Glc₂ free | Yes |
| Lactose | Galactose + Glucose | β-1,4 (C1 of Gal → C4 of Glc) | C1 of Glc free | Yes | - 5
Identification
The solution that does NOT produce a red precipitate is sucrose.
- 6
Reasoning
In sucrose, the glycosidic bond is between C1 of glucose (the anomeric carbon of the aldose) and C2 of fructose (the anomeric carbon of the ketose). With both anomeric carbons consumed in bond formation, neither sugar unit can ring-open to generate a reducing functional group. Hence, sucrose cannot reduce Fehling's reagent.
Maltose and lactose each have one anomeric carbon uninvolved in the glycosidic linkage. That free anomeric –OH allows ring-opening and reduction of Cu²⁺ → Cu₂O. - 7
Final answer
Sucrose is the non-reducing disaccharide that does not react with Fehling's solution. Maltose and lactose are reducing disaccharides that produce the red Cu₂O precipitate.
- 8
Common trap
Aspirants sometimes assume fructose (a ketose) cannot be a reducing sugar, and therefore expect sucrose's non-reducing character to stem from fructose being a ketose. This is incorrect — free fructose IS a reducing sugar (it tautomerises via an enediol intermediate). Sucrose is non-reducing because fructose's anomeric C2 is bonded, not because it's a ketose.
- 9
Similar NEET-style question
"Among sucrose, maltose, cellobiose, and trehalose, how many are non-reducing sugars?" (Answer: 2 — sucrose and trehalose, both having their anomeric carbons locked in the glycosidic bond.)
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What to remember before solving Disaccharides questions
More in Biomolecules: 4 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
Disaccharides questions from past NEET papers
1 question from NEET 2020. Answers verified against NTA official keys.
NEET 2020
1α-D-Fructose+β-D-Fructose
2β-D-Glucose+α-D-Fructose
3α-D-Glucose+β-D-Glucose
4α-D-Glucose+β-D-Fructose
NTA Answer: Option 4(final)
Sources
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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