Disaccharides

8 MCQs1 revision card9-step worked example

Source: NCERT Unit 19PYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

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, Part 2, Chapter 14, page 6).

Practice 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?

MCQ 2Easy RecallPractice

Sucrose on hydrolysis gives:

MCQ 3Easy RecallPractice

Lactose is composed of which two monosaccharides?

MCQ 4Easy RecallPractice

The glycosidic linkage in maltose is:

MCQ 5Direct ApplicationPractice

Inversion of sucrose refers to:

MCQ 6Direct ApplicationPractice

Which of the following statements about disaccharides is CORRECT?

MCQ 7Direct ApplicationPractice

Sucrose does not reduce Tollens' reagent because:

MCQ 8Concept TrapPractice

Consider the following disaccharides: (i) Sucrose (ii) Maltose (iii) Lactose Which of these will give a positive Benedict's test?

Quick recall before you leave

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.) ---

Before solving, remember these

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, p. 6

Formulas

DNA hydrogen bonds per base pair

A - T : 2; G - C : 3

Used to compute total H-bonds in a duplex of given GC%/AT% composition.

Symbol	Quantity	SI Unit
%GC	GC content	-

Valid when

Watson–Crick double helix

General formula of monosaccharides

C_{n} (H_{2} O)_{n}

Empirical formula of simple monosaccharides; glucose/fructose are C6H12O6.

Symbol	Quantity	SI Unit
n	carbon count	-

Valid when

Open-chain or cyclic forms of aldoses/ketoses

Peptide bonds in a polypeptide of N residues

# bonds = N - 1

Number of amide (peptide) bonds in a linear polypeptide of N amino acids.

Symbol	Quantity	SI Unit
N	residue count	-

Valid when

Linear (non-cyclic) polypeptide

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

DNA vs RNA base pairing

Category: Similar Terms

Student writes A=U for DNA or A=T for RNA. DNA: A=T, G≡C. RNA: A=U (no T), G≡C.

When it triggers

Question on base pairing or sugar identity.

How to avoid

DNA: deoxyribose, A-T-G-C bases. RNA: ribose, A-U-G-C bases (uracil instead of thymine). H-bond pairs: A=T (DNA) or A=U (RNA), G≡C (3 H-bonds, both).

Trap

Denaturation does NOT break primary structure

Category: Similar Terms

Student claims denaturation breaks peptide bonds. Denaturation only breaks H-bonds, ionic, hydrophobic interactions; primary structure (peptide bonds) intact.

When it triggers

Question about protein denaturation effects.

How to avoid

Denaturation: heat/pH/organic solvents disrupt secondary, tertiary, quaternary structure. Primary structure (covalent peptide bonds) requires hydrolysis to break.

Mistake

Claims denaturation breaks peptide bonds (primary structure).

Root cause: concept gap

Correction

Denaturation breaks H-bonds, ionic, hydrophobic; preserves peptide bonds. Hydrolysis (acid/base/enzyme) breaks primary structure.

Mistake

Writes A=U for DNA or A=T for RNA.

Root cause: concept gap

Correction

DNA: deoxyribose, A=T, G≡C. RNA: ribose, A=U (no T), G≡C.

Past Year Questions

6 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Match List-I with List-II. List-I List-II (Name of Vitamin) (Deficiency disease) A. Vitamin B I. Cheilosis 12 B. Vitamin D II. Convulsions C. Vitamin B III. Rickets 2 D. Vitamin B IV. Pernicious anaemia 6 Choose the correct answer from the options given below:

1A-IV, B-III, C-II, D-I

2A-I, B-III, C-II, D-IV

3A-IV, B-III, C-I, D-II

4A-II, B-III, C-I, D-IV

NTA Answer: Option 3(final)

NEET 2025

Given below are two statements : Statement-I : Benzenediazonium salt is prepared by the reaction of aniline with nitrous acid at 273 – 278 K. It decomposes easily in the dry state. Statement-II : Insertion of iodine into the benzene ring is difficult and hence iodobenzene is prepared through the reaction of benzenediazonium salt with KI. In the light of the above statements, choose the most appropriate answer from the options given below :

1Statement I is incorrect but Statement II is correct

2Both Statement I and Statement II are correct

3Both Statement I and Statement II are incorrect

4Statement I is correct but Statement II is incorrect

NTA Answer: Option 2(final)

NEET 2024Revised key

Match List I with List II. List I List II (Reaction) (Reagents/Condition) A. I. B. II. CrO 3 C. III. KMnO /KOH,  4 D. IV. (i) O 3 (ii) Zn-H O 2 Choose the correct answer from the options given below:

1A-IV, B-I, C-III, D-II

2A-III, B-I, C-II, D-IV

3A-IV, B-I, C-II, D-III

4A-I, B-IV, C-II, D-III

NTA Answer: Option 3(revised_final)

NEET 2023

Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R : Assertion A : A reaction can have zero activation energy. Reasons R : The minimum extra amount of energy absorbed by reactant molecules so that their energy becomes equal to threshold value, is called activation energy. In the light of the above statements, choose the correct answer from the options given below :

1Both A and R are true and R is NOT the correct explanation of A

2A is true but R is false

3A is false but R is true

4Both A and R are true and R is the correct explanation of A

NTA Answer: Option 3(final)

NEET 2022

The incorrect statement regarding enzymes is

1Enzymes are very specific for a particular reaction and substrate.

2Enzymes are biocatalysts.

3Like chemical catalysts enzymes reduce the activation energy of bio processes.

4Enzymes are polysaccharides.

NTA Answer: Option 4(final)

NEET 2021

An organic compound contains 78% (by wt.) carbon and remaining percentage of hydrogen. The right option for the empirical formula of this compound is : [Atomic wt. of C is 12, H is 1]

1CH

2CH 4

3CH

4CH 2 3

NTA Answer: Option 4(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Biomolecules — recall and classify carbohydrates, proteins, nucleic acids, vitamins; identify base pairs and peptide bonds.

RecallMedium

Common distractors

swapped classes

Tempts surface-level recall.

Test yourself on this topic with real past-paper questions:

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