Q.55 Chemistry ⭐ 360° Option Elimination Active

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NEET 2026 Chemistry Question 55 Solution

Topic: Organic Chemistry: Some Basic Principles and Techniques & Biomolecules · Official NTA Booklet Code 50

Question

The amino acid that gives a red-blood colour on treating its sodium fusion extract with sodium nitroprusside is

(1)leucine
(2)threonine
(3)methionine
(4)serine

✓

Verified Against NTA Final Answer Key

Option (3) : methionine

📚 Organic Chemistry: Some Basic Principles and Techniques & Biomolecules Easy

📋 Given Parameters & Problem Setup

Reaction Sodium fusion extract (Lassaigne's extract) treated with sodium nitroprusside

Observation Characteristic red/violet color indicating presence of sulfur (or blood-red SCN⁻ complex)

Options leucine, threonine, methionine, serine

⚡

The 30-Second Exam Hall Shortcut: ⚡ The Sulfur-Containing Amino Acid Identifier

⏱️ Save ~50 seconds

Only TWO standard amino acids contain sulfur: Cysteine and Methionine!

1 Recall amino acid structures: Leucine (hydrocarbon), Threonine (alcohol), Serine (alcohol), Methionine (thioether).

2 Among the four choices, only METHIONINE contains a sulfur atom (-CH₂-CH₂-S-CH₃).

3 Any qualitative test for sulfur (nitroprusside test or thiocyanate formation) can ONLY test positive for the sulfur-containing amino acid.

4 Pick Methionine (Option 3) in 5 seconds with 100% confidence!

🛡️

Why You Can 100% Trust This Shortcut: Of the 20 standard proteinogenic amino acids, only Methionine and Cysteine possess sulfur in their side chains.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Identify the Elemental Constituents of the Given Amino Acids

Examine the side chains of the four amino acids in the options: 1. Leucine: (CH₃)₂CH-CH₂-CH(NH₂)COOH (Contains C, H, N, O). 2. Threonine: CH₃-CH(OH)-CH(NH₂)COOH (Contains C, H, N, O). 3. Serine: HO-CH₂-CH(NH₂)COOH (Contains C, H, N, O). 4. Methionine: CH₃-S-CH₂-CH₂-CH(NH₂)COOH (Contains C, H, N, O, and S).

\text{Methionine} \equiv \text{CH}_3-\text{S}-\text{CH}_2-\text{CH}_2-\text{CH}(\text{NH}_2)\text{COOH}

• Methionine is the unique sulfur-containing amino acid among the choices.

Step 2

Sodium Fusion (Lassaigne's Test) Chemistry

When an organic compound containing sulfur (and nitrogen) is fused with sodium metal, covalent sulfur is converted into ionic sodium sulfide (or sodium thiocyanate if N and S are both present and Na is not in excess): 2Na + S → Na₂S Na + C + N + S → NaSCN

2\text{Na} + \text{S} \xrightarrow{\Delta} \text{Na}_2\text{S}

• Organic sulfur is mineralized into sulfide / thiocyanate ions.

Step 3

Color Reaction with Sodium Nitroprusside

Sulfide ions react with sodium nitroprusside [Fe(CN)₅NO]²⁻ in alkaline medium to form the brilliant purple-violet complex ion [Fe(CN)₅NOS]⁴⁻ (often noted alongside red-blood thiocyanate coloration in dual N+S analysis). Because leucine, threonine, and serine lack sulfur entirely, they cannot give this test.

\text{S}^{2-} + [\text{Fe}(\text{CN})_5\text{NO}]^{2-} \longrightarrow [\text{Fe}(\text{CN})_5\text{NOS}]^{4-} \quad (\text{Violet/Purple complex})

• Only methionine yields the sulfur diagnostic test.
• Corresponds to Option (3).

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Leucine has an aliphatic branched hydrocarbon side chain containing only C, H, N, O (no sulfur).

⚠️

Student Fallacy (Why candidates pick this trap): Assuming all essential amino acids contain sulfur.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Threonine is a hydroxy-containing amino acid with no sulfur.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing the thiol/thioether group with a secondary alcohol.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

Exact match. Methionine contains a thioether side chain (-S-CH₃) providing sulfur for the sodium fusion test.

💡

Why this is airtight: None; indisputable chemical fact.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Serine contains a primary hydroxyl group (-CH₂OH) and no sulfur.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing serine with cysteine.

⚠️

NTA Trap Alert: Searching for Cysteine When Only Methionine is Listed

Students often memorize cysteine as the test-tube sulfur amino acid. When cysteine is absent from the options, they hesitate—forgetting that methionine also contains sulfur!

🛡️ How to avoid the -1 mark: Remember: Cysteine and Methionine are the twin sulfur-containing amino acids.

📖

Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Biomolecules & Practical Organic Chemistry Standard Benchmark

Core Scientific Principle Tested:

"Detection of sulfur via sodium fusion extract requires a sulfur-bearing functional group in the precursor."

Syllabus Focus: Classification of amino acids and qualitative elemental detection

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Sulfur in proteins? Only Two in the scene: Cysteine and Methionine!

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NEET 2026 Chemistry Question 55 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ The Sulfur-Containing Amino Acid Identifier

Step-by-Step Rigorous Workflow & Mathematical Proof

Identify the Elemental Constituents of the Given Amino Acids

Sodium Fusion (Lassaigne's Test) Chemistry

Color Reaction with Sodium Nitroprusside

360° Option Elimination Engine Key Feature

NTA Trap Alert: Searching for Cysteine When Only Methionine is Listed

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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