Q.84 Chemistry ⭐ 360° Option Elimination Active
NEET 2023 Chemistry Question 84 Solution
Topic: Amines · Official NTA Booklet Code G5
Question
Which of the following reactions will NOT give primary amine as the product?
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Verified Against NTA Final Answer Key
Option (4) : CH₃NC —( (i) LiAlH₄ / (ii) H₂O )→ Product
📚 Amines Medium
📋 Given Parameters & Problem Setup
Reaction 1 CH₃CONH₂ + LiAlH₄ / H₂O
Reaction 2 CH₃CONH₂ + Br₂ / KOH
Reaction 3 CH₃CN + LiAlH₄ / H₂O
Reaction 4 CH₃NC + LiAlH₄ / H₂O
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⏱️ Save ~30 seconds The 30-Second Exam Hall Shortcut: ⚡ Isonitrile Functional Rule: Reduction of R–NC Yields 2° Amine (R–NH–CH₃)
Nitriles (R–CN) reduce to 1° primary amines (R–CH₂–NH₂), but isonitriles (R–NC) reduce to 2° secondary amines (R–NH–CH₃)! Acetamide reduces with LiAlH₄ to ethylamine (1°), and with Br₂/KOH undergoes Hoffmann degradation to methylamine (1°). Thus, ONLY Reaction (4) produces a secondary amine!
1 Option 1: CH₃CONH₂ + LiAlH₄ → CH₃CH₂NH₂ (1° amine).
2 Option 2: CH₃CONH₂ + Br₂/KOH → CH₃NH₂ (1° amine, Hoffmann degradation).
3 Option 3: CH₃CN + LiAlH₄ → CH₃CH₂NH₂ (1° amine).
4 Option 4: CH₃NC + LiAlH₄ → CH₃–NH–CH₃ (Dimethylamine, 2° amine).
5 Question asks which does NOT give primary amine: Option (4)!
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Why You Can 100% Trust This Shortcut: Standard textbook organic functional group reduction of nitrogen compounds.
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Analyze Reaction (1): LiAlH₄ Reduction of Amide
Reduction of acetamide by lithium aluminium hydride reduces the carbonyl group to a methylene group, producing ethanamine (1° amine):
Step 2
Analyze Reaction (2): Hoffmann Bromamide Degradation
Degradation of an aliphatic primary amide with bromine in alkaline KOH eliminates the carbonyl carbon, producing methanamine (1° amine):
Step 3
Analyze Reaction (3): LiAlH₄ Reduction of Nitrile
Reduction of acetonitrile converts the cyano group into a primary amine with the same carbon count:
Step 4
Analyze Reaction (4): LiAlH₄ Reduction of Isocyanide
Reduction of an alkyl isocyanide (methyl isonitrile) adds hydrogens across the coordinate bond, yielding a secondary amine:
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 (1)
✗ Incorrect Verdict & Scientific Reason:
Produces ethanamine (CH₃CH₂NH₂), which is a primary amine.
⚠️
Student Fallacy (Why candidates pick this trap): Candidate forgot that LiAlH₄ reduces amides to primary amines without loss of carbon.
2 (2)
✗ Incorrect Verdict & Scientific Reason:
Produces methanamine (CH₃NH₂), which is the benchmark primary amine of Hoffmann bromamide degradation.
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Student Fallacy (Why candidates pick this trap): Candidate misidentified the product of the Hoffmann bromamide reaction.
3 (3)
✗ Incorrect Verdict & Scientific Reason:
Produces ethanamine (CH₃CH₂NH₂), which is a primary amine.
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Student Fallacy (Why candidates pick this trap): Candidate confused the reduction of nitriles with the reduction of isonitriles.
4 (4)
✓ 100% Correct Verdict & Scientific Reason:
Accurate: Reduction of methyl isonitrile gives dimethylamine (CH₃-NH-CH₃), a 2° amine.
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Why this is airtight: None. Candidate correctly distinguished nitrile (-CN) from isonitrile (-NC) reduction products.
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NTA Trap Alert: Confusing Cyanide (-CN) with Isocyanide (-NC)
Nitriles (R-CN) have nitrogen at the terminus, so reduction gives R-CH₂-NH₂ (1° amine). Isocyanides (R-NC) have nitrogen in the middle between two carbon atoms, so reduction gives R-NH-CH₃ (2° amine)!
🛡️ How to avoid the -1 mark: Remember: Cyanide → 1° Amine; Isocyanide → 2° Amine.
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Core NEET Syllabus Standard
National Eligibility cum Entrance Test (NEET UG) · Amines Standard Benchmark
Core Scientific Principle Tested:
"Reduction of alkyl nitriles yields primary amines, whereas reduction of alkyl isonitriles yields secondary amines."
Syllabus Focus: Preparation of Amines – Reduction of Nitriles and Isonitriles
Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗
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