Q.96 Chemistry ⭐ 360° Option Elimination Active
NEET 2023 Chemistry Question 96 Solution
Topic: Alcohols, Phenols and Ethers · Official NTA Booklet Code G5
Question
Consider the following reaction: Identify products A and B.
✓
Verified Against NTA Final Answer Key
Option (4) : A = C₆H₅–CH₂I and B = C₆H₅–OH
📚 Alcohols, Phenols and Ethers Medium
📋 Given Parameters & Problem Setup
Substrate Benzyl phenyl ether (C₆H₅–CH₂–O–C₆H₅)
Reagent HI, heat (Δ)
Products A and B
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⏱️ Save ~30 seconds The 30-Second Exam Hall Shortcut: ⚡ Stable Benzyl Carbocation Rule: C–O Cleavage Occurs at Benzyl Carbon!
In benzyl phenyl ether (C₆H₅–CH₂–O–C₆H₅), the phenyl–oxygen bond has partial double bond character due to resonance with the benzene ring, making it extremely strong. In contrast, the benzyl C–O bond cleaves easily because the benzyl carbocation (or benzyl transition state) is resonance-stabilized! Hence, iodide attacks the benzyl carbon to form benzyl iodide (C₆H₅CH₂I), while oxygen retains the proton to form phenol (C₆H₅OH)! Option (4) in 10 seconds!
1 Protonation of oxygen gives C₆H₅–CH₂–O⁺(H)–C₆H₅.
2 Compare the two C–O bonds:
3 - Phenyl C(sp²)–O bond: has partial double-bond character; phenyl cation is extremely unstable; cannot be attacked.
4 - Benzyl C(sp³)–O bond: cleavage gives resonance-stabilized benzyl species.
5 Nucleophilic attack by I⁻ occurs at the benzyl carbon.
6 Products formed: Benzyl iodide (C₆H₅–CH₂I) and Phenol (C₆H₅–OH).
7 Match Option (4) directly.
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Why You Can 100% Trust This Shortcut: Classical NCERT ether cleavage mechanism by hydrohalic acids.
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Protonation of the Ether Linkage
In the presence of concentrated HI, the lone pair on the ether oxygen is protonated, yielding a positively charged oxonium ion:
Step 2
Bond Energy Comparison: Phenyl–O vs Benzyl–O
The bond between oxygen and the phenyl ring has partial double bond character due to resonance of oxygen lone pairs with the aromatic ring, making it resistant to cleavage. Furthermore, nucleophilic attack on an sp² phenyl carbon is energetically disfavored:
Step 3
Nucleophilic Cleavage at the Benzyl Carbon
The benzyl C(sp³)–O bond cleaves readily via SN1/SN2 pathway because the developing positive charge at the benzylic carbon is stabilized by resonance with the phenyl ring. Iodide ion attacks the benzylic carbon:
Step 4
Final Product Identification
Product A is benzyl iodide (C₆H₅–CH₂I) and Product B is phenol (C₆H₅–OH):
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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:
Toluene and iodobenzene are not formed; the phenyl–O bond cannot cleave under these conditions.
⚠️
Student Fallacy (Why candidates pick this trap): Candidate incorrectly cleaved the phenyl C–O bond and added hydride to benzyl.
2 (2)
✗ Incorrect Verdict & Scientific Reason:
Cannot form toluene by simple HI cleavage; iodide must attach to the carbon chain.
⚠️
Student Fallacy (Why candidates pick this trap): Candidate forgot the nucleophilic addition of iodide.
3 (3)
✗ Incorrect Verdict & Scientific Reason:
Requires cleavage of the partial double-bond phenyl–O bond, which is energetically forbidden.
⚠️
Student Fallacy (Why candidates pick this trap): Candidate placed iodide on the phenyl ring instead of the benzyl group.
4 (4)
✓ 100% Correct Verdict & Scientific Reason:
Accurate: Cleavage yields benzyl iodide and phenol exclusively.
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Why this is airtight: None. Candidate recognized that the phenyl–O bond remains intact while the benzyl C–O bond cleaves.
⚠️
NTA Trap Alert: Attempting to Cleave the Phenyl–Oxygen Bond
Students often incorrectly put iodide on the phenyl ring (forming iodobenzene) and keep –OH on the benzyl group. The phenyl–O bond has partial double bond character and never cleaves under these conditions; nucleophilic substitution occurs at the benzyl carbon!
🛡️ How to avoid the -1 mark: Remember: Phenolic C–O bond NEVER cleaves with HI. Phenol is ALWAYS one of the products!
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Core NEET Syllabus Standard
National Eligibility cum Entrance Test (NEET UG) · Alcohols, Phenols and Ethers Standard Benchmark
Core Scientific Principle Tested:
"Ethers with alkyl and aryl groups cleave to yield phenol and alkyl iodide due to partial double-bond character of the Ar–O bond."
Syllabus Focus: Chemical Reactions of Ethers – Cleavage of C–O Bond by HI
Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗
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