Q.75 Chemistry ⭐ 360° Option Elimination Active
NEET 2023 Chemistry Question 75 Solution
Topic: Haloalkanes and Haloarenes · Official NTA Booklet Code G5
Question
The given compound C₆H₅−CH=CH−CH(X)−CH₂CH₃ is an example of ___.
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Verified Against NTA Final Answer Key
Option (4) : allylic halide
📚 Haloalkanes and Haloarenes Easy
📋 Given Parameters & Problem Setup
Given structure C₆H₅–CH = CH–CH(X)–CH₂CH₃
Position of halogen X Attached to sp³ carbon adjacent to aliphatic C=C double bond
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⏱️ Save ~30 seconds The 30-Second Exam Hall Shortcut: ⚡ Carbon Neighborhood Check: Attached to C=C, NOT Benzene Ring!
Halogen X is attached to an sp³ carbon that is DIRECTLY adjacent to an aliphatic C=C double bond (–CH=CH–CH(X)–) ⟹ Allylic Halide! (It is NOT benzylic because it is separated from the benzene ring by two carbons). Option (4) in 5 seconds!
1 Inspect the exact carbon holding halogen X: –CH=CH–CH(X)–CH₂CH₃.
2 The carbon bearing X is sp³-hybridized.
3 It is directly bonded to an sp²-hybridized carbon of an aliphatic double bond (–CH=CH–).
4 By definition, a halide attached to an sp³ carbon adjacent to a carbon-carbon double bond is an ALLYLIC HALIDE.
5 Why is it not benzylic? Because a benzylic halide requires X to be attached to an sp³ carbon directly bonded to an aromatic benzene ring (Ar–CH(X)–). Here, the halogen is two carbons away from the benzene ring!
6 Lock Option (4) allylic halide immediately!
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Why You Can 100% Trust This Shortcut: Organic Nomenclature · Classification of Halogen Compounds (Allylic halides are compounds in which the halogen atom is bonded to an sp³-hybridized carbon atom next to a carbon-carbon double bond).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
IUPAC Definitions of Halide Classes
Halogen derivatives of hydrocarbons are classified into distinct structural categories based on the hybridization and environment of the carbon atom bearing the halogen:
Step 2
Structural Analysis of the Given Molecule
In the provided structure C₆H₅–CH=CH–CH(X)–CH₂CH₃, locate the halogen X:
Step 3
Evaluation of the Carbon-Halogen Environment
The halogen X is bonded to carbon C-γ, which is an sp³-hybridized carbon atom. C-γ is bonded directly to carbon C-β, which is an sp²-hybridized carbon participating in an aliphatic carbon-carbon double bond (C=C). This matches the exact definition of an allylic halide.
Step 4
Refutation of Benzylic Character
The benzene ring (Ar) is attached to C-α, not to C-γ. Because C-γ is not directly attached to the aromatic ring, the compound CANNOT be classified as a benzylic halide.
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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:
In a vinylic halide, X must be bonded directly to an sp² double-bonded carbon (–CH=C(X)–). Here X is on an sp³ carbon.
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Student Fallacy (Why candidates pick this trap): Candidate thought any halide near a double bond is vinylic.
2 (2)
✗ Incorrect Verdict & Scientific Reason:
In a benzylic halide, X must be on the carbon directly attached to the benzene ring (C₆H₅–CH(X)–). Here X is two carbons away.
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Student Fallacy (Why candidates pick this trap): Candidate saw the benzene ring in the drawing and reflexively chose benzylic.
3 (3)
✗ Incorrect Verdict & Scientific Reason:
In an aryl halide, X must be directly on the benzene ring (e.g., chlorobenzene).
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Student Fallacy (Why candidates pick this trap): Candidate confused presence of an aromatic ring with direct aromatic substitution.
4 (4)
✓ 100% Correct Verdict & Scientific Reason:
Accurate: X is attached to an sp³ carbon adjacent to an aliphatic C=C double bond.
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Why this is airtight: None. Candidate correctly recognized the allylic carbon position.
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NTA Trap Alert: Seeing the Benzene Ring and Blindly Choosing "Benzylic Halide" (Option 2)
The presence of the benzene ring is an intentional distractor. Students see the phenyl ring and hastily pick "benzylic halide" without checking whether the halogen-bearing carbon is actually attached to the ring. In this molecule, the halogen is attached next to the double bond, making it an ALLYLIC halide!
🛡️ How to avoid the -1 mark: Trace the direct bond: Look at the carbon attached to X. Is it attached directly to the benzene ring? NO. Is it attached to a C=C double bond? YES ⟹ Allylic!
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Core NEET Syllabus Standard
National Eligibility cum Entrance Test (NEET UG) · Haloalkanes and Haloarenes Standard Benchmark
Core Scientific Principle Tested:
"Allylic halides are compounds in which the halogen atom is bonded to an sp³-hybridized carbon atom adjacent to a carbon-carbon double bond (C=C)."
Syllabus Focus: Classification of Halogen Compounds
Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗
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