Q.77 Chemistry ⭐ 360° Option Elimination Active
NEET 2025 Chemistry Question 77 Solution
Topic: Hydrocarbons · Official NTA Booklet Code 45
Question
Which one of the following compounds does not decolourize bromine water?
✓
Verified Against NTA Final Answer Key
Option (1) : Cyclohexane (Saturated cycloalkane)
📚 Hydrocarbons Easy
📋 Given Parameters & Problem Setup
Option 1 Compound Cyclohexane (C₆H₁₂, fully saturated alicyclic hydrocarbon)
Option 2 Compound Phenol (C₆H₅OH, strongly activated aromatic ring)
Option 3 Compound Styrene (C₆H₅-CH=CH₂, unsaturated olefinic side chain)
Option 4 Compound Aniline (C₆H₅NH₂, strongly activated aromatic amine)
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⏱️ Save ~25 seconds The 30-Second Exam Hall Shortcut: ⚡ Bromine Water Decolourization Eliminator
Phenol gives white 2,4,6-tribromophenol ppt (decolourizes!). Aniline gives 2,4,6-tribromoaniline ppt (decolourizes!). Styrene adds Br₂ across alkene C=C (decolourizes!). Cyclohexane is a SATURATED ALKANE with only single bonds ⟹ DOES NOT decolourize bromine water!
1 Recall test: Bromine water (Br₂ in water, reddish-brown) is decolourized by: (1) Unsaturated compounds (alkenes, alkynes via addition), and (2) Highly activated aromatic rings (phenol, aniline via rapid polybromination).
2 Examine Option (1) Cyclohexane: Saturated ring with only sp³ C-C and C-H single bonds. It does not react with bromine in the dark/absence of UV light. Hence it DOES NOT decolourize bromine water.
3 Examine Option (2) Phenol: Highly activated by -OH, precipitates 2,4,6-tribromophenol, decolourizing bromine water.
4 Examine Option (3) Styrene: Contains an aliphatic C=C double bond, immediately adds Br₂ across the double bond.
5 Examine Option (4) Aniline: Strongly activated by -NH₂, precipitates 2,4,6-tribromoaniline, decolourizing bromine water.
6 Select Option (1) in under 10 seconds!
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Why You Can 100% Trust This Shortcut: Bromine Water Test & Electrophilic Substitution (NCERT Chemistry · Hydrocarbons, Phenols and Amines).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Reactivity of Cyclohexane with Bromine Water
Cyclohexane (C₆H₁₂) is a fully saturated cycloalkane without π-bonds or activating heteroatoms. Under normal laboratory conditions without high-intensity ultraviolet irradiation:
- • Cyclohexane cannot decolourize aqueous bromine solution.
- • Therefore, Option (1) is the correct answer.
Step 2
Reactivity of Styrene with Bromine Water
Styrene (phenylethene) possesses an aliphatic carbon-carbon double bond which acts as an electron-rich nucleophile:
- • Rapid electrophilic addition across the alkene double bond discharges the reddish-brown color.
Step 3
Reactivity of Phenol and Aniline with Bromine Water
In aqueous medium, the -OH and -NH₂ groups are exceptionally powerful activating groups through +M resonance:
- • Aniline similarly reacts with bromine water to form 2,4,6-tribromoaniline precipitate.
- • Both reactions consume bromine rapidly, completely decolourizing bromine water.
- • Conclusion: Only cyclohexane does not decolourize bromine water, matching Option (1).
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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 A
✓ 100% Correct Verdict & Scientific Reason:
100% correct. Saturated cycloalkane, unreactive towards bromine water under ordinary conditions.
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Why this is airtight: None; textbook test for unsaturation.
2 B
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Phenol decolourizes bromine water to yield a white precipitate of 2,4,6-tribromophenol.
⚠️
Student Fallacy (Why candidates pick this trap): Thinking aromatic rings never react with bromine water.
3 C
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Styrene has an alkene double bond which rapidly adds bromine.
⚠️
Student Fallacy (Why candidates pick this trap): Overlooking the side-chain double bond.
4 D
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Aniline decolourizes bromine water instantly with 2,4,6-tribromoaniline precipitation.
⚠️
Student Fallacy (Why candidates pick this trap): Unaware of aniline’s extreme ortho/para activation.
⚠️
NTA Trap Alert: Thinking Aromatic Rings Never Decolourize Bromine Water
While plain benzene does not decolourize bromine water, strongly activated benzene rings (phenol and aniline) react INSTANTANEOUSLY with bromine water without any Lewis acid catalyst to form tribromo derivatives!
🛡️ How to avoid the -1 mark: Remember: Phenol and Aniline decolourize bromine water! Only saturated alkanes do not.
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Core NEET Syllabus Standard
NEET Chemistry Core Syllabus · Hydrocarbons Standard Benchmark
Core Scientific Principle Tested:
"Saturated hydrocarbons do not decolourize bromine water; alkenes, phenols, and anilines decolourize it."
Syllabus Focus: Alkanes, Alkenes & Aromatic Electrophilic Substitution
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
No double bond, no activating group = No reaction: Cyclohexane keeps the color (Option 1)!
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