Q.54 Chemistry · Section A ⭐ 360° Option Elimination Active

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NEET 2024 Chemistry Question 54 Solution

Topic: Alcohols, Phenols and Ethers · Official NTA Booklet Code T1

Question

Which one of the following alcohols reacts instantaneously with Lucas reagent?

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Verified Against NTA Final Answer Key

Option (2) : CH3-C(CH3)(OH)-CH3 (2-methylpropan-2-ol / tert-butanol)

📚 Alcohols, Phenols and Ethers Easy

📋 Given Parameters & Problem Setup

Reagent Lucas reagent (concentrated HCl + anhydrous ZnCl2)

Observation condition Reacts instantaneously at room temperature

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The 30-Second Exam Hall Shortcut: ⚡ Lucas Turbidity Speed Rule

⏱️ Save ~30 seconds

3° Alcohol = Instant Turbidity (Immediate); 2° = ~5 minutes; 1° = Turbidity only on heating!

1 Lucas test differentiates alcohols based on the rate of formation of an insoluble alkyl chloride via an SN1 pathway.

2 Rate depends directly on the stability of the carbocation intermediate: 3° > 2° > 1°.

3 Inspect options: Option (1) is 1° isobutanol; Option (3) is 1° n-butanol; Option (4) is 2° sec-butanol.

4 Option (2) is tert-butanol (2-methylpropan-2-ol), a tertiary (3°) alcohol.

5 3° alcohol forms the stable tert-butyl cation immediately -> instantaneous turbidity -> Option (2) in 3 seconds!

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Why You Can 100% Trust This Shortcut: NCERT Chemistry · Alcohols, Phenols and Ethers (Reactions involving cleavage of C-O bond and Lucas reagent).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Mechanism of the Lucas Reaction

Lucas reagent consists of equimolar anhydrous ZnCl2 (a strong Lewis acid) and concentrated hydrochloric acid. The alcohol coordinates with ZnCl2, turning the poor leaving group -OH into a good leaving group [-O(ZnCl2)H]:

R{-}OH + \text{ZnCl}_2 \rightleftharpoons R{-}\overset{\oplus}{\text{O}}(\text{ZnCl}_2^\ominus)H

Step 2

Carbocation Generation (Rate-Determining Step)

In an SN1 pathway, departure of the leaving group produces a carbocation intermediate. The activation energy is lowest for tertiary carbocations owing to inductive stabilization (+I) and hyperconjugation (9 α-hydrogens in the tert-butyl cation):

(CH_3)_3C{-}OH \xrightarrow{\text{ZnCl}_2 / \text{HCl}} (CH_3)_3C^\oplus + [\text{ZnCl}_2\text{OH}]^\ominus

Step 3

Formation of Insoluble Alkyl Halide Layer

Rapid attack by chloride ion yields tert-butyl chloride, which is immiscible in aqueous solution, producing immediate cloudiness / turbidity:

(CH_3)_3C^\oplus + Cl^\ominus \xrightarrow{\text{fast}} (CH_3)_3C{-}Cl \downarrow \; (\text{immediate oily cloudiness})

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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 (1)

✗ Incorrect

Verdict & Scientific Reason:

Primary (1°) alcohol; does not produce cloudiness at room temperature without prolonged heating.

⚠️

Student Fallacy (Why candidates pick this trap): Focusing on the branched isopropyl group instead of the reacting carbinol carbon.

2 (2)

✓ 100% Correct

Verdict & Scientific Reason:

Tertiary (3°) alcohol reacting instantaneously at room temperature.

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Why this is airtight: None.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Primary alcohol; requires heating to react.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming all 4-carbon alcohols react similarly.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Secondary (2°) alcohol; turbidity appears only after ~5 minutes.

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Student Fallacy (Why candidates pick this trap): Confusing secondary reactivity (~5 min) with instantaneous reactivity.

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NTA Trap Alert: Counting Total Carbons Instead of Carbinol Degree

Option (1) has an isopropyl group that confuses students into thinking it is secondary or tertiary. Always examine the carbon directly bonded to -OH!

🛡️ How to avoid the -1 mark: Identify the carbinol carbon: if attached to 3 carbons -> 3° (instant); 2 carbons -> 2° (5 min); 1 carbon -> 1° (heat only).

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Core NEET Syllabus Standard

Chemistry Core Organic Chemistry · Alcohols, Phenols and Ethers Standard Benchmark

Core Scientific Principle Tested:

"Tertiary alcohols react instantaneously with Lucas reagent via an SN1 carbocation mechanism, producing an insoluble layer of alkyl chloride."

Syllabus Focus: Distinction between Primary, Secondary, and Tertiary Alcohols (Lucas Test)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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Ease of Acid-Catalysed Dehydration of Alcohols (Destabilizing Effect of Nitro Group)

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NEET 2024 Chemistry Question 54 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Lucas Turbidity Speed Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Mechanism of the Lucas Reaction

Carbocation Generation (Rate-Determining Step)

Formation of Insoluble Alkyl Halide Layer

360° Option Elimination Engine Key Feature

NTA Trap Alert: Counting Total Carbons Instead of Carbinol Degree

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Alcohols, Phenols and Ethers

Total Number of Cyclic Ether Isomers (Structural and Stereoisomers) of C₄H₈O

Reaction of 3-Methylbutan-2-ol with HBr via 1,2-Hydride Shift

Cleavage of Ethers by Hydrogen Halides (HI) – Benzyl Phenyl Ether

Ease of Acid-Catalysed Dehydration of Alcohols (Destabilizing Effect of Nitro Group)

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