Q.100 Chemistry ⭐ 360° Option Elimination Active

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NEET 2020 Chemistry Question 100 Solution

Topic: Alcohols, Phenols and Ethers · Official NTA Booklet Code E1

Question

Reaction between acetone and methylmagnesium chloride followed by hydrolysis will give:

(1)Isopropyl alcohol
(2)Sec. butyl alcohol
(3)Tert. butyl alcohol
(4)Isobutyl alcohol

✓

Verified Against NTA Final Answer Key

Option (3) : Tert. butyl alcohol

📚 Alcohols, Phenols and Ethers Easy

📋 Given Parameters & Problem Setup

Substrate Acetone (CH3-CO-CH3, propan-2-one)

Grignard Reagent Methylmagnesium chloride (CH3MgCl)

Second Step Aqueous acid hydrolysis (H3O+)

Question Objective Identify the alcohol produced

⚡

The 30-Second Exam Hall Shortcut: ⚡ "Formaldehyde -> 1° Alcohol; Other Aldehydes -> 2° Alcohol; Ketones -> 3° Alcohol!"

⏱️ 20 seconds

Acetone is a ketone ( $CH_3-CO-CH_3$ ). Reaction of any ketone with a Grignard reagent yields a TERTIARY (3°) alcohol upon hydrolysis. Here, acetone (3 carbons) + methyl Grignard (1 carbon) produces 2-methylpropan-2-ol, commonly known as Tertiary Butyl Alcohol (4 carbons)! Mark Tert. butyl alcohol (Option 3) in 5 seconds.

1 Formaldehyde + RMgX -> 1° alcohol.

2 Any other aldehyde + RMgX -> 2° alcohol.

3 Ketone (Acetone) + RMgX -> 3° alcohol.

4 Acetone (CH3COCH3) + CH3MgCl -> (CH3)3C-OMgCl.

5 Hydrolysis with H2O -> (CH3)3C-OH (Tert-butyl alcohol).

6 Select Option (3).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · Alcohols, Phenols and Ethers (Preparation of Alcohols: From Grignard Reagents)

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Nucleophilic Addition of Grignard Reagent

The polarized carbon-magnesium bond in methylmagnesium chloride ( $\text{CH}_3^{\delta-} - \text{MgCl}^{\delta+}$ ) acts as a strong nucleophile. The carbanion ( $\text{CH}_3^-$ ) attacks the electrophilic carbonyl carbon of acetone: $\text{CH}_3-\text{CO}-\text{CH}_3 + \text{CH}_3\text{MgCl} \xrightarrow{\text{dry ether}} (\text{CH}_3)_3\text{C-OMgCl}$ This forms a magnesium alkoxide addition complex.

Step 2

Hydrolysis to Alcohol

Subsequent hydrolysis with water in mildly acidic medium cleaves the oxygen-magnesium bond: $(\text{CH}_3)_3\text{C-OMgCl} + \text{H}_2\text{O} \to (\text{CH}_3)_3\text{C-OH} + \text{Mg(OH)Cl}$ The resulting product is 2-methylpropan-2-ol, whose common IUPAC/trivial name is tertiary butyl alcohol (tert-butyl alcohol).

Step 3

Classification of the Product Alcohol

The hydroxyl (-OH) group is attached to a tertiary carbon that is bonded to three other methyl carbon atoms. Thus, it is an unambiguous 3° alcohol.

Step 4

Eliminate Other Options

Isopropyl alcohol (2-propanol) is a 2° alcohol formed from acetaldehyde + methyl Grignard; sec-butyl alcohol (2-butanol) is formed from propanal + methyl Grignard or acetaldehyde + ethyl Grignard; isobutyl alcohol is a 1° alcohol.

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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 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; isopropyl alcohol ( $CH_3-CH(OH)-CH_3$ ) has 3 carbons and is formed from acetaldehyde ( $CH_3CHO$ ) + $CH_3MgX$ , not acetone.

⚠️

Student Fallacy (Why candidates pick this trap): Forgetting that acetone already contains 3 carbons, so adding a methyl group gives 4 carbons.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; sec-butyl alcohol ( $CH_3-CH(OH)-CH_2-CH_3$ ) is a 2° alcohol formed by reacting acetaldehyde with ethyl Grignard.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing 2° butyl alcohol with 3° butyl alcohol.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

Factually and chemically exact; nucleophilic addition of methyl group to acetone followed by protonation gives 2-methylpropan-2-ol (tert-butyl alcohol).

💡

Why this is airtight: None; classic organic synthesis rule.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; isobutyl alcohol ( $(CH_3)_2CH-CH_2OH$ ) is a primary alcohol, which cannot be formed from a ketone.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing branched primary alcohol with tertiary alcohol.

⚠️

NTA Trap Alert: Carbon Count & Degree of Alcohol with Grignard

Ketone + Grignard = ALWAYS 3° (Tertiary) alcohol! Acetone has 3 carbons + Grignard has 1 carbon = 4 carbons (Tert-butyl alcohol).

🛡️ How to avoid the -1 mark: Ketone gives TERTIARY alcohol! Acetone gives Tert-butyl alcohol!

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Core NEET Syllabus Standard

Chemistry · Alcohols, Phenols and Ethers Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Grignard Reaction with Ketones: Acetone Yields Tertiary Butyl Alcohol

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Chemistry Question 100 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Formaldehyde -> 1° Alcohol; Other Aldehydes -> 2° Alcohol; Ketones -> 3° Alcohol!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Nucleophilic Addition of Grignard Reagent

Hydrolysis to Alcohol

Classification of the Product Alcohol

Eliminate Other Options

360° Option Elimination Engine Key Feature

NTA Trap Alert: Carbon Count & Degree of Alcohol with Grignard

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Alcohols, Phenols and Ethers

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