Q.92 Chemistry ⭐ 360° Option Elimination Active

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NEET 2021 Chemistry Question 92 Solution

Topic: Aldehydes, Ketones and Carboxylic Acids · Official NTA Booklet Code M1

Question

The intermediate compound 'X' in the following chemical reaction is:

✓

Verified Against NTA Final Answer Key

Option (1) : Ph-CH(OCrOHCl2)2 (Structure 1)

📚 Aldehydes, Ketones and Carboxylic Acids Easy

📋 Given Parameters & Problem Setup

Substrate Toluene (C6H5CH3)

Reagent Chromyl chloride (CrO2Cl2) in CS2

Final Product Benzaldehyde (C6H5CHO) upon aqueous hydrolysis (H3O+)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Étard Reaction Intermediate: Brown Chromium Complex Ph-CH(OCrOHCl2)2

⏱️ Save ~20 seconds

Verbatim NCERT Aldehydes: Chromyl chloride ( $CrO_2Cl_2$ ) oxidizes the methyl group of toluene in $CS_2$ solvent to form a brown chromium complex intermediate $C_6H_5-CH(OCrOHCl_2)_2$ , which upon acidic hydrolysis yields benzaldehyde. This uniquely matches Structure (1)! Match Option (1)!

1 Recognize the classic Étard reaction: Toluene

\xrightarrow{CrO_2Cl_2 / CS_2} X \xrightarrow{H_3O^+} C_6H_5CHO

2 Chromyl chloride (

CrO_2Cl_2

) reacts with the methyl group to insert into two

C-H

bonds.

3 The structure of the intermediate is the brown chromium complex:

C_6H_5-CH(OCrOHCl_2)_2

4 Match Structure (1).

5 Structure (2) is benzylidene diacetate formed with

CrO_3 / (CH_3CO)_2O

; Structure (3) is benzal chloride.

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Aldehydes, Ketones and Carboxylic Acids named reaction (Étard Reaction).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Identification of the Named Reaction

The oxidation of toluene to benzaldehyde using chromyl chloride ( $CrO_2Cl_2$ ) dissolved in carbon disulfide ( $CS_2$ ) or carbon tetrachloride ( $CCl_4$ ), followed by aqueous hydrolysis, is the Étard Reaction.

Step 2

Mechanism of Formation of Intermediate X

Chromyl chloride is a mild oxidizing agent that selectively oxidizes the benzylic methyl group without over-oxidizing the ring to benzoic acid. Two moles of $CrO_2Cl_2$ react with one mole of toluene to form a distinctive brown precipitated chromium complex:

C_6H_5CH_3 + 2\,\text{CrO}_2\text{Cl}_2 \xrightarrow{\text{CS}_2} C_6H_5-\text{CH}(\text{OCrOHCl}_2)_2 \quad (\mathbf{\text{Intermediate X}})

Step 3

Aqueous Acid Hydrolysis to Benzaldehyde

Hydrolysis of this complex in acidic water ( $H_3O^+$ ) cleaves the $Cr-O$ and $C-O$ bonds, releasing chromium(III) inorganic salts and furnishing benzaldehyde:

C_6H_5-\text{CH}(\text{OCrOHCl}_2)_2 \xrightarrow{\text{H}_3\text{O}^+} C_6H_5\text{CHO} + 2\,\text{Cr(OH)}_2\text{Cl}_2

⭐

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

✓ 100% Correct

Verdict & Scientific Reason:

Verbatim NCERT formula and structure for the Étard reaction chromium complex.

💡

Why this is airtight: None; textbook organic named reaction intermediate recall.

2 2

✗ Incorrect

Verdict & Scientific Reason:

This gem-diacetate intermediate is formed when toluene is oxidized by chromium trioxide in acetic anhydride ( $CrO_3 / Ac_2O$ ), not chromyl chloride.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing Étard reaction with

CrO_3 / Ac_2O

oxidation.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Benzal chloride, formed by side-chain free-radical chlorination ( $Cl_2 / h\nu$ ), not Étard oxidation.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing Étard reaction with photochemical chlorination.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Benzyl chloride, not formed in the Étard reaction.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming single chlorination of benzylic carbon.

⚠️

NTA Trap Alert: Étard Complex vs. Gem-Diacetate Intermediate Trap

NCERT presents two methods for commercial preparation of benzaldehyde from toluene: 1. $CrO_2Cl_2$ in $CS_2 \implies C_6H_5CH(OCrOHCl_2)_2$ (Étard reaction, Option 1). 2. $CrO_3$ in $(CH_3CO)_2O \implies C_6H_5CH(OCOCH_3)_2$ (Benzylidene diacetate, Option 2). Candidates who mix up the reagents select Option 2.

🛡️ How to avoid the -1 mark:

CrO_2Cl_2

contains chromium and chlorine

\implies

intermediate MUST contain

Cr

and

Cl

C_6H_5CH(OCrOHCl_2)_2

📖

Core NEET Syllabus Standard

NCERT Chemistry · Aldehydes, Ketones and Carboxylic Acids Standard Benchmark

Core Scientific Principle Tested:

"Chromyl chloride oxidizes the methyl group to a brown chromium complex C6H5CH(OCrOHCl2)2, which on hydrolysis yields benzaldehyde."

Syllabus Focus: Preparation of Aldehydes (From Hydrocarbons - Étard Reaction)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Chemistry Question 92 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Étard Reaction Intermediate: Brown Chromium Complex Ph-CH(OCrOHCl2)2

Step-by-Step Rigorous Workflow & Mathematical Proof

Identification of the Named Reaction

Mechanism of Formation of Intermediate X

Aqueous Acid Hydrolysis to Benzaldehyde

360° Option Elimination Engine Key Feature

NTA Trap Alert: Étard Complex vs. Gem-Diacetate Intermediate Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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