Q.87 Chemistry ⭐ 360° Option Elimination Active

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NEET 2021 Chemistry Question 87 Solution

Topic: Amines · Official NTA Booklet Code M1

Question

The reagent 'R' in the given sequence of chemical reaction is:

(1)H2O
(2)CH3CH2OH
(3)HI
(4)CuCN/KCN

✓

Verified Against NTA Final Answer Key

Option (2) : CH3CH2OH

📚 Amines Easy

📋 Given Parameters & Problem Setup

Substrate 2,4,6-Tribromobenzenediazonium chloride

Reagent R Mild reducing agent for replacement of -N2+Cl- by -H

Product 1,3,5-Tribromobenzene (symmetrical tribromobenzene)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Deamination of -N2⁺Cl⁻ to -H: Use H3PO2 or CH3CH2OH!

⏱️ Save ~20 seconds

Replacement of the diazonium group ( $-N_2^+Cl^-$ ) by hydrogen ( $-H$ ) requires a mild reducing agent. Verbatim NCERT Amines: "Certain mild reducing agents like hypophosphorous acid ( $H_3PO_2$ ) or ethanol ( $CH_3CH_2OH$ ) reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively." Among the choices, the reducing agent is ethanol ( $CH_3CH_2OH$ )! Match Option (2)!

1 Inspect conversion: The diazonium group (

-N_2^+Cl^-

) at position 1 is replaced by hydrogen (

-H

2 This reductive deamination reaction is carried out using mild reducing agents:

H_3PO_2 / H_2O

CH_3CH_2OH

3 Ethanol acts as hydride donor, reducing the diazonium arene to the parent hydrocarbon while being oxidized to acetaldehyde (

CH_3CHO

4 Reaction:

Ar-N_2^+Cl^- + CH_3CH_2OH \longrightarrow Ar-H + N_2\uparrow + HCl + CH_3CHO

5 Match Option (2) CH3CH2OH.

6 Other reagents:

H_2O

with heat gives phenol;

HI

gives aryl iodide;

CuCN/KCN

gives benzonitrile (Sandmeyer).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Amines section on Reactions Involving Displacement of Nitrogen (Reduction).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Identifying the Target Chemical Transformation

The reaction sequence shows: 1. 2,4,6-tribromoaniline undergoes diazotization with nitrous acid ( $\text{NaNO}_2 + \text{HCl}$ at $0-5^\circ\text{C}$ ) to form 2,4,6-tribromobenzenediazonium chloride. 2. The diazonium chloride reacts with reagent 'R' to lose the diazonium function, yielding 1,3,5-tribromobenzene. This is a classic reductive deamination where the $-N_2^+Cl^-$ group is replaced by a hydrogen atom ( $-H$ ).

Step 2

Evaluating Reducing Agents for Diazonium Salts in NCERT

According to NCERT Chapter on Amines: *"Certain mild reducing agents like hypophosphorous acid ( $H_3PO_2$ ) or ethanol ( $CH_3CH_2OH$ ) reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively."* The balanced chemical equation for the reduction by ethanol is:

Ar-N_2^+Cl^- + CH_3CH_2OH \longrightarrow Ar-H + N_2\uparrow + HCl + CH_3CHO

Step 3

Eliminating Alternative Reagents

- $H_2O$ (Option 1): Warming diazonium salts with water leads to hydrolysis, replacing $-N_2^+Cl^-$ with a phenolic $-OH$ group (forming 2,4,6-tribromophenol). - $HI$ (Option 3): Replaces $-N_2^+Cl^-$ with an iodine atom (forming 1-iodo-2,4,6-tribromobenzene). - $CuCN/KCN$ (Option 4): Sandmeyer reaction replacing $-N_2^+Cl^-$ with a cyano group ( $-CN$ ). Therefore, reagent 'R' is uniquely ethanol ( $CH_3CH_2OH$ ).

⭐

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:

Hydrolyzes the diazonium group into a phenol ( $-OH$ ), not a hydrocarbon ( $-H$ ).

⚠️

Student Fallacy (Why candidates pick this trap): Assuming water simply protonates the ring.

2 2

✓ 100% Correct

Verdict & Scientific Reason:

Exact reducing agent specified in NCERT that reduces diazonium salts to arenes, forming ethanal byproduct.

💡

Why this is airtight: None; textbook diazonium reduction.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Converts diazonium salt to iodobenzene derivative.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing reduction to arene with iodination.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Sandmeyer cyanation reaction yielding a nitrile.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing reduction with cyanation.

⚠️

NTA Trap Alert: Water Hydrolysis vs. Ethanol Reduction Trap

Students frequently remember that diazonium salts react with water, and assume water provides the $H$ to make the benzene ring. Water introduces an $-OH$ group to make phenol! Only ETHANOL ( $CH_3CH_2OH$ ) or $H_3PO_2$ reduces the diazonium salt to the hydrocarbon.

🛡️ How to avoid the -1 mark:

-N_2^+Cl^- \xrightarrow{H_2O, \Delta} -OH

(Phenol).

-N_2^+Cl^- \xrightarrow{CH_3CH_2OH} -H

(Arene).

📖

Core NEET Syllabus Standard

NCERT Chemistry · Amines Standard Benchmark

Core Scientific Principle Tested:

"Ethanol reduces diazonium salts to arenes while being oxidized to ethanal."

Syllabus Focus: Chemical Reactions of Diazonium Salts (Reduction / Replacement by H)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Chemistry Question 87 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Deamination of -N2⁺Cl⁻ to -H: Use H3PO2 or CH3CH2OH!

Step-by-Step Rigorous Workflow & Mathematical Proof

Identifying the Target Chemical Transformation

Evaluating Reducing Agents for Diazonium Salts in NCERT

Eliminating Alternative Reagents

360° Option Elimination Engine Key Feature

NTA Trap Alert: Water Hydrolysis vs. Ethanol Reduction Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Amines

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Multistep Synthesis: Cyanation, Partial Hydrolysis, and Hoffmann Degradation

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