Q.87 Chemistry ⭐ 360° Option Elimination Active

Vault ↗

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 ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 200-minute timed NTA simulator.

Launch NEET 2021 CBT Test →

← Previous (Q.86) All Chemistry Questions Grid Next (Q.88) →

Spotted a mistake in this solution? Report an error ([email protected])

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2021 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

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

Preparation Methods of Primary Aromatic Amines (Aniline)

Decreasing Order of Basic Strength of Substituted Amines in Aqueous Medium

Limitations of Friedel-Crafts and Gabriel Phthalimide Syntheses with Aniline

Multistep Synthesis: Cyanation, Partial Hydrolysis, and Hoffmann Degradation

Solve 4 NEET Questions a Day — One per Subject