Q.57 Chemistry ⭐ 360° Option Elimination Active

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NEET 2021 Chemistry Question 57 Solution

Topic: Amines · Official NTA Booklet Code M1

Question

Identify the compound that will react with Hinsberg's reagent to give a solid which dissolves in alkali.

✓

Verified Against NTA Final Answer Key

Option (3) : CH3 - CH2 - NH2

📚 Amines Easy

📋 Given Parameters & Problem Setup

Reagent Hinsberg's reagent (benzenesulfonyl chloride, C6H5SO2Cl)

Observation Forms a solid which dissolves in alkali (aqueous KOH/NaOH)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Hinsberg Signature: 1° Amine Dissolves in Alkali!

⏱️ Save ~20 seconds

Primary amines ( $1^\circ$ , $R-NH_2$ ) react with Hinsberg's reagent to form N-alkylbenzenesulfonamides that retain an acidic hydrogen on nitrogen, dissolving readily in aqueous alkali. Secondary amines lack this acidic proton and remain insoluble. Tertiary amines do not react. The only primary amine among the choices is ethylamine ( $CH_3-CH_2-NH_2$ )! Match Option (3)!

1^\circ

Amine (

R-NH_2

) +

C_6H_5SO_2Cl \to C_6H_5SO_2-NH-R

(solid with acidic H attached to N)

\xrightarrow{\text{KOH}}

soluble salt.

2^\circ

Amine (

R_2NH

) +

C_6H_5SO_2Cl \to C_6H_5SO_2-NR_2

(no acidic H attached to N)

\xrightarrow{\text{KOH}}

insoluble.

3^\circ

Amine (

R_3N

) +

C_6H_5SO_2Cl \to

No reaction.

4 Identify the

1^\circ

amine: Option (3) is

CH_3-CH_2-\ddot{N}H_2

(ethylamine).

5 Match Option (3).

🛡️

Why You Can 100% Trust This Shortcut: Canonical Hinsberg test chemistry from NCERT Amines.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Chemistry of Hinsberg's Reagent

Hinsberg's reagent is benzenesulfonyl chloride ( $C_6H_5SO_2Cl$ ). It is used to distinguish and separate primary ( $1^\circ$ ), secondary ( $2^\circ$ ), and tertiary ( $3^\circ$ ) amines based on the presence of replaceable hydrogen atoms on nitrogen.

Step 2

Reaction with a Primary Amine (CH3CH2NH2)

Ethylamine ( $CH_3CH_2NH_2$ , a primary amine) reacts with benzenesulfonyl chloride to yield N-ethylbenzenesulfonamide:

C_6H_5SO_2Cl + H_2N-CH_2CH_3 \longrightarrow C_6H_5SO_2-NH-CH_2CH_3 + HCl

Step 3

Acidity and Solubility in Aqueous Alkali

The hydrogen atom attached to the nitrogen atom in N-ethylbenzenesulfonamide is strongly acidic because of the strong electron-withdrawing inductive and resonance effects of the sulfonyl ( $-SO_2-$ ) group. In the presence of base (alkali, such as aqueous $KOH$ or $NaOH$ ), it readily deprotonates to form a water-soluble potassium/sodium sulfonamide salt:

C_6H_5SO_2-\overset{\delta-}{N}H-CH_2CH_3 + KOH \longrightarrow [C_6H_5SO_2-\bar{N}-CH_2CH_3] K^+ + H_2O \quad (\mathbf{\text{Soluble in alkali}})

⭐

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:

Nitroethane is a nitro compound, not an amine; does not participate in the Hinsberg amine test.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing nitroalkanes with primary amines.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Secondary amine ( $N$ -methylethanamine): forms N-ethyl-N-methylbenzenesulfonamide which has NO acidic hydrogen on nitrogen and remains INSOLUBLE in alkali.

⚠️

Student Fallacy (Why candidates pick this trap): Forgetting that secondary amine sulfonamides lack acidic protons.

3 3

✓ 100% Correct

Verdict & Scientific Reason:

Primary amine ( $1^circ$ ): forms N-ethylbenzenesulfonamide containing an acidic hydrogen that dissolves in aqueous alkali.

💡

Why this is airtight: None; textbook Hinsberg test reaction.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Tertiary amine ( $3^circ$ ): has no hydrogen on nitrogen and does not react with Hinsberg's reagent at all.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming tertiary amines react with benzenesulfonyl chloride.

⚠️

NTA Trap Alert: Secondary Amine Insoluble Precipitate Trap

Students frequently remember that secondary amines (Option 2) also react with Hinsberg's reagent, but forget that the resulting product has NO proton on nitrogen and is completely INSOLUBLE in alkali. Only primary amine sulfonamides dissolve in alkali.

🛡️ How to avoid the -1 mark: Soluble in alkali

\implies 1^\circ

amine. Insoluble in alkali

\implies 2^\circ

amine. No reaction

\implies 3^\circ

amine.

📖

Core NEET Syllabus Standard

NCERT Chemistry · Amines Standard Benchmark

Core Scientific Principle Tested:

"Primary amines react with benzenesulfonyl chloride to form N-alkylbenzenesulfonamides, which dissolve in alkali due to the presence of an acidic hydrogen on nitrogen."

Syllabus Focus: Reaction with Arylsulphonyl Chloride (Hinsberg's Test)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Chemistry Question 57 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Hinsberg Signature: 1° Amine Dissolves in Alkali!

Step-by-Step Rigorous Workflow & Mathematical Proof

Chemistry of Hinsberg's Reagent

Reaction with a Primary Amine (CH3CH2NH2)

Acidity and Solubility in Aqueous Alkali

360° Option Elimination Engine Key Feature

NTA Trap Alert: Secondary Amine Insoluble Precipitate Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Amines

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