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NEET 2025 Chemistry Question 67 Solution

Topic: Amines · Official NTA Booklet Code 45

Question

The correct order of decreasing basic strength of the given amines is:

(1)N-methylaniline > benzenamine > ethanamine > N-ethylethanamine
(2)N-ethylethanamine > ethanamine > benzenamine > N-methylaniline
(3)N-ethylethanamine > ethanamine > N-methylaniline > benzenamine
(4)benzenamine > ethanamine > N-methylaniline > N-ethylethanamine

✓

Verified Against NTA Final Answer Key

Option (3) : N-ethylethanamine > ethanamine > N-methylaniline > benzenamine

📚 Amines Medium

📋 Given Parameters & Problem Setup

Amine 1 N-ethylethanamine (Diethylamine, (CH₃CH₂)₂NH, 2° aliphatic amine)

Amine 2 Ethanamine (Ethylamine, CH₃CH₂NH₂, 1° aliphatic amine)

Amine 3 N-methylaniline (C₆H₅NH(CH₃), secondary aromatic-aliphatic amine)

Amine 4 Benzenamine (Aniline, C₆H₅NH₂, 1° aromatic amine)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Aliphatic Beats Aromatic Basicity Rule

⏱️ Save ~45 seconds

Rule 1: Aliphatic amines are 10⁶ times more basic than aromatic amines (resonance delocalizes nitrogen lone pair into benzene ring). Rule 2: In ethyl amines, 2° > 1° (Diethylamine > Ethylamine). Rule 3: N-methylaniline has +I methyl pushing electron density into N > Aniline! Order: N-ethylethanamine > ethanamine > N-methylaniline > benzenamine!

1 Classify amines: Aliphatic (N-ethylethanamine, ethanamine) vs Aromatic (N-methylaniline, benzenamine).

2 Aliphatic amines have localized nitrogen lone pairs (+I alkyl groups), making them far more basic than aromatic amines (where the lone pair is delocalized into the aromatic π-system).

3 Compare aliphatic pair: In aqueous solution, ethyl-substituted amines follow the canonical 231 order: 2° > 1° ⟹ N-ethylethanamine > ethanamine.

4 Compare aromatic pair: N-methylaniline contains an electron-donating methyl group (+I effect) attached to nitrogen, which increases electron density on N relative to unsubstituted benzenamine (aniline). Thus N-methylaniline > benzenamine.

5 Combined decreasing basicity: N-ethylethanamine > ethanamine > N-methylaniline > benzenamine.

6 Matches Option (3) in under 15 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Inductive, Steric, and Solvation Effects on Amine Basicity (NCERT Chemistry · Amines).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Contrast Aliphatic vs. Aromatic Amines

In aromatic amines (benzenamine and N-methylaniline), the lone pair of electrons on the nitrogen atom is conjugated with the aromatic π-electron system and delocalized over the ortho and para positions:

pK_b(\text{Aliphatic}) \approx 3.0 - 3.3 \ll pK_b(\text{Aromatic}) \approx 9.3 - 9.4

• Because lower pK_b corresponds to stronger basicity, aliphatic amines are over six orders of magnitude more basic than aromatic amines.
• Therefore, both aliphatic amines stand ahead of both aromatic amines in basic strength.

Step 2

Compare Aliphatic Amines (Inductive, Steric & Solvation Effects)

In aqueous medium, the basic strength of alkylamines is governed by a balance of inductive (+I), steric hindrance, and hydration of the conjugate cation:

\text{For ethyl group: } 2^\circ > 3^\circ > 1^\circ > \text{NH}_3 \implies (\text{C}_2\text{H}_5)_2\text{NH} > \text{C}_2\text{H}_5\text{NH}_2

• N-ethylethanamine (pK_b = 3.00) is a stronger base than ethanamine (pK_b = 3.29).

Step 3

Compare Aromatic Amines and Synthesize the Hierarchy

In N-methylaniline, the methyl group exerts an electron-releasing (+I) inductive effect, increasing electron density on the nitrogen atom and stabilizing the protonated anilinium ion compared to aniline:

pK_b(\text{N-methylaniline}) = 9.30 < pK_b(\text{benzenamine}) = 9.38 \implies \text{N-methylaniline} > \text{benzenamine}

• Full decreasing order of basic strength:
• \text{N-ethylethanamine} > \text{ethanamine} > \text{N-methylaniline} > \text{benzenamine}.
• Matches Option (3) perfectly.

⭐

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. Falsely claims aromatic amines are more basic than aliphatic amines.

⚠️

Student Fallacy (Why candidates pick this trap): Ignoring resonance delocalization of aromatic nitrogen lone pairs.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Inverts the relative basicity of benzenamine and N-methylaniline.

⚠️

Student Fallacy (Why candidates pick this trap): Ignoring the electron-donating +I effect of the methyl group in N-methylaniline.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. 2° aliphatic > 1° aliphatic > substituted aromatic (+I) > unsubstituted aromatic.

💡

Why this is airtight: None; textbook mastery of organic basicity.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Scrambled order with aniline ahead of aliphatic amines.

⚠️

Student Fallacy (Why candidates pick this trap): Total misunderstanding of amine basicity.

⚠️

NTA Trap Alert: Aniline vs N-Methylaniline Basicity Trap

Option 2 is identical to Option 3 except it places benzenamine ahead of N-methylaniline. Remember that the methyl group in N-methylaniline supplies electron density (+I effect), making N-methylaniline slightly MORE basic than aniline (pK_b 9.30 vs 9.38)!

🛡️ How to avoid the -1 mark: Electron donating groups on nitrogen increase basicity: N-methylaniline > Aniline!

📖

Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Amines Standard Benchmark

Core Scientific Principle Tested:

"Aliphatic amines are stronger bases than aromatic amines; ethyl amines follow 2° > 1°; +I groups increase aromatic amine basicity."

Syllabus Focus: Structure-Basicity Relationships of Amines

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Aliphatic beats Aromatic: 2° > 1° > N-methyl > Aniline (Option 3)!

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NEET 2025 Chemistry Question 67 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Aliphatic Beats Aromatic Basicity Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Contrast Aliphatic vs. Aromatic Amines

Compare Aliphatic Amines (Inductive, Steric & Solvation Effects)

Compare Aromatic Amines and Synthesize the Hierarchy

360° Option Elimination Engine Key Feature

NTA Trap Alert: Aniline vs N-Methylaniline Basicity Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Amines

Preparation Methods of Primary Aromatic Amines (Aniline)

Limitations of Friedel-Crafts and Gabriel Phthalimide Syntheses with Aniline

Multistep Synthesis: Cyanation, Partial Hydrolysis, and Hoffmann Degradation

Sandmeyer Reaction Followed by Grignard Hydrolysis to Benzene

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