Q.70 Chemistry ⭐ 360° Option Elimination Active

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NEET 2025 Chemistry Question 70 Solution

Topic: Aldehydes, Ketones and Carboxylic Acids · Official NTA Booklet Code 45

Question

The correct order of decreasing acidity of the following aliphatic acids is:

✓

Verified Against NTA Final Answer Key

Option (3) : HCOOH > CH₃COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH

📚 Aldehydes, Ketones and Carboxylic Acids Easy

📋 Given Parameters & Problem Setup

Acid 1 HCOOH (Formic acid / Methanoic acid)

Acid 2 CH₃COOH (Acetic acid / Ethanoic acid)

Acid 3 (CH₃)₂CHCOOH (Isobutyric acid / 2-Methylpropanoic acid)

Acid 4 (CH₃)₃CCOOH (Pivalic acid / 2,2-Dimethylpropanoic acid)

⚡

The 30-Second Exam Hall Shortcut: ⚡ +I Inductive Charge Destabilization Rule

⏱️ Save ~30 seconds

Acidity ∝ 1 / (+I effect)! Alkyl groups push electron density into -COO⁻, destabilizing the conjugate base and reducing acidity! HCOOH has zero alkyl groups (STRONGEST). CH₃ has 1 methyl. (CH₃)₂CH has 2 methyls. (CH₃)₃C has 3 methyls (WEAKEST)! ⟹ HCOOH > CH₃COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH!

1 Recall acidity determinant: Stronger acid produces a more stable conjugate base (carboxylate anion R-COO⁻).

2 Electron-donating groups (+I effect) intensify negative charge on the carboxylate oxygen, destabilizing the anion and decreasing acid strength.

3 Order of +I inductive effect: -C(CH₃)₃ (tertiary butyl) > -CH(CH₃)₂ (isopropyl) > -CH₃ (methyl) > -H.

4 Invert for acid strength: HCOOH > CH₃COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH.

5 Select Option (3) in under 10 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Electron-Withdrawing Substituent Effects on Carboxylic Acid Acidity (NCERT Chemistry · Aldehydes, Ketones and Carboxylic Acids).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Role of Inductive Effect in Carboxylate Anion Stability

Carboxylic acids dissociate in water to generate carboxylate anions and hydronium ions:

\text{R-COOH} + \text{H}_2\text{O} \rightleftharpoons \text{R-COO}^- + \text{H}_3\text{O}^+

• Any factor that disperses the negative charge stabilizes the carboxylate anion and increases acidity.
• Any factor that concentrates or intensifies the negative charge destabilizes the anion and decreases acidity.

Step 2

Compare +I Power of Alkyl Substituents

Alkyl groups are electron-releasing (+I) groups. As the size and branching of the alkyl group increases, its +I electron-donating power increases dramatically:

+I \text{ effect: } -\text{C(CH}_3)_3 > -\text{CH(CH}_3)_2 > -\text{CH}_3 > -\text{H}

• In (CH₃)₃CCOOH, three electron-releasing methyl groups pump electron density into the carboxylate group, creating maximum destabilization of the anion (weakest acid).
• In HCOOH, there is no alkyl group attached to the carboxyl carbon, so there is no +I destabilization (strongest acid).

Step 3

Synthesize the Complete Acidity Order

Invert the +I hierarchy to obtain the decreasing acidity series:

\text{Decreasing Acidity: } \text{HCOOH} > \text{CH}_3\text{COOH} > (\text{CH}_3)_2\text{CHCOOH} > (\text{CH}_3)_3\text{CCOOH}

• pK_a values: HCOOH (3.75) < CH₃COOH (4.76) < (CH₃)₂CHCOOH (4.86) < (CH₃)₃CCOOH (5.05).
• 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. Completely inverts the acidity order, treating +I groups as acid-strengthening.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing electron-donating groups with electron-withdrawing groups.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Falsely places formic acid (HCOOH) as the weakest acid.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking formic acid is weaker than acetic acid.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. HCOOH is strongest, followed by acetic, isobutyric, and pivalic acid.

💡

Why this is airtight: None; flawless mastery of inductive effects.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Scrambled middle order.

⚠️

Student Fallacy (Why candidates pick this trap): Inverting the branched alkyl sequence.

⚠️

NTA Trap Alert: Placing Acetic Acid Ahead of Formic Acid

Some students memorize that substituted acids can be stronger and mistakenly think CH₃COOH is stronger than HCOOH. In reality, HCOOH is significantly stronger than acetic acid (pK_a 3.75 vs 4.76) because H does not donate electrons by +I effect!

🛡️ How to avoid the -1 mark: Formic acid is ALWAYS the strongest among unsubstituted simple aliphatic acids!

📖

Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Aldehydes, Ketones and Carboxylic Acids Standard Benchmark

Core Scientific Principle Tested:

"+I electron donating groups decrease acidity: HCOOH > CH₃COOH > R₂CHCOOH > R₃CCOOH."

Syllabus Focus: Acidity of Carboxylic Acids & Effect of Substituents

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Less Alkyl = Stronger Acid: Formic > Acetic > Isobutyric > Pivalic (Option 3)!

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NEET 2025 Chemistry Question 70 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ +I Inductive Charge Destabilization Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Role of Inductive Effect in Carboxylate Anion Stability

Compare +I Power of Alkyl Substituents

Synthesize the Complete Acidity Order

360° Option Elimination Engine Key Feature

NTA Trap Alert: Placing Acetic Acid Ahead of Formic Acid

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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