Q.70 Chemistry ⭐ 360° Option Elimination Active

Vault ↗

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).

📐

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)!

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 180-minute timed NTA simulator.

Launch NEET 2025 CBT Test →

← Previous (Q.69) All Chemistry Questions Grid Next (Q.71) →

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

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2025 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 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?

🔗 Related Questions — Aldehydes, Ketones and Carboxylic Acids

Friedel-Crafts Acylation Followed by Haloform Cleavage

Regiochemical Selectivity in Aldol Dehydration Products

Étard Reaction & Rosenmund Reduction of Benzaldehyde

Reagents for Carbonyl Synthesis and Side-Chain Oxidation

Solve 4 NEET Questions a Day — One per Subject