Q.83 Chemistry ⭐ 360° Option Elimination Active

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NEET 2025 Chemistry Question 83 Solution

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

Question

Identify the suitable reagent for the following conversion. Methyl benzoate (C₆H₅COOCH₃) → Benzaldehyde (C₆H₅CHO)

✓

Verified Against NTA Final Answer Key

Option (2) : (i) AlH(iBu)₂ (ii) H₂O

📚 Aldehydes, Ketones and Carboxylic Acids Easy

📋 Given Parameters & Problem Setup

Substrate Methyl benzoate (Ph-COOCH₃, aromatic ester)

Target Product Benzaldehyde (Ph-CHO, aromatic aldehyde)

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The 30-Second Exam Hall Shortcut: ⚡ DIBAL-H Ester-to-Aldehyde Trademark

⏱️ Save ~30 seconds

Ester ⟶ Aldehyde requires DIBAL-H [AlH(iBu)₂]! LiAlH₄ over-reduces esters all the way to primary alcohols (Ph-CH₂OH). NaBH₄ does not reduce esters. H₂/Pd-BaSO₄ reduces acid chlorides, not esters! ⟹ (i) AlH(iBu)₂ (ii) H₂O!

1 Notice transformation: Ester (Ph-COOCH₃) is selectively reduced to an Aldehyde (Ph-CHO).

2 Recall canonical reagent: Diisobutylaluminium hydride, DIBAL-H or AlH(iBu)₂, selectively reduces esters to aldehydes at low temperature followed by hydrolysis.

3 Evaluate alternatives:

4 - LiAlH₄: Powerful reducing agent; completely reduces esters to primary alcohol (benzyl alcohol Ph-CH₂OH).

5 - NaBH₄: Mild reducing agent; unreactive towards esters.

6 - H₂ / Pd-BaSO₄: Rosenmund reduction, specifically reduces acyl chlorides (R-COCl) to aldehydes, not esters.

7 Only DIBAL-H [AlH(iBu)₂] accomplishes selective reduction of esters to aldehydes.

8 Select Option (2) in under 10 seconds!

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Why You Can 100% Trust This Shortcut: Selective DIBAL-H Reduction of Esters and Nitriles (NCERT Chemistry · Aldehydes, Ketones and Carboxylic Acids).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Mechanism of DIBAL-H Selective Reduction of Esters

Diisobutylaluminium hydride, [(CH₃)₂CH-CH₂]₂AlH or AlH(iBu)₂, delivers exactly one hydride ion to the carbonyl carbon of the ester at low temperature (-78 °C):

\text{Ph-COOCH}_3 + \text{AlH}(i\text{Bu})_2 \xrightarrow{-78^\circ\text{C}} \text{Ph}-\text{CH}(\text{OCH}_3)(\text{O}-\text{Al}(i\text{Bu})_2) \text{ (Stable hemiacetal intermediate)}

• The tetrahedral intermediate is stable at low temperatures, preventing further hydride attack.

Step 2

Aqueous Hydrolysis to Aldehyde

Upon addition of water (H₂O workup), the tetrahedral aluminium intermediate collapses, releasing the aldehyde and methanol:

\text{Ph}-\text{CH}(\text{OCH}_3)(\text{OAlR}_2) \xrightarrow{\text{H}_2\text{O}} \text{Ph-CHO} + \text{CH}_3\text{OH} + \text{Al(OH)R}_2

• The reaction stops cleanly at benzaldehyde without over-reduction.

Step 3

Contrast with Other Reagents

Verify why the other reagents fail for this specific transformation:

\text{LiAlH}_4: \; \text{Ph-COOCH}_3 \longrightarrow \text{Ph-CH}_2\text{OH} \quad (\text{Over-reduction to alcohol})

• NaBH₄ does not react with esters at a measurable rate.
• H₂/Pd-BaSO₄ requires an acid chloride (Ph-COCl), not an ester.
• Therefore, (i) AlH(iBu)₂ (ii) H₂O is the unique correct reagent, matching Option (2).

⭐

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. LiAlH₄ reduces esters completely to primary alcohols (benzyl alcohol Ph-CH₂OH).

⚠️

Student Fallacy (Why candidates pick this trap): Thinking LiAlH₄ stops at the aldehyde stage.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. DIBAL-H specifically reduces esters to aldehydes.

💡

Why this is airtight: None; textbook organic synthesis reaction.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. NaBH₄ cannot reduce esters.

⚠️

Student Fallacy (Why candidates pick this trap): Believing NaBH₄ reduces esters.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Rosenmund catalyst reduces acyl chlorides, not esters.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing ester substrate with acid chloride.

⚠️

NTA Trap Alert: Choosing LiAlH₄ for Aldehyde Preparation

LiAlH₄ is too reactive to prepare aldehydes from carboxylic acids or esters—it inevitably reduces the carbonyl all the way down to a primary alcohol. DIBAL-H is specifically designed to stop at the aldehyde!

🛡️ How to avoid the -1 mark: Ester to Aldehyde = DIBAL-H [AlH(iBu)₂]!

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Core NEET Syllabus Standard

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

Core Scientific Principle Tested:

"DIBAL-H selectively reduces esters to aldehydes upon hydrolysis."

Syllabus Focus: Preparation of Aldehydes by Reduction of Esters

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

DIBAL-H = Die (Stop) at ALdehyde: Ester ⟶ Aldehyde (Option 2)!

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NEET 2025 Chemistry Question 83 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ DIBAL-H Ester-to-Aldehyde Trademark

Step-by-Step Rigorous Workflow & Mathematical Proof

Mechanism of DIBAL-H Selective Reduction of Esters

Aqueous Hydrolysis to Aldehyde

Contrast with Other Reagents

360° Option Elimination Engine Key Feature

NTA Trap Alert: Choosing LiAlH₄ for Aldehyde Preparation

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Aldehydes, Ketones and Carboxylic Acids

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Étard Reaction & Rosenmund Reduction of Benzaldehyde

Reagents for Carbonyl Synthesis and Side-Chain Oxidation

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