Q.92 Chemistry ⭐ 360° Option Elimination Active

NEET 2023 Chemistry Question 92 Solution

Topic: Aldehydes, Ketones and Carboxylic Acids · Official NTA Booklet Code G5

Question

Identify the major product obtained in the following reaction: 2-acetylbenzaldehyde + 2[Ag(NH₃)₂]⁺ + 3 ⁻OH → (Δ) major product

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Verified Against NTA Final Answer Key

Option (4) : Ortho-acetylbenzoate (ketone unchanged, aldehyde oxidized to -COO⁻)

📚 Aldehydes, Ketones and Carboxylic Acids Medium

📋 Given Parameters & Problem Setup

Substrate Ortho-formylacetophenone (benzene ring with ortho -COCH₃ and -CHO)

Reagent 2 [Ag(NH₃)₂]⁺ + 3 OH⁻ (Tollens’ reagent) with heat

Reaction type Mild chemoselective oxidation

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The 30-Second Exam Hall Shortcut: ⚡ Chemoselective Reagent Filter: Tollens’ Oxidizes –CHO to –COO⁻, Leaves Ketone (–COCH₃) Untouched!

⏱️ Save ~35 seconds

Tollens’ reagent [Ag(NH₃)₂]⁺ is a mild, selective oxidizing agent that oxidizes aldehydes (–CHO ⟶ –COO⁻) but cannot oxidize ketones! Therefore, the –CHO group becomes –COO⁻, and the –COCH₃ group remains 100% intact. Option (4) is immediately selected in 5 seconds!

1 Identify functional groups on benzene ring: one ketone (–COCH₃) and one aldehyde (–CHO).

2 Identify reagent: [Ag(NH₃)₂]⁺ in alkaline medium is Tollens’ reagent.

3 Recall selectivity: Tollens’ reagent selectively oxidizes –CHO to carboxylate –COO⁻.

4 Ketones (aliphatic or aromatic) do NOT react with Tollens’ reagent.

5 Expected product retains –COCH₃ and has –COO⁻ (Option 4).

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Why You Can 100% Trust This Shortcut: Classical carbonyl differentiation test and mild oxidation selectivity in NCERT Organic Chemistry.

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Identify Functional Groups of the Reactant

The reactant is 2-acetylbenzaldehyde, containing two different carbonyl functional groups on adjacent positions of a benzene ring: an aldehyde (–CHO) and a methyl ketone (–COCH₃):

\text{Reactant: } o\text{-}\text{C}_6\text{H}_4(\text{COCH}_3)(\text{CHO})

Step 2

Evaluate Reagent and Reaction Conditions

Ammoniacal silver nitrate solution [Ag(NH₃)₂]⁺ in the presence of base (OH⁻) is Tollens’ reagent, a classic mild oxidizing agent:

2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \quad (\text{Tollens' Reagent})

Step 3

Chemoselective Oxidation Mechanism

Tollens’ reagent readily oxidizes the formyl group (–CHO) to the corresponding carboxylate anion (–COO⁻) while metallic silver is precipitated as a silver mirror:

\text{R–CHO} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \xrightarrow{\Delta} \text{R–COO}^- + 2\text{Ag}\downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O}

Step 4

Inertness of the Ketone Carbonyl Group

Under these mild basic oxidizing conditions, ketones do not have an oxidizable hydrogen atom on the carbonyl carbon and remain completely unreacted. The –COCH₃ group persists unchanged in the product:

\text{Product: } o\text{-}\text{C}_6\text{H}_4(\text{COCH}_3)(\text{COO}^-) \quad (\text{Matching Option 4})

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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:

The ketone is not reduced under Tollens’ oxidation conditions.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate incorrectly assumed Tollens’ reagent reduces ketones to alcohols.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Neither group undergoes reduction; Tollens’ is an oxidizing agent, not a reducing hydride.

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Student Fallacy (Why candidates pick this trap): Candidate confused Tollens’ reagent with LiAlH₄ reduction.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Aldehyde is oxidized to carboxylate, not reduced to primary alcohol.

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Student Fallacy (Why candidates pick this trap): Candidate treated the aldehyde as being reduced to alcohol.

4 (4)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate: Chemoselective oxidation of aldehyde to carboxylate while ketone remains completely intact.

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Why this is airtight: None. Candidate correctly recognized the chemoselectivity of Tollens’ reagent.

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NTA Trap Alert: Confusing Redox Reagents (Oxidation vs Reduction)

Some students see [Ag(NH₃)₂]⁺ and confuse it with complex metal hydrides (NaBH₄ or LiAlH₄), mistakenly drawing reduced alcohol groups. Tollens’ is an OXIDIZING agent that converts –CHO to –COO⁻.

🛡️ How to avoid the -1 mark: Remember: Tollens’ reagent oxidizes aldehydes to carboxylate; it never reduces carbonyl groups!

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Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Aldehydes, Ketones and Carboxylic Acids Standard Benchmark

Core Scientific Principle Tested:

"Tollens’ reagent selectively oxidizes aldehydes to carboxylate anions without affecting ketonic carbonyls."

Syllabus Focus: Oxidation of Aldehydes and Ketones – Tollens’ Test

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Chemistry Question 92 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Chemoselective Reagent Filter: Tollens’ Oxidizes –CHO to –COO⁻, Leaves Ketone (–COCH₃) Untouched!

Step-by-Step Rigorous Workflow & Mathematical Proof

Identify Functional Groups of the Reactant

Evaluate Reagent and Reaction Conditions

Chemoselective Oxidation Mechanism

Inertness of the Ketone Carbonyl Group

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing Redox Reagents (Oxidation vs Reduction)

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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