Q.71 Chemistry ⭐ 360° Option Elimination Active

NEET 2023 Chemistry Question 71 Solution

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

Question

Identify product (A) in the following reaction: 1-[3-(3-acetylcyclohexyl)phenyl]ethan-1-one (a diketone) → (Zn−Hg, conc. HCl) (A) + 2H₂O

✓

Verified Against NTA Final Answer Key

Option (2) : Di-ethyl substituted product (Both -COCH₃ reduced to -CH₂CH₃)

📚 Aldehydes, Ketones and Carboxylic Acids Easy

📋 Given Parameters & Problem Setup

Substrate Compound containing two acetyl groups (–COCH₃): one on cyclohexyl ring, one on benzene ring

Reagent Zn–Hg / conc. HCl (Clemmensen reduction)

Byproduct stated 2 H₂O

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The 30-Second Exam Hall Shortcut: ⚡ Clemmensen Carbonyl-to-Methylene Rule: >C=O ⟶ >CH₂

⏱️ Save ~35 seconds

Clemmensen reduction (Zn-Hg / conc. HCl) converts every ketone group >C=O into a methylene group >CH₂! Thus both –COCH₃ (acetyl) groups become –CH₂CH₃ (ethyl) groups in 10 seconds! (Option 2)

1 Identify the reducing reagent: Zn-Hg in concentrated HCl is the classic CLEMMENSEN REDUCTION.

2 Recall what Clemmensen reduction does: It selectively deoxygenates aldehydes and ketones into hydrocarbons: >C=O ⟶ >CH₂.

3 Inspect the functional groups in the substrate:

4 - Acetyl group on cyclohexyl ring: –COCH₃ ⟶ –CH₂CH₃ (ethyl group).

5 - Acetyl group on benzene ring: –COCH₃ ⟶ –CH₂CH₃ (ethyl group).

6 Carbon count per group: Each –COCH₃ has 2 carbons and becomes an ethyl group –CH₂CH₃ (2 carbons).

7 Option (1) has methyl groups (lost a carbon, wrong). Option (3) has alcohol groups (wrong reagent).

8 Option (2) contains both ethyl groups and is the exact correct product!

9 Lock Option (2) immediately!

🛡️

Why You Can 100% Trust This Shortcut: Organic Functional Group Transformations · Clemmensen Reduction (Zinc amalgam and hydrochloric acid reduce the carbonyl group of aldehydes and ketones to a CH₂ group with loss of H₂O).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Identification of Reaction Type and Mechanism

The reaction utilizes zinc amalgam in concentrated hydrochloric acid (Zn–Hg / conc. HCl), which is the standard protocol for the Clemmensen reduction. This heterogeneous single-electron transfer reduction converts carbonyl groups (>C=O) of aldehydes and ketones directly into methylene groups (>CH₂):

\text{R}-\overset{\text{O}}{\text{C}}-\text{R}' + 4[\text{H}] \xrightarrow{\text{Zn-Hg / conc. HCl}} \text{R}-\text{CH}_2-\text{R}' + \text{H}_2\text{O}

Step 2

Transformation of the Alicyclic Carbonyl Group

The acetyl group attached to the cyclohexyl ring is converted into an ethyl group:

-\text{C}(=\text{O})\text{CH}_3 \xrightarrow{\text{Zn-Hg / conc. HCl}} -\text{CH}_2\text{CH}_3 \quad (\text{Ethyl substituent})

Step 3

Transformation of the Aromatic Carbonyl Group

Similarly, the acetyl group attached to the meta position of the benzene ring is reduced to an ethyl group:

-\text{C}(=\text{O})\text{CH}_3 \xrightarrow{\text{Zn-Hg / conc. HCl}} -\text{CH}_2\text{CH}_3 \quad (\text{Ethyl substituent})

Step 4

Overall Stoichiometry and Final Product Formula

Both ketone groups are fully reduced, producing 2 moles of H₂O as explicitly stated in the balanced equation. The final product features ethyl groups at both positions on the unchanged ring skeletons, corresponding identically to Option (2).

\text{Product (A)} = \text{1-ethyl-3-(3-ethylcyclohexyl)benzene}

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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:

Loss of a carbon atom: –COCH₃ contains 2 carbons, so it cannot become a 1-carbon –CH₃ group.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate forgot that the carbonyl carbon itself is reduced to CH₂, retaining both carbons as –CH₂CH₃.

2 (2)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate product: Clemmensen reduction transforms both –COCH₃ groups into –CH₂CH₃ groups.

💡

Why this is airtight: None. Candidate tracked carbon counts and Clemmensen selectivity.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Represents partial reduction by NaBH₄ or LiAlH₄, not complete deoxygenation by Zn-Hg/HCl.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate confused Clemmensen reduction with hydride reduction to alcohols.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Incorrect structure with wrong carbon count and wrong oxidation state.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate drew an unphysical partial reduction product.

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NTA Trap Alert: Counting Carbons: Turning –COCH₃ into –CH₃ Instead of –CH₂CH₃

A common slip is thinking the ketone "disappears" leaving only the methyl group (Option 1). Remember that the carbonyl carbon does NOT leave; it gets hydrogenated into a –CH₂– group, forming –CH₂–CH₃ (ethyl).

🛡️ How to avoid the -1 mark: Count total carbons: –C(=O)CH₃ has TWO carbons. In the product, it must have TWO carbons: –CH₂CH₃!

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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:

"The carbonyl group of aldehydes and ketones is reduced to CH₂ group on treatment with zinc amalgam and concentrated hydrochloric acid."

Syllabus Focus: Reduction of Carbonyl Compounds – Clemmensen Reduction

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Chemistry Question 71 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Clemmensen Carbonyl-to-Methylene Rule: >C=O ⟶ >CH₂

Step-by-Step Rigorous Workflow & Mathematical Proof

Identification of Reaction Type and Mechanism

Transformation of the Alicyclic Carbonyl Group

Transformation of the Aromatic Carbonyl Group

Overall Stoichiometry and Final Product Formula

360° Option Elimination Engine Key Feature

NTA Trap Alert: Counting Carbons: Turning –COCH₃ into –CH₃ Instead of –CH₂CH₃

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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