Q.107 Chemistry ⭐ 360° Option Elimination Active

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NEET 2020 Chemistry Question 107 Solution

Topic: Aldehydes, Ketones and Carboxylic Acids · Official NTA Booklet Code E1

Question

Reaction between benzaldehyde and acetophenone in presence of dilute NaOH is known as:

(1)Aldol condensation
(2)Cannizzaro's reaction
(3)Cross Cannizzaro's reaction
(4)Cross Aldol condensation

✓

Verified Against NTA Final Answer Key

Option (4) : Cross Aldol condensation

📚 Aldehydes, Ketones and Carboxylic Acids Easy

📋 Given Parameters & Problem Setup

Reactant 1 Benzaldehyde (C6H5-CHO, aldehyde with no α-H)

Reactant 2 Acetophenone (C6H5-CO-CH3, ketone with 3 α-H)

Catalyst Dilute NaOH (basic medium)

Question Objective Identify the name of this organic reaction

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The 30-Second Exam Hall Shortcut: ⚡ "Two Different Carbonyl Compounds + Dilute Base = CROSS ALDOL CONDENSATION!"

⏱️ 20 seconds

Benzaldehyde has no α-hydrogen atoms, while acetophenone has α-hydrogen atoms on its methyl group. In the presence of dilute base (NaOH), acetophenone forms an enolate ion that attacks the carbonyl carbon of benzaldehyde. Condensation between two DIFFERENT carbonyl compounds is by definition CROSS ALDOL CONDENSATION (Claisen-Schmidt reaction)! Mark Option (4) in 5 seconds.

1 Reactants: Benzaldehyde (aldehyde) + Acetophenone (ketone).

2 Base: Dilute NaOH.

3 Benzaldehyde has no α-H; Acetophenone has 3 α-H.

4 Reaction between two different carbonyls in dilute base = Cross Aldol Condensation.

5 Product formed upon heating: 1,3-diphenylprop-2-en-1-one (benzalacetophenone / chalcone).

6 Select Option (4) Cross Aldol condensation.

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · Aldehydes, Ketones and Carboxylic Acids (Chemical Reactions: Cross Aldol Condensation)

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Identify Reaction Partners and α-Hydrogens

Benzaldehyde ( $\text{C}_6\text{H}_5\text{CHO}$ ) is an aromatic aldehyde with zero $\alpha$ -hydrogens on the formyl carbon. Acetophenone ( $\text{C}_6\text{H}_5\text{COCH}_3$ ) is an aromatic methyl ketone with three active $\alpha$ -hydrogens on the methyl group.

Step 2

Enolate Formation and Nucleophilic Addition

Dilute $\text{NaOH}$ abstracts an $\alpha$ -proton from the methyl group of acetophenone to generate a resonance-stabilized carbanion/enolate: $\text{C}_6\text{H}_5-\text{CO}-\text{CH}_2^- + \text{C}_6\text{H}_5-\text{CHO} \to \text{C}_6\text{H}_5-\text{CH(O}^-) - \text{CH}_2-\text{CO}-\text{C}_6\text{H}_5$ Proton transfer yields a $\beta$ -hydroxy ketone: $\text{C}_6\text{H}_5-\text{CH(OH)}-\text{CH}_2-\text{CO}-\text{C}_6\text{H}_5$

Step 3

Dehydration to Chalcone

The $\beta$ -hydroxy ketone readily undergoes dehydration (loss of $\text{H}_2\text{O}$ ) to form an $\alpha,\beta$ -unsaturated ketone with an extended conjugated system, 1,3-diphenylprop-2-en-1-one (benzalacetophenone / chalcone): $\text{C}_6\text{H}_5-\text{CH}=\text{CH}-\text{CO}-\text{C}_6\text{H}_5$

Step 4

Classify Reaction Name

Because aldol condensation occurs between two DIFFERENT carbonyl compounds (an aldehyde and a ketone), it is specifically termed Cross Aldol Condensation (or the Claisen-Schmidt reaction).

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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 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; simple aldol condensation refers to self-condensation between two identical molecules of an aldehyde or ketone.

⚠️

Student Fallacy (Why candidates pick this trap): Omitting the "Cross" designation when two different carbonyls react.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; Cannizzaro's reaction is a disproportionation of aldehydes lacking α-H in CONCENTRATED alkali, producing alcohol and carboxylate salt.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking benzaldehyde lacks α-H so it must be Cannizzaro.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; Cross Cannizzaro occurs between two different aldehydes BOTH lacking α-H (e.g. benzaldehyde and formaldehyde). Here, acetophenone has α-H.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming presence of benzaldehyde forces a Cannizzaro pathway.

4 D

✓ 100% Correct

Verdict & Scientific Reason:

Factually and mechanistically exact; base-catalyzed reaction between an aldehyde and a ketone with α-H is Cross Aldol condensation.

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Why this is airtight: None; explicit textbook reaction example.

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NTA Trap Alert: Aldol vs Cannizzaro Decision Rule

If AT LEAST ONE reactant has an α-hydrogen (acetophenone has 3 α-H), ALDOL CONDENSATION occurs! Cannizzaro reaction only occurs when NEITHER reactant possesses α-hydrogens in concentrated alkali.

🛡️ How to avoid the -1 mark: Look at acetophenone: Has α-hydrogens -> CANNOT be Cannizzaro! Must be CROSS ALDOL!

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Core NEET Syllabus Standard

Chemistry · Aldehydes, Ketones and Carboxylic Acids Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Claisen-Schmidt Reaction: Cross Aldol Condensation between Benzaldehyde and Acetophenone

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Chemistry Question 107 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Two Different Carbonyl Compounds + Dilute Base = CROSS ALDOL CONDENSATION!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Identify Reaction Partners and α-Hydrogens

Enolate Formation and Nucleophilic Addition

Dehydration to Chalcone

Classify Reaction Name

360° Option Elimination Engine Key Feature

NTA Trap Alert: Aldol vs Cannizzaro Decision Rule

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Aldehydes, Ketones and Carboxylic Acids

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Regiochemical Selectivity in Aldol Dehydration Products

Étard Reaction & Rosenmund Reduction of Benzaldehyde

Decreasing Order of Acidity of Aliphatic Carboxylic Acids (+I Inductive Effect)

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