Q.63 Chemistry ⭐ 360° Option Elimination Active

NEET 2023 Chemistry Question 63 Solution

Topic: Amines · Official NTA Booklet Code G5

Question

Identify the product in the following reaction: Benzenediazonium chloride (C₆H₅N₂⁺Cl⁻) → (i) Cu₂Br₂/HBr (ii) Mg/dry ether (iii) H₂O → Product

✓

Verified Against NTA Final Answer Key

Option (3) : Benzene

📚 Amines Easy

📋 Given Parameters & Problem Setup

Starting material Benzenediazonium chloride, C₆H₅N₂⁺ Cl⁻

Reagent (i) Cu₂Br₂ / HBr (Sandmeyer reaction)

Reagent (ii) Mg / dry ether (Grignard reagent preparation)

Reagent (iii) H₂O (Protonation of Grignard reagent)

⚡

The 30-Second Exam Hall Shortcut: ⚡ 3-Step Reaction Roadmap: Diazonium ⟶ Bromobenzene ⟶ Grignard ⟶ Benzene

⏱️ Save ~40 seconds

(i) Cu₂Br₂/HBr gives Bromobenzene. (ii) Mg/ether gives Phenylmagnesium bromide (C₆H₅MgBr). (iii) H₂O protonates Grignard to yield pure BENZENE in 10 seconds!

1 Step (i): Benzenediazonium chloride undergoes Sandmeyer reaction with Cu₂Br₂/HBr to replace the diazonium group with bromine ⟶ Bromobenzene (C₆H₅Br).

2 Step (ii): Bromobenzene reacts with magnesium turnings in dry ether to form a Grignard reagent ⟶ Phenylmagnesium bromide (C₆H₅MgBr).

3 Step (iii): Grignard reagents are extremely strong bases/nucleophiles. Addition of water (H₂O) protonates the carbanionic phenyl group: C₆H₅⁻ + H⁺ ⟶ C₆H₆ (Benzene).

4 Final product is Benzene!

5 Lock Option (3) Benzene immediately!

🛡️

Why You Can 100% Trust This Shortcut: Organic Synthetic Conversions · Grignard Hydrolysis (Organomagnesium halides react instantaneously with active hydrogen compounds like H₂O to generate the parent hydrocarbon).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Step (i): Sandmeyer Reaction to Form Bromobenzene

Benzenediazonium chloride treated with cuprous bromide in hydrobromic acid (Cu₂Br₂ / HBr) undergoes replacement of the diazonium group by bromine with evolution of nitrogen gas (Sandmeyer reaction):

\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{Cu}_2\text{Br}_2 / \text{HBr}} \text{C}_6\text{H}_5\text{Br} + \text{N}_2 \uparrow

Step 2

Step (ii): Preparation of Phenylmagnesium Bromide

The resulting bromobenzene is treated with magnesium turnings in anhydrous dry ether to insert magnesium into the carbon-bromine bond, forming the Grignard reagent:

\text{C}_6\text{H}_5\text{Br} + \text{Mg} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_5\text{MgBr} \quad (\text{Phenylmagnesium bromide})

Step 3

Step (iii): Hydrolysis of the Organometallic Intermediate

Grignard reagents are highly reactive toward protic sources. Water hydrolyzes phenylmagnesium bromide, transferring a proton to the highly basic phenyl carbon to produce benzene and basic magnesium bromide:

\text{C}_6\text{H}_5\text{MgBr} + \text{H}_2\text{O} \longrightarrow \text{C}_6\text{H}_6 + \text{Mg}(\text{OH})\text{Br}

Step 4

Identification of Final Product

The final product obtained after all three sequential transformations is benzene (C₆H₆).

\text{Product} = \text{Benzene}

⭐

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:

Phenol does not form under these conditions; water does not substitute on bromobenzene.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate thought H₂O displaced Br by nucleophilic aromatic substitution.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Phenol would only form if diazonium salt were directly warmed with water without steps (i) and (ii).

⚠️

Student Fallacy (Why candidates pick this trap): Candidate ignored the Cu₂Br₂ and Mg steps.

3 (3)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate product resulting from complete hydrolysis of phenylmagnesium bromide.

💡

Why this is airtight: None. Candidate traced all three reaction steps accurately.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Intermediate formed after step (ii), which reacts further with water in step (iii).

⚠️

Student Fallacy (Why candidates pick this trap): Candidate forgot to carry out the third step (hydrolysis).

⚠️

NTA Trap Alert: Stopping at Step (ii) and Choosing Phenylmagnesium Bromide (Option 4)

Many students recognize the Grignard formation in step (ii) and hastily pick Option (4) without completing step (iii) H₂O, which destroys the Grignard reagent to form benzene.

🛡️ How to avoid the -1 mark: Always cross off each step as you execute it: (i) Done ⟶ C₆H₅Br, (ii) Done ⟶ C₆H₅MgBr, (iii) Done ⟶ C₆H₆.

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Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Amines Standard Benchmark

Core Scientific Principle Tested:

"Diazonium salts undergo Sandmeyer substitution to aryl halides; Grignard reagents protonate with water to yield hydrocarbons."

Syllabus Focus: Synthetic Importance of Diazonium Salts and Grignard Reactions

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Chemistry Question 63 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ 3-Step Reaction Roadmap: Diazonium ⟶ Bromobenzene ⟶ Grignard ⟶ Benzene

Step-by-Step Rigorous Workflow & Mathematical Proof

Step (i): Sandmeyer Reaction to Form Bromobenzene

Step (ii): Preparation of Phenylmagnesium Bromide

Step (iii): Hydrolysis of the Organometallic Intermediate

Identification of Final Product

360° Option Elimination Engine Key Feature

NTA Trap Alert: Stopping at Step (ii) and Choosing Phenylmagnesium Bromide (Option 4)

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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