Q.49 Chemistry ⭐ 360° Option Elimination Active

NEET 2025 Chemistry Question 49 Solution

Topic: Hydrocarbons & Amines · Official NTA Booklet Code 45

Question

Predict the major product 'P' in the following sequence of reactions - 1-methylcyclopentene → (i) HBr, benzoyl peroxide (ii) KCN (iii) Na(Hg)/C₂H₅OH → P (Major)

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Verified Against NTA Final Answer Key

Option (1) : 1-methyl-2-(aminomethyl)cyclopentane (Adjacent 1,2-substituted cyclopentane derivative)

📚 Hydrocarbons & Amines Medium

📋 Given Parameters & Problem Setup

Substrate 1-Methylcyclopentene

Step (i) Reagents HBr, benzoyl peroxide (Peroxide effect / Kharasch addition)

Step (ii) Reagent KCN (Nucleophilic substitution with cyanide)

Step (iii) Reagents Na(Hg) / C₂H₅OH (Mendius reduction of nitrile)

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The 30-Second Exam Hall Shortcut: ⚡ Anti-Markovnikov to Primary Amine Shortcut

⏱️ Save ~60 seconds

Step 1: HBr + peroxide gives Anti-Markovnikov addition: Br attaches to the LESS substituted C2 (adjacent to C1-methyl). Step 2: KCN replaces Br by CN at C2. Step 3: Reduction converts -CN into -CH₂NH₂ at C2 ⟹ 1,2-disubstituted product (Option 1)!

1 Step (i): 1-Methylcyclopentene + HBr in presence of benzoyl peroxide proceeds via free radical mechanism.

2 Br• radical attacks C2 (the less substituted secondary carbon) to create the more stable tertiary free radical at C1.

3 Hydrogen then attaches to C1, yielding 1-bromo-2-methylcyclopentane (1,2-relationship between -CH₃ and -Br).

4 Step (ii): Nucleophilic substitution of Br by CN⁻ using KCN yields 2-methylcyclopentanecarbonitrile (still 1,2-disubstituted!).

5 Step (iii): Mendius reduction (Na/Hg in ethanol) reduces -C≡N to -CH₂NH₂.

6 The final product has -CH₃ at C1 and -CH₂NH₂ at C2 (adjacent 1,2-relationship).

7 Look at Option (1): 1,2-disubstituted cyclopentane ring. Matches Option (1) instantly!

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Why You Can 100% Trust This Shortcut: Kharasch Peroxide Effect & Nitrile Reduction (NCERT Chemistry · Hydrocarbons and Amines).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Anti-Markovnikov Addition of HBr via Free Radical Mechanism

In the presence of benzoyl peroxide, HBr undergoes homolytic cleavage to produce bromine free radicals (Br•). The Br• radical adds regioselectively to the less substituted double-bond carbon (C2) to generate the more stable 3° tertiary radical at C1:

\text{1-methylcyclopentene} + \text{Br}^\bullet \longrightarrow \text{3}^\circ \text{ radical at C1} \xrightarrow{\text{HBr}} \text{1-bromo-2-methylcyclopentane}

• Bromine is positioned on C2, and the methyl group remains on C1 (1,2-regiochemistry).

Step 2

Nucleophilic Substitution with KCN

Potassium cyanide (KCN) is an ionic cyanide source. The cyanide ion (CN⁻) acts as an ambident nucleophile and attacks through the carbon atom, displacing bromide in an S_N2 reaction:

\text{1-bromo-2-methylcyclopentane} + \text{KCN} \longrightarrow \text{2-methylcyclopentanecarbonitrile} + \text{KBr}

• The nitrile group (-C≡N) is now located at C2.

Step 3

Mendius Reduction of Nitrile to Primary Amine

Sodium amalgam in ethanol (Na(Hg) / C₂H₅OH) provides nascent hydrogen to reduce the nitrile group completely to a primary aminomethyl group (-CH₂NH₂):

\text{R}-\text{C}\equiv\text{N} + 4[\text{H}] \xrightarrow{\text{Na(Hg)/EtOH}} \text{R}-\text{CH}_2\text{NH}_2

• The product is (2-methylcyclopentyl)methanamine, represented by Option (1) with adjacent -CH₃ and -CH₂NH₂ groups.

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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

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. Anti-Markovnikov addition places Br at C2, giving 1,2-substitution.

💡

Why this is airtight: None; rigorous multi-step organic reaction mechanism.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Corresponds to Markovnikov addition (without peroxide) where Br attaches to C1.

⚠️

Student Fallacy (Why candidates pick this trap): Ignoring the presence of benzoyl peroxide.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Contains an isonitrile group (-NC) and omits the reduction step.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing KCN with AgCN and forgetting Mendius reduction.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Combines Markovnikov regiochemistry with isonitrile formation without reduction.

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Student Fallacy (Why candidates pick this trap): Multiple mechanistic errors.

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NTA Trap Alert: Ignoring Peroxide (Markovnikov vs Anti-Markovnikov)

Without peroxide, HBr adds according to Markovnikov’s rule, placing Br at C1 (tertiary carbon), leading to 1,1-substitution (Option 2). The presence of benzoyl peroxide completely reverses regiochemistry to 1,2-substitution (Option 1)!

🛡️ How to avoid the -1 mark: Always spot peroxide: HBr + peroxide = Anti-Markovnikov addition!

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Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Hydrocarbons Standard Benchmark

Core Scientific Principle Tested:

"HBr + peroxide undergoes free radical Anti-Markovnikov addition."

Syllabus Focus: Addition Reactions of Alkenes & Peroxide Effect

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Peroxide puts Br on the neighbor (C2) ⟹ -CH₂NH₂ on neighbor ⟹ 1,2-ring (Option 1)!

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NEET 2025 Chemistry Question 49 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Anti-Markovnikov to Primary Amine Shortcut

Step-by-Step Rigorous Workflow & Mathematical Proof

Anti-Markovnikov Addition of HBr via Free Radical Mechanism

Nucleophilic Substitution with KCN

Mendius Reduction of Nitrile to Primary Amine

360° Option Elimination Engine Key Feature

NTA Trap Alert: Ignoring Peroxide (Markovnikov vs Anti-Markovnikov)

Core NEET Syllabus Standard

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