Q.53 Chemistry ⭐ 360° Option Elimination Active

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NEET 2021 Chemistry Question 53 Solution

Topic: Hydrocarbons · Official NTA Booklet Code M1

Question

The major product of the following chemical reaction is:

✓

Verified Against NTA Final Answer Key

Option (1) : (CH3)2CH - CH2 - CH2 - Br

📚 Hydrocarbons Easy

📋 Given Parameters & Problem Setup

Reactant Alkene 3-methylbut-1-ene ((CH3)2CH - CH = CH2)

Reagent HBr in presence of benzoyl peroxide ((C6H5CO)2O2)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Peroxide Effect on HBr = Anti-Markovnikov Product!

⏱️ Save ~25 seconds

Addition of $HBr$ in the presence of organic peroxides exclusively follows the free radical Anti-Markovnikov pathway (Kharasch effect). The bromine radical ( $\dot{\text{Br}}$ ) attacks the less substituted terminal carbon ( $C_1$ ) to form the more stable $2^\circ$ free radical, yielding 1-bromo-3-methylbutane: $(CH_3)_2CH-CH_2-CH_2-Br$ ! Match Option (1)!

1 Reagent is

HBr + \text{peroxide} \implies

Peroxide effect (Anti-Markovnikov addition).

2 In free-radical addition,

\dot{\text{Br}}

attacks first at the terminal carbon (

CH_2

) to generate the more stable secondary free radical:

(CH_3)_2CH - \dot{C}H - CH_2Br

3 Subsequent reaction with

HBr

abstracts a hydrogen atom to yield the

1

-bromo alkane.

4 Result:

(CH_3)_2CH - CH_2 - CH_2 - Br

(1-bromo-3-methylbutane).

5 Match Option (1).

🛡️

Why You Can 100% Trust This Shortcut: Canonical Kharasch peroxide effect mechanism in NCERT Hydrocarbons.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Initiation Step: Generation of Bromine Free Radical

Benzoyl peroxide undergoes homolytic cleavage upon heating or exposure to light to yield benzoyloxy radicals, which decarboxylate into phenyl radicals and abstract hydrogen from $HBr$ to generate bromine radicals:

(C_6H_5CO)_2O_2 \longrightarrow 2 C_6H_5\dot{C}O \longrightarrow 2 \dot{C}_6H_5 + 2 CO_2, \quad \dot{C}_6H_5 + HBr \longrightarrow C_6H_6 + \dot{\text{Br}}

Step 2

Propagation Step 1: Regioselective Addition of Bromine Radical

The electrophilic bromine radical attacks the unsymmetrical double bond of 3-methylbut-1-ene. Addition at the less hindered terminal $CH_2$ carbon generates a stable secondary ( $2^\circ$ ) free radical rather than an unstable primary ( $1^\circ$ ) radical:

(CH_3)_2CH - CH = CH_2 + \dot{\text{Br}} \longrightarrow (CH_3)_2CH - \dot{C}H - CH_2 - Br \quad (\text{more stable } 2^\circ \text{ radical})

Step 3

Propagation Step 2: Hydrogen Abstraction to Form Final Product

The secondary radical abstracts a hydrogen atom from another $HBr$ molecule to produce 1-bromo-3-methylbutane and regenerates a bromine radical:

(CH_3)_2CH - \dot{C}H - CH_2 - Br + HBr \longrightarrow (CH_3)_2CH - CH_2 - CH_2 - Br + \dot{\text{Br}}

⭐

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

✓ 100% Correct

Verdict & Scientific Reason:

Exact Anti-Markovnikov major product obtained via free-radical peroxide mechanism.

💡

Why this is airtight: None; rigorous organic mechanism execution.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Erroneously incorporates the peroxide catalyst into the organic skeleton.

⚠️

Student Fallacy (Why candidates pick this trap): Believing the peroxide acts as a nucleophile rather than a radical initiator.

3 3

✗ Incorrect

Verdict & Scientific Reason:

This is the un-rearranged Markovnikov addition product formed in the ABSENCE of peroxides.

⚠️

Student Fallacy (Why candidates pick this trap): Ignoring the presence of benzoyl peroxide and applying standard Markovnikov rule.

4 4

✗ Incorrect

Verdict & Scientific Reason:

This is the rearranged tertiary carbocation product formed during carbocationic Markovnikov addition (in absence of peroxides).

⚠️

Student Fallacy (Why candidates pick this trap): Assuming carbocation intermediate with hydride shift occurs under free radical conditions.

⚠️

NTA Trap Alert: Markovnikov vs. Peroxide Effect Trap

Without peroxide, $HBr$ addition proceeds via carbocations, undergoing a 1,2-hydride shift to yield 2-bromo-2-methylbutane (Option 4). In the PRESENCE of peroxide, free radicals are formed, which DO NOT undergo hydride rearrangements and strictly yield the Anti-Markovnikov terminal bromide (Option 1).

🛡️ How to avoid the -1 mark: Identify

(C_6H_5CO)_2O_2

! Peroxide +

HBr \implies

Anti-Markovnikov (terminal Br).

📖

Core NEET Syllabus Standard

NCERT Chemistry · Hydrocarbons Standard Benchmark

Core Scientific Principle Tested:

"In the presence of peroxide, addition of HBr to unsymmetrical alkenes takes place contrary to Markovnikov's rule via a free-radical mechanism."

Syllabus Focus: Chemical Reactions of Alkenes (Peroxide Effect / Kharasch Effect)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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Sequential Reaction: Anti-Markovnikov HBr Addition, Cyanide Substitution & Mendius Reduction

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NEET 2021 Chemistry Question 53 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Peroxide Effect on HBr = Anti-Markovnikov Product!

Step-by-Step Rigorous Workflow & Mathematical Proof

Initiation Step: Generation of Bromine Free Radical

Propagation Step 1: Regioselective Addition of Bromine Radical

Propagation Step 2: Hydrogen Abstraction to Form Final Product

360° Option Elimination Engine Key Feature

NTA Trap Alert: Markovnikov vs. Peroxide Effect Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Hydrocarbons

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Sequential Reaction: Anti-Markovnikov HBr Addition, Cyanide Substitution & Mendius Reduction

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