Q.75 Chemistry ⭐ 360° Option Elimination Active

NEET 2025 Chemistry Question 75 Solution

Topic: Haloalkanes and Haloarenes · Official NTA Booklet Code 45

Question

How many products (including stereoisomers) are expected from monochlorination of the following compound? (CH₃)₂CH−CH₂−CH₃

✓

Verified Against NTA Final Answer Key

Option (4) : 6

📚 Haloalkanes and Haloarenes Medium

📋 Given Parameters & Problem Setup

Substrate 2-Methylbutane (Isopentane, (CH₃)₂CH-CH₂-CH₃)

Reaction Photochemical Monochlorination (Cl₂ / hν)

Inclusion All products INCLUDING stereoisomers (enantiomers and diastereomers)

⚡

The 30-Second Exam Hall Shortcut: ⚡ 4 Regioisomers + 2 Chiral Centers Rule

⏱️ Save ~45 seconds

There are 4 constitutionally distinct carbons: C1 gives 1-chloro-2-methylbutane (C2 is chiral ⟹ 2 enantiomers: d + l). C2 gives 2-chloro-2-methylbutane (achiral ⟹ 1). C3 gives 2-chloro-3-methylbutane (C3 is chiral ⟹ 2 enantiomers: d + l). C4 gives 1-chloro-3-methylbutane (achiral ⟹ 1). Total = 2 + 1 + 2 + 1 = 6!

1 Write the 4 constitutional isomers from monochlorination of 2-methylbutane:

2 Isomer 1 (at C1): ClCH₂-CH(CH₃)-CH₂-CH₃ (1-chloro-2-methylbutane). C2 is bonded to -H, -CH₃, -CH₂Cl, -CH₂CH₃ (4 distinct groups ⟹ CHIRAL!). Produces 2 stereoisomers (d and l pair).

3 Isomer 2 (at C2): CH₃-C(Cl)(CH₃)-CH₂-CH₃ (2-chloro-2-methylbutane). C2 has two identical methyl groups ⟹ Achiral (1 product).

4 Isomer 3 (at C3): CH₃-CH(CH₃)-CH(Cl)-CH₃ (2-chloro-3-methylbutane). C3 is bonded to -H, -Cl, -CH₃, -CH(CH₃)₂ (4 distinct groups ⟹ CHIRAL!). Produces 2 stereoisomers (d and l pair).

5 Isomer 4 (at C4): CH₃-CH(CH₃)-CH₂-CH₂Cl (1-chloro-3-methylbutane). No chiral carbon ⟹ Achiral (1 product).

6 Sum total: 2 + 1 + 2 + 1 = 6 products including stereoisomers.

7 Select Option (4) in under 20 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Free Radical Halogenation and Chiral Stereoisomerism (NCERT Chemistry · Haloalkanes and Haloarenes).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Identify Constitutionally Distinct Hydrogen Sets in 2-Methylbutane

2-Methylbutane has the structure (CH₃)₂CH-CH₂-CH₃. There are four distinct types of hydrogen atoms available for substitution:

\text{Substitutable sites: } \text{C1 (primary)}, \; \text{C2 (tertiary)}, \; \text{C3 (secondary)}, \; \text{C4 (primary)}

• This yields 4 constitutional (structural) isomers.

Step 2

Evaluate Chirality of Monochloro Derivatives at C1 and C2

Examine each constitutional isomer for chiral stereocenters (asymmetric carbons):

(1) \; \text{ClCH}_2-\text{C}^*\text{H(CH}_3)\text{CH}_2\text{CH}_3 \implies \text{C2 is chiral} \implies 2\text{ stereoisomers } (d, l)

• (2) CH₃-C(Cl)(CH₃)-CH₂-CH₃: C2 is bonded to two identical methyl groups, hence achiral ⟹ 1 isomer.

Step 3

Evaluate Chirality at C3 and C4 and Calculate Total

Examine the remaining two constitutional isomers:

(3) \; (\text{CH}_3)_2\text{CH}-\text{C}^*\text{H(Cl)}-\text{CH}_3 \implies \text{C3 is chiral} \implies 2\text{ stereoisomers } (d, l)

• (4) (CH₃)₂CH-CH₂-CH₂Cl: Contains no asymmetric carbon atom, hence achiral ⟹ 1 isomer.
• Total number of products including stereoisomers:
• \text{Total} = 2 + 1 + 2 + 1 = 6.
• Matches Option (4) perfectly.

⭐

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. Severe undercount.

⚠️

Student Fallacy (Why candidates pick this trap): Only considering major thermodynamic products.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Omitting primary chlorination and stereoisomers.

⚠️

Student Fallacy (Why candidates pick this trap): Ignoring primary C-H substitution.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Forgetting the chiral center in one of the two chiral isomers.

⚠️

Student Fallacy (Why candidates pick this trap): Counting only one isomer as chiral (4 + 1 = 5).

4 D

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. 4 constitutional isomers, two of which have an asymmetric carbon, giving 2 + 1 + 2 + 1 = 6 stereoisomers.

💡

Why this is airtight: None; rigorous stereochemical analysis.

⚠️

NTA Trap Alert: Counting Structural Isomers Only (4 vs 6)

There are 4 STRUCTURAL (constitutional) isomers. Students who fail to read "(including stereoisomers)" look for 4, and seeing no 4 in the options, guess 5 or 3. Recognizing that C1 and C3 chlorination each creates a pair of enantiomers yields 2 + 1 + 2 + 1 = 6!

🛡️ How to avoid the -1 mark: Always scan for asymmetric carbons (4 different groups) in each constitutional isomer!

📖

Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Haloalkanes and Haloarenes Standard Benchmark

Core Scientific Principle Tested:

"Monochlorination of 2-methylbutane produces 4 constitutional isomers yielding 6 total stereoisomers."

Syllabus Focus: Free Radical Halogenation & Stereochemistry

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

4 positions, 2 are chiral (d+l): 2 + 1 + 2 + 1 = 6 products (Option 4)!

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NEET 2025 Chemistry Question 75 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ 4 Regioisomers + 2 Chiral Centers Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Identify Constitutionally Distinct Hydrogen Sets in 2-Methylbutane

Evaluate Chirality of Monochloro Derivatives at C1 and C2

Evaluate Chirality at C3 and C4 and Calculate Total

360° Option Elimination Engine Key Feature

NTA Trap Alert: Counting Structural Isomers Only (4 vs 6)

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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