Q.75 Chemistry · Section A ⭐ 360° Option Elimination Active

Vault ↗

NEET 2024 Chemistry Question 75 Solution

Topic: Organic Chemistry - Some Basic Principles and Techniques · Official NTA Booklet Code T1

Question

The most stable carbocation among the following is: E)

✓

Verified Against NTA Final Answer Key

Option (2) : 1-methylcyclohexyl carbocation (3° carbocation with 7 α-hydrogens)

📚 Organic Chemistry - Some Basic Principles and Techniques Easy

📋 Given Parameters & Problem Setup

Cation 1 (Cyclopentylmethyl) carbocation (1°)

Cation 2 1-methylcyclohexyl carbocation (3°)

Cation 3 2-methylpentan-3-yl carbocation (2°)

Cation 4 4-methylpentan-2-yl carbocation (2°)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Degree of Carbocation Instant Win

⏱️ Save ~30 seconds

Tertiary (3°) > Secondary (2°) > Primary (1°)! Structure (2) is a TERTIARY (3°) carbocation with 7 α-hydrogens -> Unrivaled stability!

1 Inspect the degree of each carbocation center:

2 - Option (1): Positive charge is on -CH2⁺ attached to cyclopentyl -> Primary (1°) carbocation.

3 - Option (2): Positive charge is on ring carbon bonded to a methyl group and two ring CH2 carbons -> Tertiary (3°) carbocation.

4 - Option (3): Positive charge is on -C⁺H- attached to -CH2CH3 and -CH(CH3)2 -> Secondary (2°) carbocation.

5 - Option (4): Positive charge is on -C⁺H- attached to -CH3 and -CH2CH(CH3)2 -> Secondary (2°) carbocation.

6 Tertiary carbocations are vastly more stable than secondary or primary carbocations due to maximum hyperconjugative structures and +I inductive effects.

7 1-methylcyclohexyl cation has 3 (from methyl) + 2 (ring C2) + 2 (ring C6) = 7 α-hydrogens.

8 Option (2) in 3 seconds!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · Organic Chemistry - Some Basic Principles and Techniques (Carbocation intermediates, inductive effect, and hyperconjugation stability order: 3° > 2° > 1°).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Fundamental Factors Governing Carbocation Stability

A carbocation is an sp²-hybridized, planar, sextet electron-deficient carbon. Stability is enhanced by electron donation through: 1. Inductive effect (+I): Alkyl groups push electron density toward the positive center. 2. Hyperconjugation: Delocalization of σ(C-H) electrons of α-carbons into the empty 2pz orbital of the carbocation. The more α-hydrogens, the greater the number of hyperconjugative resonance structures.

\text{Carbocation Stability} \propto \text{Number of } \alpha\text{-Hydrogens} \; (3^\circ > 2^\circ > 1^\circ > \overset{\oplus}{\text{C}}\text{H}_3)

Step 2

Classification of the Four Given Structures

Evaluating the α-hydrogen count and degree of each carbocation: - Option (1): Cyclopentyl-CH2⁺: Primary (1°) carbocation. Only 1 α-hydrogen (at the methine bridgehead). Very unstable. - Option (2): 1-Methylcyclohexyl cation: Tertiary (3°) carbocation. C-1 is bonded to three carbon atoms: the exocyclic methyl carbon (3 α-H) and two endocyclic methylene carbons at C-2 (2 α-H) and C-6 (2 α-H). Total α-H = 3 + 2 + 2 = 7 α-hydrogens! - Option (3): (CH3)2CH-C⁺H-CH2CH3: Secondary (2°) carbocation. α-hydrogens = 1 (from isopropyl CH) + 2 (from ethyl CH2) = 3 α-hydrogens. - Option (4): CH3-C⁺H-CH2-CH(CH3)2: Secondary (2°) carbocation. α-hydrogens = 3 (from terminal methyl) + 2 (from methylene CH2) = 5 α-hydrogens.

n_\alpha(\text{Option 2}) = 7 \; (3^\circ) \;>\; n_\alpha(\text{Option 4}) = 5 \; (2^\circ) \;>\; n_\alpha(\text{Option 3}) = 3 \; (2^\circ) \;>\; n_\alpha(\text{Option 1}) = 1 \; (1^\circ)

Step 3

Conclusion

Structure (2) is the only tertiary (3°) carbocation in the set. Its tertiary status combined with 7 hyperconjugative α-hydrogens makes it by far the most stable species.

\text{Most Stable} = \text{1-methylcyclohexyl cation (Option 2)}

⭐

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:

Primary carbocation with only 1 α-hydrogen; extremely unstable.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking ring size alone imparts stability.

2 (2)

✓ 100% Correct

Verdict & Scientific Reason:

Tertiary (3°) carbocation with 7 hyperconjugative α-hydrogens and favorable six-membered chair relaxation.

💡

Why this is airtight: None.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Secondary (2°) carbocation with only 3 α-hydrogens.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming branched chain makes it tertiary.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Secondary (2°) carbocation with 5 α-hydrogens.

⚠️

Student Fallacy (Why candidates pick this trap): Overlooking that positive charge is secondary, not tertiary.

⚠️

NTA Trap Alert: Confusing Branched Aliphatic Chain with Tertiary Carbocation

In Option (3), the isopropyl group is branched, but the carbon bearing the positive charge is bonded to only 2 carbons (secondary). In Option (2), the carbocation carbon itself is bonded to 3 carbons (tertiary).

🛡️ How to avoid the -1 mark: Count how many carbons are directly attached to the positive carbon: 3 = tertiary (most stable).

📖

Core NEET Syllabus Standard

Chemistry Core Organic Chemistry · Organic Chemistry - Some Basic Principles and Techniques Standard Benchmark

Core Scientific Principle Tested:

"Carbocation stability follows the order 3° > 2° > 1° > methyl due to hyperconjugation and +I inductive effects of adjacent alkyl substituents."

Syllabus Focus: Carbocation Intermediates: Structure and Relative Stability

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Three-degree with seven alpha-H: Stability crowned in the cyclohexyl cage!

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 200-minute timed NTA simulator.

Launch NEET 2024 CBT Test →

← Previous (Q.74) All Chemistry Questions Grid Next (Q.76) →

🔗 Related Questions — Organic Chemistry: Some Basic Principles and Techniques

All 12 Organic Chemistry: Some Basic Principles and Techniques PYQs →

NEET 2026 Q48

Orbital Interactions in Carbocation Stabilization (Resonance & Hyperconjugation)

View Solution →

NEET 2026 Q83

Spotted a mistake in this solution? Report an error ([email protected])

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2024 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

NEET 2024 Chemistry Question 75 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Degree of Carbocation Instant Win

Step-by-Step Rigorous Workflow & Mathematical Proof

Fundamental Factors Governing Carbocation Stability

Classification of the Four Given Structures

Conclusion

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing Branched Aliphatic Chain with Tertiary Carbocation

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Organic Chemistry: Some Basic Principles and Techniques

Orbital Interactions in Carbocation Stabilization (Resonance & Hyperconjugation)

Classification of Polyenes (Conjugated vs Isolated Dienes)

Increasing Order of Polarity of Organic Functional Groups

Matching Organic Mixtures with Specific Distillation Techniques

Solve 4 NEET Questions a Day — One per Subject