Q. 20 The two major products of the reaction

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Question Overview

The given reaction involves a secondary amine attached to a cyclohexyl-containing alkyl chain. The reaction conditions are:

Reagents:

1. Excess CH₃I

2. Ag₂O, H₂O, Δ (heat)

You are asked to identify the two major products formed in this reaction.

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Key Concept: Hofmann Elimination

🔬 What Happens in Hofmann Elimination?

This reaction proceeds in three main steps:

1. Exhaustive methylation

   • Excess CH₃I converts the amine into a quaternary ammonium iodide

2. Formation of quaternary ammonium hydroxide

   • Ag₂O replaces I⁻ with OH⁻

3. β-Elimination (E2) on heating

   • Produces:

     • An alkene

     • A tertiary amine

📌 Important Rule (Hofmann Rule):
The least substituted alkene is formed as the major product.

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Analysis of the Given Substrate

From the structure:

• Nitrogen is attached to:

  • An ethyl group

  • A secondary alkyl chain linked to cyclohexane

• After exhaustive methylation, nitrogen becomes:

  • N(CH₃)₃⁺ (quaternary ammonium)

During elimination:

• β-Hydrogen is removed from the less hindered side

• This gives the least substituted alkene

• The leaving group forms a tertiary amine

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✅ Correct Answer

✔️ Option (B)

Products formed:

• Least substituted alkene (terminal alkene)

• N(CH₃)₂CH₂CH₃ (tertiary amine)

This perfectly follows Hofmann elimination rules.

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Why Option (B) Is Correct

✔️ Produces the least substituted alkene
✔️ Forms a tertiary amine, not secondary
✔️ Matches E2 elimination under Hofmann conditions
✔️ Sterically favored β-hydrogen abstraction

Hence, Option (B) is the correct answer.

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Why Other Options Are Incorrect

❌ Option (A)

• Shows ethylene (CH₂=CH₂), which cannot form directly from this substrate

• Alkene carbon count does not match

• Incorrect elimination pathway

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❌ Option (C)

• Alkene shown is more substituted

• Violates Hofmann rule

• Less stable transition state under steric control

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❌ Option (D)

• Produces a secondary amine

• Hofmann elimination always gives a tertiary amine

• Mechanistically incorrect

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Final Conclusion

✔️ Under excess CH₃I followed by Ag₂O, H₂O, and heat, the reaction proceeds via Hofmann elimination, giving:

• Least substituted alkene

• Tertiary amine

✅ Correct Answer: Option (B)