Conformational Isomerism in n-Butane (C₄H₁₀)

Conformational Isomerism in n-Butane (C₄H₁₀)

Conformational Isomerism in n-Butane (C₄H₁₀) arises from C–C bond rotation, giving anti, gauche, and eclipsed forms with different stabilities.

Rotation around the C₂–C₃ Bond

• n-Butane has a straight chain: CH₃–CH₂–CH₂–CH₃.
• Rotation about the C₂–C₃ bond generates multiple conformations.

Key Conformations

1. Anti Conformation (Staggered):

   • The two methyl groups are 180° apart.
   • Most stable due to minimal steric hindrance.

2. Gauche Conformation (Staggered):

   • Methyl groups are 60° apart.
   • Less stable than anti due to steric strain between CH₃ groups.

3. Eclipsed (H–CH₃):

   • Methyl on one carbon is aligned with a hydrogen on the other.
   • Higher energy than gauche.

4. Fully Eclipsed (CH₃–CH₃):

   • Methyl groups are aligned.
   • Highest energy due to maximum steric repulsion.

Stability Order

Anti > Gauche > Eclipsed (H–CH₃) > Fully Eclipsed (CH₃–CH₃)

Energy Difference

• Anti to Gauche: ~3.8 kJ/mol
• Anti to Fully Eclipsed: ~18–19 kJ/mol

Potential Energy Diagram

• A full 360° rotation around the C₂–C₃ bond produces a sinusoidal energy profile with:
  • Minima at staggered conformations (especially anti and gauche)
  • Maxima at eclipsed conformations (especially fully eclipsed)

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