Conformational Isomerism in Cyclohexane (C₆H₁₂)

Conformational Isomerism in Cyclohexane

Conformational Isomerism in Cyclohexane (C₆H₁₂) involves chair, boat, twist, and half-chair forms, with the chair conformation being the most stable.

Why Is Cyclohexane Special?

• Although cyclohexane forms a ring, it avoids angle strain (unlike smaller rings) by adopting non-planar conformations.
• The ideal bond angle (109.5°) is maintained through specific 3D shapes, minimizing strain.

Main Conformations

1. Chair Conformation:

   • Most stable.
   • Bond angles are ~109.5° (ideal tetrahedral).
   • All C–H bonds are staggered.
   • Hydrogens alternate between axial (up/down) and equatorial (around the ring) positions.

2. Boat Conformation:

   • Less stable due to:
     • Steric strain (flagpole interactions between axial Hs on C1 and C4).
     • Torsional strain (some eclipsing).

3. Twist-Boat Conformation:

   • Intermediate in stability.
   • Some relief of flagpole and torsional strain.
   • More stable than boat, less than chair.

Chair Flip

• Cyclohexane can undergo a chair flip:
  • Axial positions become equatorial and vice versa.
  • Important when evaluating substituted cyclohexanes.

Substituent Preference

• Large groups prefer the equatorial position to reduce 1,3-diaxial interactions.
• Example: In methylcyclohexane, the equatorial CH₃ is more stable than axial by ~7.6 kJ/mol.

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