Propane (C₃H₈) shows conformational isomerism due to free rotation around the C–C single bond (σ-bond).

---

Key Idea

Rotation is considered around the C₁–C₂ bond (or C₂–C₃; both are equivalent).

Use Newman projection to understand conformations.

Types of Conformations in Propane

1. Staggered Conformation (Most Stable)

• Hydrogen atoms on front and back carbon are as far apart as possible
• Dihedral angle = 60°
• Minimum torsional strain

This is the lowest energy conformation

---

2. Eclipsed Conformation (Least Stable)

• Hydrogen atoms on front and back carbon overlap each other
• Dihedral angle = 0°
• Maximum torsional strain

This is the highest energy conformation

Important Difference from Ethane

• In propane, one carbon has CH₃ group, so:
  • There are H–H eclipsing interactions
  • And also H–CH₃ eclipsing interactions (more repulsion)

Hence, eclipsed propane is slightly more unstable than ethane eclipsed

---

Energy Profile of Propane Rotation

• Energy varies as bond rotates:
  • Maxima → Eclipsed
  • Minima → Staggered

There are:

• 3 staggered conformations (all equivalent)
• 3 eclipsed conformations (all equivalent)

---

JEE/NEET Important Points

• Only two types → staggered & eclipsed
• Staggered = most stable
• Eclipsed = least stable
• Rotation barrier ≈ 14 kJ/mol (slightly higher than ethane)
• Stability depends on torsional strain

---

Quick Memory Trick

“Spread = Stable, Clash = Unstable”

Why is propane eclipsed more unstable than ethane?”
Answer: Because of H–CH₃ repulsion > H–H repulsion