Conformational Isomerism
Rotamers & Stability Analysis | Organic Chemistry
1. What are Conformers?
Since the rotation around a single bond is not completely free due to repulsive interactions, these isomers have different energies. However, the energy barrier is usually small, so they interconvert rapidly at room temperature.
2. Ways to Represent
- Sawhorse Projection: Viewed from an oblique angle. Shows the C-C bond length.
- Newman Projection: Viewed along the C-C bond axis. The front carbon is a dot, the back carbon is a circle. This is most used for stability analysis.
Dihedral Angle ($\theta$): The angle between the Hydrogen (or group) on the front carbon and the Hydrogen (or group) on the back carbon.
3. Conformations of Ethane ($C_2H_6$)
There are infinite conformations, but two extremes are important:
| Form | Dihedral Angle ($\theta$) | Stability | Reason |
|---|---|---|---|
| Staggered | $60^\circ$ | Maximum (Most Stable) | Minimum Torsional Strain (e- pair repulsion) |
| Eclipsed | $0^\circ$ | Minimum (Least Stable) | Maximum Torsional Strain |
Torsional Strain: Repulsion between the bond pair electrons of C-H bonds on adjacent carbons.
4. Conformations of n-Butane ($C_4H_{10}$)
Rotation around the $C_2-C_3$ bond gives four key conformers:
- Anti (Staggered): $\theta = 180^\circ$. Methyl groups are opposite. Most Stable. (Min Torsional & Steric strain).
- Gauche (Skew): $\theta = 60^\circ$. Methyl groups are adjacent. Less stable than Anti due to Van der Waals (Steric) repulsion.
- Partially Eclipsed: $\theta = 120^\circ$. Methyl eclipses Hydrogen.
- Fully Eclipsed: $\theta = 0^\circ$. Methyl eclipses Methyl. Least Stable (Max Steric & Torsional strain).
5. The Gauche Effect (Exception)
Generally, Anti form is more stable. However, if Hydrogen Bonding or ion-dipole attraction is possible, the Gauche form becomes more stable.
Example: Ethylene Glycol ($CH_2OH-CH_2OH$)
- In the Gauche form ($\theta = 60^\circ$), intramolecular Hydrogen Bonding forms between the two -OH groups.
- Stability: Gauche > Anti.
6. Conformations of Cyclohexane
Cyclohexane is not planar; it puckers to relieve angle strain (Baeyer's Strain) and torsional strain.
- Chair Form: Most stable. All angles $109.5^\circ$. All bonds staggered.
- Boat Form: Less stable due to "Flagpole" repulsion and eclipsing.
- Stability Order: Chair > Twist Boat > Boat > Half Chair.
Practice Quiz
Test your knowledge on Conformers.
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