Conformations of Ethane
Rotation around ethane's C–C single bond produces different spatial arrangements of its hydrogen atoms. These arrangements, called conformations, aren't different molecules; they're different orientations of the same molecule that interconvert constantly at room temperature. Understanding ethane's conformations builds the foundation for analyzing more complex molecules like butane and cyclohexane.
Conformations in Ethane
A conformation is any spatial arrangement of atoms that results from rotation around a single bond. Because C–C single bonds allow relatively free rotation, ethane can adopt an infinite number of conformations as one methyl group spins relative to the other.
Out of that infinite set, two extremes matter most:
- Staggered conformation: the C–H bonds on the front carbon are offset by 60° from those on the back carbon.
- Eclipsed conformation: the C–H bonds on the front and back carbons line up directly behind one another (0° offset).
You can visualize these using Newman projections, where you look straight down the C–C bond axis. In a Newman projection, the front carbon is drawn as a dot and the back carbon as a circle, with three bonds radiating from each.
Staggered vs. Eclipsed Conformations
The staggered conformation is more stable than the eclipsed conformation. Here's why:
- In the staggered form, bonding electrons on adjacent carbons are as far apart as possible. This minimizes repulsion between the electron clouds of neighboring C–H bonds, placing the molecule at an energy minimum.
- In the eclipsed form, C–H bonds on adjacent carbons are directly aligned. The electron clouds are forced closer together, increasing repulsion and placing the molecule at an energy maximum.
The energy difference between staggered and eclipsed ethane is approximately 12 kJ/mol (about 3 kcal/mol). This value is called the torsional barrier (or rotational barrier). Each eclipsed H–H interaction contributes roughly 4 kJ/mol of torsional strain, and since three pairs of C–H bonds eclipse simultaneously, the total comes to about 12 kJ/mol.
Torsional strain is the resistance to rotation caused by repulsion between bonding electrons on adjacent atoms. It's distinct from steric strain, which involves repulsion between the electron clouds of bulky groups. In ethane, the hydrogens are small enough that the strain is almost entirely torsional rather than steric.
Ethane Rotation Energy Diagram
A plot of potential energy versus dihedral angle (the angle between C–H bonds on the front and back carbons, measured in a Newman projection) reveals a repeating pattern:
- The curve is sinusoidal, completing three full cycles over a 360° rotation.
- Energy minima (staggered conformations) occur at dihedral angles of 60°, 180°, and 300°.
- Energy maxima (eclipsed conformations) occur at dihedral angles of 0°, 120°, and 240°.
- The height of each peak above the neighboring valley is the torsional barrier of ~12 kJ/mol.
Because 12 kJ/mol is relatively small compared to the thermal energy available at room temperature (~2.5 kJ/mol average per degree of freedom), ethane molecules rotate through all conformations rapidly. You can't isolate a single conformation. However, at any given instant, more molecules will be found near the staggered arrangement than near the eclipsed one.
Structural Considerations
Each carbon in ethane has tetrahedral geometry ( hybridization), with bond angles close to 109.5°. This tetrahedral arrangement is what produces the 60° offset between adjacent C–H bonds in the staggered form.
Because of ethane's molecular symmetry, all three staggered conformations are identical in energy, and all three eclipsed conformations are identical in energy. This equivalence won't hold for larger alkanes like butane, where different staggered conformations (anti vs. gauche) have different energies due to steric interactions between larger substituents.