Animated Solution for Chemistry - Organic Chemistry: Staggered and eclipsed conformers of ethane are

The 3D world of molecules is far from static; it is a dynamic, constantly moving environment. When we draw a molecule like ethane ($C_2{H}_6$) on a flat piece of paper, it is easy to imagine it as a rigid structure. However, the reality is much more energetic.

The two carbon atoms in ethane are connected by a single sigma ($\sigma$) bond. Because a sigma bond is cylindrically symmetrical along the internuclear axis, it allows the two carbon atoms to rotate freely relative to one another. Imagine two wheels connected by a single axle; you can hold one wheel still and spin the other without breaking the axle. This free rotation gives rise to an infinite number of spatial arrangements.

Eclipsed

As the carbon atoms rotate, the hydrogen atoms attached to them change their relative positions. We often visualize this using a Newman projection, looking straight down the carbon-carbon bond.

In the staggered conformation, the hydrogen atoms on the front carbon are perfectly positioned between the hydrogen atoms on the back carbon. This maximizes the distance between the electron clouds of the $C-H$ bonds, minimizing repulsion and making it the most stable, lowest-energy state.

Conversely, in the eclipsed conformation, the hydrogen atoms on the front carbon directly align with those on the back carbon. This alignment forces the electron clouds into close proximity, creating maximum torsional strain and making it the least stable, highest-energy state.

Because these different spatial arrangements—staggered, eclipsed, and everything in between—are generated purely by rotation around a single bond, they are classified as conformational isomers. In organic chemistry, we use a beautiful portmanteau to describe them: Rotational Isomers, or simply, Rotamers.

At room temperature, the energy barrier between the staggered and eclipsed forms is merely $12.5\text{ kJ/mol}$. This barrier is so low that the thermal energy of the surroundings is more than enough to overcome it, causing the ethane molecule to spin millions of times per second! Therefore, while we can draw them as distinct structures, they are inseparable in practice.