Question

Why is benzene planar and cyclohexane nonplanar?

Answer 1

Short answer: Benzene is planar because its carbon atoms are `"sp"^2` hybridized, and cyclohexane is nonplanar because its carbon atoms are `"sp"^3` hybridized.

Explanation:

Benzene

The structure of benzene is

Each carbon atom is bonded to three other atoms, so it is `"sp"^2` hybridized: trigonal planar with all bond angles equal to 120°.

The interior angles of a regular hexagon are 120°.

This exactly matches the `"sp"^2` bond angles, so benzene can be planar with no angle strain.

Cyclohexane

The structure of cyclohexane is

Each carbon atom is bonded to four other atoms, so it is `"sp"^3` hybridized: tetrahedral with all bond angles equal to 109.5°.

If the carbon atoms in cyclohexane were arranged as a planar hexagon, the bond angles would have to be 120°.

This would introduce a large amount of angle strain (and other types of strain) into the molecule.

The molecule can relieve this strain if it puckers into a three-dimensional chair shape.

This brings the bond angles back to 109.5 ° and minimizes all the strains in the molecule.