Why is benzene planar and cyclohexane nonplanar?
Short answer: Benzene is planar because its carbon atoms are
The structure of benzene is
Each carbon atom is bonded to three other atoms, so it is
The interior angles of a regular hexagon are 120°.
This exactly matches the
The structure of cyclohexane is
Each carbon atom is bonded to four other atoms, so it is
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.