Why is benzene planar and cyclohexane nonplanar?

1 Answer
Ernest Z. · Truong-Son N.
Dec 2, 2015

Short answer: Benzene is planar because its carbon atoms are "sp"^2sp2 hybridized, and cyclohexane is nonplanar because its carbon atoms are "sp"^3sp3 hybridized.

Explanation:

Benzene

The structure of benzene is

asbury.com

Each carbon atom is bonded to three other atoms, so it is "sp"^2sp2 hybridized: trigonal planar with all bond angles equal to 120°.

The interior angles of a regular hexagon are 120°.

upload.wikimedia.org

This exactly matches the "sp"^2sp2 bond angles, so benzene can be planar with no angle strain.

Cyclohexane

The structure of cyclohexane is

chemistry2.csudh.edu

Each carbon atom is bonded to four other atoms, so it is "sp"^3sp3 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.

kb.chem.iupui.edu

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

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