Why are aromatic rings stable?

1 Answer
Jan 6, 2017

Aromatic rings are stable because they are cyclic, conjugated molecules.

Explanation:

Conjugation of π orbitals lowers the energy of a molecule.

Thus, the order of stability is ethene < buta-1,3-diene < hexa-1,3,5-triene ≪ benzene.

Energy levels
(From Kshitij IIT JEE Online Coaching)

Benzene is different because it is a cyclic conjugated molecule.

In a cyclic conjugated molecule, each energy level above the first occurs in pairs.

Thus, the total energy of the six π electrons in benzene is much less than the energy of three separate ethene molecules or even of a hexa-1,3,5-triene molecule.

A Frost circle reflects the pattern of orbital energies.

ursula.chem.yale.edu

You place the ring inside a circle with one of its vertices pointing downwards.

Then you draw a horizontal line though the vertices. This represents the orbital energy levels.

If there isn't one already, you draw a horizontal dashed line through the circle's (and molecule's) centre. This represents the nonbonding energy level.

archive.uea.ac.uk

Qualitatively, a Frost circle explains the #(4n+2)# rule.

You need 2 electrons to fill the first level and 4 electrons to fill each level above the first.