What is the difference between nonaromatic heterocycles and aromatic heterocycles?

Heterocycles are cyclic compounds with more than one type of atom in the ring (hetero = different).

Obviously the difference between nonaromatic and aromatic heterocycles is that aromatic heterocycles are... well... aromatic. So now we should see what that really means.

Let's take a look at an example.

PYRIDINE IS AROMATIC

Pyridine is an aromatic heterocycle.

• It has `4n+2` delocalized `p` electrons in the ring (the lone pair does NOT count; it is in an `sp^2` orbital parallel to the ring, perpendicular to each of the 6 `2p_z` orbitals from each atom around the ring, and hence external to the ring), where `n = 1`.
• It is fully conjugated throughout the ring.
• It is cyclic and planar.

Hence, it meets all requirements for aromaticity.

PYRIDINE LOOK-ALIKE? ...NOT AROMATIC!

I thought this example of a photopyridone looked cool. It kinda looks like pyridine, but it is a nonaromatic heterocycle. This is a tricky one, but let's see.

• The `sp^2` orbital on nitrogen is bonding with the hydrogen, so it just so happens that the lone pair is in a `2p_z` orbital perpendicular to the ring, and hence it is participating in the ring. Therefore, there are `4n+2` `p` electrons in the ring, where `n = 1`. Do not count the carbonyl `p` electrons outside of the ring. They don't qualify in assigning aromaticity.

• It is cyclic, and it is actually fairly planar because of how the carbonyl and other double bonds lock the bonds in place.

• Here is why it is not aromatic. Really, at the carbon nearest the carbonyl carbon from the counterclockwise direction, we have drawn a resonance structure in which the `pi` electrons have gone outside of the ring. So by a technicality, it fails to be aromatic since it is not fully conjugated all the way around the ring.