What is the difference between nonaromatic heterocycles and aromatic heterocycles?
1 Answer
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.