The 4 conditions a molecule must fulfil in order to be aromatic.
- It must be cyclic
- every atom around the ring must have an available p-orbital
- The number of electrons in the pi system must be 2, 6, 10, 14, 18, or a higher number in the set that increases from 18 in increments of 4 (22, 26, 30…. Etc). We usually abbreviate this as [4n+2] pi electrons.
- The molecule must be flat.
The structure of pyridine considerably resembles that of benzene. Pyridine is aromatic based on the following facts.
- The protons of pyridine display chemical shifts in the NMR spectrum that are typical of aromatic protons.
- The nitrogen of pyridine is sp2-hybridized and possesses one lone electron pair. This electron pair is located in a sp2 orbital that is parallel to the ring plane.
- As an aromatic compound, pyridine may participate in electrophilic substitutions as an electrophile.
- Pyridine is cyclic, conjugated, and has three pi bonds. There is a total of six pi electrons, which is a Huckel number and the molecule is aromatic.