# What is an aromatic compound?

May 7, 2015

An aromatic compound is a cyclic compound that contains $4 n + 2$ electrons in a planar conjugated $\pi$ system (n = 0, 1, 2, …), meaning the electrons are delocalized throughout the molecule, promoting resonance stabilization.

The most common aromatic compounds contain benzene rings ($n = 1$).

But any planar cyclic compound that contains 2, 6, or 10 $\pi$ electrons is aromatic.

Examples are azulene, with 10 $\pi$ electrons, and pyridine, thiophene, and imidazole, each with 6 $\pi$ electrons---but only those electrons that are in the ring count as being delocalized.

In thiophene, one pair of sulfur's lone electrons resides in a $p$ orbital coplanar with the ring, protruding outwards, and is thus localized (if you consider those delocalized $\pi$ electrons in the ring to be in ${p}_{z}$ orbitals, then the coplanar electrons are in a $p$ orbital that lies on the $x y$-plane).

Similarly, pyridine's $s {p}^{2}$-nitrogen's lone pair and imidazole's $s {p}^{3}$-nitrogen's lone pair of electrons are also coplanar with the rest of the ring, on a $p$ orbital protruding outwards.

You can see this from pyridine's MO depiction, for example: