# What are aromatic hydrocarbons?

May 7, 2015

An aromatic hydrocarbon is a hydrocarbon that contains $4 n + 2$ electrons in a planar conjugated π system (n = 0, 1, 2, …).

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

Examples are toluene and trinitrotoluene (TNT).

But any planar cyclic hydrocarbon that contains 2, 6, 10, 14 … π electrons is aromatic.

Examples are naphthalene (10 π electrons) and phenanthrene (14 π electrons).

The ring doesn't have to contain a 6-membered ring, as long as it is planar contains $4 n + 2$ π electrons

Thus, [18]annulene is aromatic. It is planar and possesses $4 n + 2$ π electrons (with $n = 4$).