Aromatic Hydrocarbons

1. Introduction

Aromatic hydrocarbons (also called arenes) are cyclic, planar hydrocarbons that contain one or more benzene rings and show a special stability called aromaticity. The name "aromatic" originally referred to their pleasant smell, but today it refers to their characteristic chemistry. The parent aromatic compound is benzene, $\ce{C6H6}$.

2. Characteristics of Aromatic Compounds

Aromatic compounds are recognized by these features:

1. They are cyclic and planar.
2. They contain alternate single and double bonds (a fully conjugated system of $\pi$ electrons) around the ring.
3. The $\pi$ electrons are delocalized over the whole ring, giving unusual stability (resonance energy).
4. They undergo substitution rather than addition (they preserve the stable ring), unlike ordinary unsaturated compounds.
5. They obey Huckel's rule of aromaticity.

3. Huckel's Rule of Aromaticity

Huckel's rule states that a planar, cyclic, fully conjugated molecule is aromatic if it contains

$(4n + 2)\ \pi\ \text{electrons}, \qquad n = 0, 1, 2, 3, \dots$

For benzene, the three double bonds provide $6$ delocalized $\pi$ electrons; taking $n = 1$ gives $4(1) + 2 = 6$, so benzene is aromatic. Species with $4n$ $\pi$ electrons (e.g. cyclobutadiene) are anti-aromatic and unstable.