Aromatic ring

An aromatic ring is a hydrocarbon, such as benzene, that has a low hydrogen to carbon ratio, and has low chemical reactivity due to aromatic resonance. In the case of benzene (formula C₆H₆), the carbon atoms form a hexagonal ring, with each carbon atom bonded in series at each point of the hexagon. Because of orbital hybridization, each carbon atom can bond to only one hydrogen. Although the traditional illustration of an aromatic ring appears to contain double bonds, in reality the extra pi electrons are equally distributed around the ring because of resonance. This results in an exceptionally stable molecule which does not undergo markovnikovian addition reactions. There are other shapes as well, including cyclooctatetraene, an octagonal aromatic ring, where the same ratio applies.

There are variations to the benzene molecule. One such variation is toluene (formula C₇H₈, also called methylbenzene) where a methyl group replaces a hydrogen atom. Other variations can have non-carbon substituents in the ring whose free electron pairs contribute to the de-localized aromatic field.

Aromatic rings can also form in groups or chains, where each ring is "joined at the hip." These are called polycyclic benzenoid aromatic hydrocarbons. One such case is naphthalene (formula C₁₀H₈), where two benzene rings share one carbon bond.

Aromatic rings were called aromatic in the early days of their discovery because they are fragrant, as they were obtained from resins or oils.