Aromatic compounds such as benzene are more stable than suggested from their structure. They undergo reactions which retain the aromatic ring system, and behave differently from alkenes or polyenes.
Aromatic compounds are cyclic and planar with sp2 hybridized atoms. They also obey the Hückel rule and have 4n2 π electrons where n1, 2, 3, ... Aromatic systems can be monocyclic or polycyclic, neutral, or charged.
The term aromatic was originally applied to benzene-like structures because of The distinctive aroma of these compounds, but the term now means something different in modern chemistry. Aromatic compounds undergo distinctive reactions which set them apart from other functional groups. They are highlyunsaturated compounds, but unlike alkenes and alkynes, they are relatively unreactive and will tend to undergo reactions which involve a retention of their unsaturation. We have already discussed the reasons for the stability of benzenein. Benzene is a six-membered ring structurewith three formal doublebonds (Fig. 1a). However, the six π electrons involved are not localized betweenany two carbon atoms. Instead, they are delocalized around the ring which results in an increased stability. This is why benzene is often written with a circle in the center of the ring to signify the delocalization of the six π electrons (Fig. 1b).
Reactions which disrupt this delocalization are not favored since it means a loss of stability, so benzene undergoes reactions where the aromatic ring system is retained. All six carbon atoms in benzene are sp2 hybridized, and the molecule itself is cyclic and planar – the planarity being necessary if the 2p atomic orbitals on each carbon atom are to overlap and result in delocalization.
An aromatic molecule must be cyclic and planar with sp2 hybridized atoms (i.e. conjugated), but it must also obey what is known as the Hückel rule. This rule states that the ring system must have 4n+2π electrons where n1, 2, 3, etc.
Therefore, ring systems which have 6, 10, 14, ... π electrons are aromatic. Benzene fits the Hückel rule since it has six π electrons. Cyclooctatetraene has eight π electrons and does not obey the Hückel rule. Although all the carbon atoms in the ring are sp2 hybridized, cyclooctatetraene reacts like a conjugated alkene. It is not planar, the π electrons are not delocalized and the molecule consists of alternating single and double bonds (Fig. 2a). However, the 18-membered cyclic system (Fig.2b) does fit the Hückel rule (n=4) and is a planar molecule with aromaticproperties and a delocalized π system.
It is also possible to get aromatic ions. The cyclopentadienyl anion and the cycloheptatrienyl cation are both aromatic (Fig. 3). Both are cyclic and planar, con-taining six π electrons, and all the atoms in the ring are sp2 hybridized.
Bicyclic and polycyclic systems can also be aromatic (Fig. 4).