20.1 Introduction
Benzene is a compound with six carbon atoms and six hydrogen atoms. It contains three pi bonds. Benzene and its derivatives are often called aromatic compounds. They were originally called this because of the odor these compounds had. But, more recently, the term “aromatic” has come to mean that these compounds are unusually stable.
When three pi bonds are drawn in the benzene ring of dimethylbenzene, it would incorrectly predict that two different isomers exist. One of the isomers would have a double bond between the methyl groups and the other would have a single bond between the methyl groups. In fact, there is only one type of dimethylbenzene. Somehow, both of these forms of dimethylbenzene are the same.
One way of thinking about how these two are the same is to consider them two resonance forms of each other. Each bond of the benzene ring is halfway between a double and single bond. We say that each of these bonds has a bond order of 1.5. The resonance hybrid of a benzene compound is sometimes drawn with a circle in the benzene ring. This is sometimes called the toilet bowl structure.
The toilet bowl structure is a pretty good approximation for the bonding in a benzene ring. With three pi bonds, each carbon atom of the benzene ring is sp2 hybridized with an unhybridized p-orbital on it. These p-orbitals can all overlap causing equally sharing the pi electrons around the ring.
The unusual stability of benzene
The pi bonds in benzene do not react like normal pi bonds. Many of the alkene reactions we learned previously do not apply to benzene rings. For instance, the bromination of an alkene is simple, yet it does not occur with a benzene ring.
We see that the benzene ring is unusually stable. Its pi bonds do not behave like regular pi bonds. Why is benzene so stable? How can we explain that? At first glance, you might think benzene is so stable because we can draw resonance forms with it. But, this is incorrect. We can draw resonance forms with other similar compounds, yet they are not very stable.