20.09 Aromatic Hydrocarbons

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Friedrich Kekulé was a German chemist in the 1800's. He supposedly was thinking about the structure of the benzene ring as he fell asleep. While asleep, he dreamed of a snake eating its own tail. He used this idea to propose the cyclic structure of benzene. Whether or not he actually had this dream has been debated ever since. Whatever really happened, the tale has persisted until today.

Aromatic Hydrocarbons

Benzene is the parent compound of the large family of organic compounds known as aromatic compounds. Unlike cyclohexane, benzene only contains six hydrogen atoms, giving the impression that the ring is unsaturated and each carbon atom participates in one double bond. Two different structures with alternating single and double bonds around the ring can be written for benzene.

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In benzene, the true bonding between carbon atoms is neither a single nor a double bond. Rather, all of the bonds are a hybrid of a single and double bond. In benzene, the pi bonding electrons are free to move completely around the ring. Delocalized electrons are electrons that are not confined to the bond between two atoms, but are instead allowed to move between three or more. The delocalization of the electrons in benzene can best be shown by showing benzene with a ring inside the hexagon, with the hydrogen atoms understood.

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Delocalization of the electrons makes for a more stable molecule than a similar molecule that does not have delocalized electrons. Benzene is a more stable and less reactive compound than straight-chain hexenes. The \(sp^2\) (double bonds) hybridization of the carbon atoms results in a planar molecule as opposed to the puckered structure of cyclohexane. Benzene rings are common in a great number of natural substances and biomolecules. The figure below shows the structural formulas for vanilla and naphthalene. Naphthalene is a chemical which is commonly used in mothballs.

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