Chapter 18 · Organic Chemistry
18.10Aromatic Hydrocarbons
8 min · two checks
Predict
Why doesn’t benzene behave like an alkene with three double bonds?
The idea
- Describe benzene’s bonding.
- Recognize aromatic substitution as the typical reaction.
Benzene is C₆H₆, a six-membered ring with every bond between a single and a double. Drawing it as alternating double bonds is a resonance picture; the real molecule is the blend, often drawn as a hexagon with a circle inside. The delocalized electrons make the ring unusually stable. Compounds built on this ring are aromatic hydrocarbons. Toluene is benzene with a methyl group. Naphthalene is two rings fused.
Because addition would break the aromatic set of electrons, benzene prefers substitution: an H on the ring is replaced by another group and the ring survives. That is why bromine water, which adds to alkenes quickly, does not decolorize with benzene under the same gentle conditions. Aromatic does not mean “smells nice,” though many of these compounds do have strong odors. It means this kind of ring.
Keep these
- Benzene’s bonds are equivalent; the ring is resonance-stabilized.
- Aromatic rings prefer substitution over addition.
- The circle-in-a-hexagon drawing means delocalized electrons.