Key ideas
Aromatic compounds such as benzene are unusually stable because of a ring of delocalized pi electrons. They keep the ring intact and undergo substitution rather than addition.
- Hückel’s rule: a planar, cyclic, fully conjugated ring with 4n + 2 pi electrons (2, 6, 10 …) is aromatic. 4n electrons make it antiaromatic.
- Benzene undergoes electrophilic substitution: nitration (HNO₃/H₂SO₄), halogenation (X₂/FeX₃), sulfonation and Friedel–Crafts alkylation and acylation.
- Activating groups (–OH, –NH₂, –OR, –CH₃) direct the next group to the ortho and para positions. Deactivating groups (–NO₂, –COOH, –CN, –CHO) direct meta. Halogens deactivate but are ortho/para directors.
- A side chain with a benzylic hydrogen is oxidized by hot KMnO₄ to –COOH, whatever its length.
Key equations
aromatic: planar, cyclic, conjugated, 4n + 2 π electronsC₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (H₂SO₄)ArCH₃ → ArCOOH (KMnO₄)
Worked example
Nitration of toluene (–CH₃ activates) gives mostly ortho- and para-nitrotoluene, and it is faster than for benzene. Nitration of nitrobenzene (–NO₂ deactivates) goes slowly and gives mainly 1,3-dinitrobenzene. The cyclopentadienyl anion has six pi electrons (4n + 2 with n = 1) and is aromatic, but cyclobutadiene has four and is antiaromatic.
Common mistakes
- Expecting benzene to add bromine like an alkene. It substitutes and needs a catalyst.
- Using a meta director to predict ortho/para products, or the reverse.
- Counting a lone pair that is not in the pi system toward the 4n + 2 electrons.
Practice
Test yourself with practice questions on this topic: Aromatic compounds