Why benzene does not behave like an alkene. Its structure and resonance, and Hückel's rule for aromatic rings. The electrophilic substitution reactions, and how a group already on the ring steers the next one. The addition reactions of benzene, and why some aromatic compounds cause cancer.
3 lessons, about 1 h 25 min3 simulations10 problems3 practice questionsPython in every lesson
Created by StemPy LabBenzene, , looks like a ring of three double bonds, but it does not react like an alkene. All its C-C bonds are long, and its six pi electrons are shared by the whole ring. This delocalisation makes it about more stable than three separate double bonds. A flat, fully conjugated ring with pi electrons is aromatic.
The pi cloud of benzene attacks electron-poor reagents, and the ring keeps its stability by substituting a hydrogen atom. The attack makes an arenium ion, and the loss of a proton restores the ring. Nitration uses nitric and sulphuric acids, halogenation needs a Lewis acid, sulphonation uses fuming sulphuric acid, and Friedel-Crafts reactions use an alkyl or acyl chloride with aluminium chloride.
A group already on the ring decides how fast the next substitution goes and where it goes. Electron-donating groups activate the ring and direct to the ortho and para positions. Electron-withdrawing groups deactivate it and direct to the meta position. Halogens deactivate but direct ortho and para. Benzene adds hydrogen or chlorine only under severe conditions. Large flat rings of carbon, such as benzo[a]pyrene, cause cancer.
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