Aromatic hydrocarbons: preparation and properties
Aromatic hydrocarbons contain a benzene ring; the first member is benzene C₆H₆ and the general formula of benzene homologues is CₙH₂ₙ₋₆. In the structure proposed by Kekulé benzene is a planar ring of six carbons; modern data show that all C–C bonds are equal in length and the π-electrons are spread evenly around the ring, which makes the ring very stable. Names: methylbenzene (toluene) C₆H₅–CH₃, ethylbenzene; with two substituents there are 1,2- (ortho-), 1,3- (meta-) and 1,4- (para-) dimethylbenzene (xylenes). The C₆H₅– radical is phenyl and C₆H₅–CH₂– is benzyl. Preparation: dehydrogenating cyclohexane C₆H₁₂ → C₆H₆ + 3H₂, trimerizing acetylene 3C₂H₂ → C₆H₆; it is also isolated from coal tar and petroleum. Benzene undergoes substitution under ordinary conditions: halogenation C₆H₆ + Br₂ → C₆H₅Br + HBr (FeBr₃) and nitration C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (with H₂SO₄). It adds only under harsh conditions: hydrogenation over Ni C₆H₆ + 3H₂ → C₆H₁₂, chlorination in light gives hexachlorocyclohexane. Benzene resists KMnO₄, whereas in toluene only the side chain is oxidized, giving benzoic acid. Benzene is toxic and carcinogenic: it is handled only by specialists in ventilated places.
Draw Kekulé’s formula of benzene and the version with a circle inside the ring on the board and explain the difference. No experiments with benzene or its derivatives are done in class.