Electrophilic substitution and directive influence

Arenes are non-polar, colourless liquids or solids with a typical smell. They do not mix with water but mix readily with organic solvents, and they burn with a sooty flame. Naphthalene balls repel moths.

Their typical reactions are electrophilic substitutions. Nitration uses conc. HNO₃ + conc. H₂SO₄; halogenation uses a Lewis acid (anhydrous FeCl₃, FeBr₃ or AlCl₃); sulphonation uses oleum; Friedel-Crafts alkylation uses R–X with anhydrous AlCl₃; Friedel-Crafts acylation uses an acyl halide or anhydride with AlCl₃. Excess Cl₂ with AlCl₃ goes all the way to C₆Cl₆.

The mechanism has three steps: the electrophile is generated; it attacks the ring to form an arenium ion (σ-complex) with one sp³ carbon, which is resonance-stabilised but no longer aromatic; then a proton is lost, removed by [AlCl₄]⁻ or [HSO₄]⁻, and aromaticity returns.

In nitration the electrophile is NO₂⁺ (nitronium ion), formed when H₂SO₄ gives a proton to HNO₃; here sulphuric acid acts as the acid and nitric acid as the base. AlCl₃ generates Cl⁺, R⁺ or RC⁺O (acylium ion).

Under severe conditions benzene adds too: H₂ over Ni at high temperature and pressure gives cyclohexane, and three Cl₂ under UV light give benzene hexachloride, C₆H₆Cl₆ (gammaxane). Combustion: C₆H₆ + 15/2 O₂ → 6CO₂ + 3H₂O.

In a monosubstituted benzene the group already present, not the incoming one, decides where the next group goes. This is the directive influence.

Ortho and para directors that activate the ring: –OH, –NH₂, –NHR, –NHCOCH₃, –OCH₃, –CH₃, –C₂H₅. In phenol, resonance raises electron density at o- and p-positions more than the –I effect lowers it.

Halogens are o- and p-directing but deactivating: their strong –I effect lowers electron density on the whole ring, while resonance keeps o- and p-positions richer than meta.

Meta directors deactivate the ring: –NO₂, –CN, –CHO, –COR, –COOH, –COOR, –SO₃H. In nitrobenzene, resonance and the –I effect pull electron density mostly from o- and p-positions, so the meta position is attacked.

Benzene and polynuclear hydrocarbons with more than two fused benzene rings are toxic and carcinogenic. They form when tobacco, coal or petroleum burns incompletely and, inside the body, can damage DNA.