Reactions of benzene and directive influence

Benzene is electron-rich (its π cloud), so it is attacked by electrophiles. But because its ring is so stable, it undergoes substitution (one H is replaced, and the aromatic ring is kept) rather than addition. The main electrophilic substitution reactions:

• Nitration: concentrated HNO₃ + concentrated H₂SO₄ give the electrophile NO₂⁺; the product is nitrobenzene.
• Halogenation: a halogen with a Lewis acid such as FeCl₃ or FeBr₃.
• Sulphonation: with oleum (fuming sulphuric acid).
• Friedel-Crafts alkylation: an alkyl halide with anhydrous AlCl₃ adds an alkyl group.
• Friedel-Crafts acylation: an acyl chloride (RCOCl) with AlCl₃ adds an acyl group (R–C=O).

The mechanism has three steps: (1) the electrophile is generated; (2) it attacks the ring, forming a carbocation called the arenium ion (σ-complex); (3) the ring loses H⁺ and regains its aromatic stability. Carbocations can rearrange to more stable forms: alkylation with 1-chloropropane gives isopropylbenzene, not propylbenzene.

Addition happens only under harsh conditions: hydrogen with a nickel catalyst gives cyclohexane; chlorine in UV light gives benzene hexachloride (gammaxane, once used as an insecticide). Benzene burns with a sooty flame, because of its high carbon content.

A group already on the ring decides where the next group goes (directive influence) and how fast:

• ortho/para directors, which are also activating (they push electrons into the ring, making it react faster): –OH, –NH₂, –NHR, –NHCOCH₃, –OCH₃, –CH₃, –C₂H₅. The halogens are the exception: ortho/para directing, but deactivating.
• meta directors, which are deactivating (they pull electrons out of the ring, making it react slower): –NO₂, –CN, –CHO, –COR, –COOH, –COOR, –SO₃H.

A health warning: benzene and polynuclear hydrocarbons (with more than two fused benzene rings) are toxic and can cause cancer (they are carcinogenic). They are formed by the incomplete burning of tobacco, coal and petroleum, which is one reason why cigarette smoke is so harmful.