Haloarenes and polyhalogen compounds

Why aryl halides resist nucleophilic substitution. Chlorobenzene does not react with NaOH under ordinary conditions (it needs 623 K and high pressure). Three reasons:

• Resonance: a lone pair on the chlorine spreads into the ring, giving the C–Cl bond partial double-bond character, so it is harder to break.
• The carbon is sp², which holds the bonding electrons more tightly and makes the bond shorter and stronger.
• The phenyl cation that SN1 would need is very unstable, and the ring blocks a backside SN2 attack.

Electron-withdrawing groups such as –NO₂ at the ortho or para positions make the substitution much easier: they draw negative charge away from the ring in the intermediate. The more nitro groups, the milder the conditions needed. A nitro group at the meta position has no such effect.

Electrophilic substitution of the ring. Halogens are an unusual case:

• By the inductive effect, they pull electrons out of the ring, so they deactivate it: haloarenes react more slowly than benzene.
• By resonance, their lone pairs push electron density back into the ortho and para positions, so substitution happens there.

Haloarenes undergo halogenation, nitration, sulphonation and Friedel-Crafts alkylation and acylation, and the para product is the main one (the ortho position is more crowded).

Polyhalogen compounds have several halogen atoms. They are very useful, but many are long-lasting and harmful:

• Dichloromethane (CH₂Cl₂) is a solvent and paint remover, but affects the nervous system.
• Chloroform (CHCl₃) is a solvent; it was once an anaesthetic. In air and light it slowly forms poisonous phosgene, so it is stored in dark, completely filled bottles.
• Carbon tetrachloride (CCl₄) was used in refrigerants and fire extinguishers.
• Freons (chlorofluorocarbons) were used in fridges, air-conditioners and aerosol sprays. They are very stable, so they drift up to the stratosphere, where they damage the ozone layer.
• DDT was the first chlorinated insecticide, used against the mosquitoes that spread malaria. But it is not broken down in nature: it builds up in the fat of animals along the food chain, harming birds in particular, and has been banned in many countries.