Benzene's π electron cloud acts as a nucleophile toward electrophiles. Unlike alkenes, benzene undergoes substitution (not addition) to maintain aromaticity — because aromaticity provides exceptional stability. The general mechanism: (1) Electrophile (E⁺) attacks benzene → arenium ion (σ-complex/Wheland intermediate — loses aromaticity temporarily). (2) Loss of H⁺ restores aromaticity → substituted product. In this case: AlCl₃ activates Cl₂ → Cl⁺ (electrophile). Cl⁺ substitutes each H on benzene progressively until all 6 H atoms are replaced → hexachlorobenzene.

Under UV light, Cl₂ undergoes homolytic cleavage → Cl• radicals. These radicals add across the double bonds of benzene (which temporarily loses its aromaticity). Three moles of Cl₂ add to one mole of benzene (across 3 double bonds): C₆H₆ + 3Cl₂ → C₆H₆Cl₆. The product BHC (Benzene HexaChloride) has 6 C atoms, 6 H atoms, and 6 Cl atoms — formula C₆H₆Cl₆. γ-isomer (lindane) was used as a pesticide/insecticide (now banned in many countries due to toxicity and persistence).

Substituents already on benzene ring direct incoming electrophile to ortho/para or meta positions and either activate (increase reactivity) or deactivate (decrease reactivity) the ring. Ortho/para directors (activating): −NH₂, −OH, −OCH₃, −NHCOCH₃, −alkyl. These donate electrons to ring via resonance or induction. Ortho/para directors (deactivating): −X (halogens) — withdraw by induction but donate by resonance (net deactivating). Meta directors (deactivating): −NO₂, −CN, −COOH, −CHO, −SO₃H, −COOR. These withdraw electrons from ring, making meta position relatively less deactivated.

Benzene (C₆H₆) is a flat, regular hexagon. All C–C bond lengths are equal (1·40 Å — between single 1·54 Å and double 1·34 Å). All bond angles = 120°. The 6 π electrons are fully delocalised over the ring — one electron per carbon in p-orbital, all perpendicular to ring plane. Hückel's rule: aromaticity requires (4n+2) π electrons (n = 0, 1, 2...). For benzene: 6 = 4(1)+2 → n=1 ✓. Resonance energy = ~150 kJ/mol — this is why benzene prefers substitution over addition (to retain aromaticity and this stabilisation).

DDT (dichlorodiphenyltrichloroethane) and BHC (lindane) are organochlorine pesticides. They were widely used in agriculture and malaria control (DDT kills Anopheles mosquitoes). Problems: extremely persistent in environment (don't biodegrade easily), accumulate in food chains (biomagnification) — higher concentrations at each trophic level. Found in human fat tissue, breast milk. DDT banned in most countries (Stockholm Convention, 2004). Still used in some tropical countries for malaria vector control — no alternatives are as cheap/effective. Environmental impact: caused decline of birds of prey (thinning of eggshells).

Unlike aliphatic halides, aryl halides (C₆H₅Cl) are very resistant to nucleophilic substitution because: (1) Cl is attached to sp² carbon (stronger C–Cl bond). (2) Cl lone pairs are delocalised into ring (partial double bond character). (3) Nucleophile approach is blocked by ring π electrons. NAS occurs only when strong electron-withdrawing groups (−NO₂) are present ortho/para to the leaving group — they stabilise the negative intermediate (Meisenheimer complex). Example: 2,4-dinitrochlorobenzene + NaOH → 2,4-dinitrophenol (NAS).