Polyhalogen Compounds

DDT has the molecular formula C₁₄H₉Cl₅. Its IUPAC name is 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane. Structurally, it consists of two para-chlorophenyl rings bonded to a trichloromethyl-substituted central carbon. DDT was first synthesised in 1874 by Othmar Zeidler, but its insecticidal properties were discovered in 1939 by Paul Hermann Müller. During World War II, it was used extensively to control malaria and typhus by killing mosquitoes and lice. After the war, it became the world's most widely used agricultural pesticide. The molecule works by interfering with the sodium ion channels in insect nerve cell membranes, keeping them open and causing continuous nerve firing, paralysis, and death. However, because DDT is highly lipophilic (log K_ow ≈ 6.9), it accumulates in fatty tissues and undergoes biomagnification — concentrating up to 10 million times from water to apex predators like eagles. Rachel Carson's 1962 book Silent Spring documented how DDT caused thinning of bird eggshells (by inhibiting calcium-ATPase enzymes), leading to reproductive failure. Most countries banned DDT for agricultural use in the 1970s–1980s, though limited use for disease vector control continues in some regions under WHO guidelines.

Freons are a trade name (DuPont) for chlorofluorocarbons (CFCs), compounds containing carbon, fluorine, and chlorine. Common examples include Freon-11 (CCl₃F, trichlorofluoromethane), Freon-12 (CCl₂F₂, dichlorodifluoromethane), and Freon-22 (CHClF₂, chlorodifluoromethane). They were developed in the 1920s–1930s as safe refrigerants, propellants, and solvents, replacing toxic gases like ammonia and sulfur dioxide. Their appeal was their low toxicity, non-flammability, and chemical inertness in the lower atmosphere. However, Sherwood Rowland and Mario Molina showed in 1974 that CFCs migrate to the stratosphere, where UV radiation causes homolytic cleavage of the C–Cl bond: CCl₂F₂ + hν → •CClF₂ + Cl•. The chlorine radical then catalytically destroys ozone: Cl• + O₃ → ClO• + O₂, followed by ClO• + O• → Cl• + O₂. A single chlorine radical can destroy up to 100,000 ozone molecules before being deactivated. Ozone depletion increases UV-B radiation reaching Earth's surface, elevating rates of skin cancer and cataracts. The Montreal Protocol (1987) mandated the global phase-out of CFCs, which have been replaced by hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), though these are potent greenhouse gases themselves.

PCBs are a family of 209 compounds (congeners) derived from biphenyl (two connected benzene rings) with 1 to 10 chlorine atoms substituted at various positions. General formula: C₁₂H₁₀₋ₙClₙ, where n = 1–10. PCBs were manufactured from 1929 until the late 1970s under trade names such as Aroclor. They were used as electrical transformer insulating fluids, hydraulic fluids, and plasticisers due to their thermal stability and electrical insulating properties. Like DDT, PCBs are persistent organic pollutants (POPs) — they resist biodegradation, bioaccumulate in food chains, and are toxic to mammals. Certain congeners are dioxin-like and act as ligands for the aryl hydrocarbon receptor (AhR), disrupting hormone signalling and development. PCBs are classified as probable human carcinogens by the IARC. Their production and new use are banned under the Stockholm Convention.

Hexachlorobenzene (C₆Cl₆) has all six hydrogen atoms of benzene replaced by chlorine. It was used as a fungicide to protect wheat seeds from fungal disease (bunt). A major public health disaster occurred in Turkey between 1955 and 1959 when HCB-treated seed grain was consumed, causing toxic porphyria in over 3,000 people with a mortality rate of about 10%. HCB is an extremely stable aromatic compound due to the combined electron-withdrawing effect of six chlorine substituents, which makes the ring highly resistant to electrophilic attack. Its environmental persistence and toxicity resulted in a worldwide ban, and it is listed as one of the 'dirty dozen' under the Stockholm Convention.

• \ce{CCl2F2} — Freon-12 (Dichlorodifluoromethane) (mol): A chlorofluorocarbon refrigerant gas; the reactant that undergoes photolysis.
• h\nu — Photon energy (J): Ultraviolet radiation from the sun with sufficient energy to break the C–Cl bond (bond energy ≈ 339 kJ/mol).
• \ce{Cl^{\bullet}} — Chlorine radical (mol): Highly reactive free radical that catalytically destroys ozone molecules in a chain reaction.

Problem. A sample of lake water contains DDT at a concentration of 0.000003 g/L (3 × 10⁻⁶ g/L). Small fish feeding in the lake accumulate DDT at a concentration 10,000 times greater than the water. A large fish eats 200 small fish. What is the approximate DDT concentration (in g/kg, assuming fish density ≈ 1 kg/L) in the large fish if we assume 100% retention and the large fish has a body mass of 2 kg?

Answer. The large fish contains approximately 0.15 g/kg of DDT, representing a biomagnification factor of 50,000× relative to the surrounding water.

• Nucleophilic Substitution in Haloarenes — Understanding why polyhalogen aromatic compounds are so chemically stable requires knowledge of how electron-withdrawing halogens deactivate the ring toward electrophilic attack and can activate it (under forcing conditions) toward nucleophilic substitution.
• Bonding and Resonance in Haloarenes — The resonance donation of lone pairs from chlorine into the aromatic ring explains both the partial double-bond character of the C–Cl bond and the reluctance of haloarenes to undergo typical reactions, contributing to their environmental persistence.
• Persistent Organic Pollutants (POPs) — DDT, PCBs, and HCB are all classified as POPs. Understanding the chemical basis for persistence, bioaccumulation, and toxicity connects haloarene chemistry to real-world environmental regulation.
• Free Radical Reactions — The photolytic decomposition of CFCs and the subsequent ozone destruction cycle are driven by free radical chain reactions — a key mechanism in organic chemistry.