What are polyhalogen compounds?
Compounds with more than one halogen atom are polyhalogen compounds. Many are useful in industry and farming, but many also harm health or the environment. Count halogens on methane: CH₃Cl (1), CH₂Cl₂ (2), CHCl₃ (3), CCl₄ (4).
Dichloromethane (methylene chloride)
Uses: solvent, paint remover, propellant in aerosols, and a solvent in making medicines; also used to clean metal.
Effects: breathing it harms the central nervous system: low levels give poor hearing and vision; more gives dizziness, nausea, and tingling in fingers and toes. On the skin it burns and reddens; in the eyes it can burn the cornea.
Chloroform (trichloromethane)
Uses: solvent for fats, alkaloids and iodine; used to make freon refrigerant R-22. It was once a general anaesthetic but has been replaced by safer ones like ether.
Effects: breathing it depresses the nervous system; long exposure damages the liver and kidneys.
Storage: in air and light it oxidises to phosgene (COCl₂), a very poisonous gas: 2CHCl₃ + O₂ → 2COCl₂ + 2HCl. So it is kept in dark coloured, completely filled, closed bottles (no air), and a little ethanol is added, which turns any phosgene into harmless diethyl carbonate.
Iodoform (triiodomethane)
A yellow crystalline solid with a strong smell. It was used as an antiseptic for wound dressings; the antiseptic action comes from the iodine it slowly releases, not from the iodoform itself. Because of its bad smell it has been replaced by other iodine formulations.
The iodoform test (I₂ + NaOH gives yellow CHI₃) is used to detect a CH₃CO– or CH₃CH(OH)– group, such as in ethanol or acetone.
Carbon tetrachloride (tetrachloromethane)
Uses: was used to make refrigerants and aerosol propellants, as a solvent for oils and fats, in dry cleaning, and in fire extinguishers.
Effects: may cause liver cancer; harms the nervous system (dizziness, headache, nausea); can make the heart beat irregularly. When released, it reaches the upper atmosphere and depletes the ozone layer, letting more UV reach us (skin cancer, eye damage).
Freons (chlorofluorocarbons, CFCs)
Freons are chlorofluoro compounds of methane and ethane, such as CCl₂F₂ (freon-12). They are stable, unreactive, non-toxic, non-corrosive and easily liquefied, so they were used in fridges, ACs, aerosol sprays and foam making.
Effect: they drift up to the stratosphere. UV light breaks them into free radicals: CF₂Cl₂ → •CF₂Cl + Cl•. The Cl• starts a chain: Cl• + O₃ → ClO• + O₂; ClO• + O → Cl• + O₂. The Cl• is set free again, so one Cl can destroy many thousands of ozone molecules.
DDT (p,p′-dichlorodiphenyltrichloroethane)
DDT was the first chlorinated organic insecticide. Its insect-killing power was found in 1939 (Paul Müller received the Nobel Prize in 1948). It greatly reduced malaria (mosquitoes) and typhus (lice).
Problems: many insects became resistant. It is not easily broken down (persists for years) and it is soluble in fat, not water. So it collects in animal fat and increases at each level of a food chain: biomagnification. Fish-eating birds laid eggs with thin shells, and their numbers fell. DDT was banned in the USA in 1973; many countries now restrict it.
Board exam focus
Common questions (1–3 marks): uses and harmful effects of each compound; "why is chloroform stored in dark bottles?"; "how do freons deplete ozone?"; "why is DDT harmful even though it is not very toxic to humans at once?"; percentage composition numericals.
Key formulas and definitions
- CH₂Cl₂ dichloromethane; CHCl₃ chloroform; CHI₃ iodoform; CCl₄ carbon tetrachloride; CCl₂F₂ freon-12
- 2CHCl₃ + O₂ → 2COCl₂ + 2HCl (light)
- CF₂Cl₂ → •CF₂Cl + Cl• (UV)
- Cl• + O₃ → ClO• + O₂; ClO• + O → Cl• + O₂
- Mass % of element = (mass of element in 1 mol ÷ molar mass) × 100
Worked examples
1. Why is chloroform stored in dark coloured, fully filled bottles?
Light and air oxidise chloroform to poisonous phosgene. A dark bottle blocks light; a full bottle leaves no air. A little ethanol is also added to destroy any phosgene formed.
2. Find the percentage of chlorine in chloroform (C = 12, H = 1, Cl = 35.5).
M = 12 + 1 + 3×35.5 = 119.5 g/mol. Mass of Cl = 106.5 g. %Cl = 106.5 ÷ 119.5 × 100 = 89.1%.
3. Find the molar mass of freon-12, CCl₂F₂ (F = 19).
M = 12 + 2×35.5 + 2×19 = 12 + 71 + 38 = 121 g/mol.
4. Find the percentage of iodine in iodoform (I = 127).
M = 12 + 1 + 3×127 = 394 g/mol. %I = 381 ÷ 394 × 100 = 96.7%.
5. How many moles of phosgene form from 23.9 g of chloroform if it all oxidises?
Moles CHCl₃ = 23.9 ÷ 119.5 = 0.2 mol. 2CHCl₃ → 2COCl₂, so 1:1 → 0.2 mol COCl₂ (0.2 × 99 = 19.8 g).
6. DDT in water is 0.00005 ppm and in fish-eating birds 25 ppm. By what factor has it been magnified?
25 ÷ 0.00005 = 500 000 times. This build-up along the food chain is biomagnification.
Common mistakes
- Saying iodoform itself kills germs: its antiseptic effect comes from the iodine it releases.
- Thinking freons are harmful because they are poisonous: they are non-toxic; the harm is ozone depletion in the stratosphere.
- Writing chloroform oxidises to CO₂: the dangerous product is phosgene, COCl₂.
- Thinking DDT is dangerous only to insects: it builds up in fat of fish and birds (biomagnification).