Polyhalogen compounds
Carbon compounds with several halogen atoms are useful solvents, refrigerants and insecticides, but many harm health or the ozone layer, so their use is now restricted.
Uses and harmful effects
| Compound | Uses | Harmful effects |
|---|---|---|
| Dichloromethane, CH₂Cl₂ | Solvent, paint remover, aerosol propellant, solvent in making medicines, cleaning and finishing metals | Harms the central nervous system: low levels affect hearing and vision; higher levels cause dizziness, nausea and numb fingers and toes. Burns skin; can burn the cornea |
| Trichloromethane (chloroform), CHCl₃ | Solvent for fats, alkaloids and iodine; mainly used to make the refrigerant freon R-22; once a general anaesthetic | Vapour depresses the nervous system (dizziness, tiredness, headache). Long exposure can damage the liver and kidneys; skin sores |
| Triiodomethane (iodoform), CHI₃ | Once used as an antiseptic; it works because it releases free iodine | Objectionable smell, so it has been replaced by other iodine formulations |
| Tetrachloromethane (carbon tetrachloride), CCl₄ | Making refrigerants and aerosol propellants; raw material for CFCs and medicines; solvent; earlier a cleaning fluid and fire extinguisher | Some evidence of liver cancer; dizziness, nausea and vomiting; nerve damage; can make the heart beat irregularly; depletes the ozone layer |
| Freons (CFCs), e.g. CCl₂F₂ | Aerosol propellants, refrigeration and air conditioning | Very stable, so they reach the stratosphere and start reactions that destroy ozone |
| DDT | Insecticide against the Anopheles mosquito (malaria) and lice (typhus) | Insects became resistant; highly toxic to fish; broken down slowly and stored in fatty tissue, so it builds up along food chains |
Why chloroform is stored in dark bottles
- 1★2CHCl₃ + O₂ → 2COCl₂ + 2HClSlow oxidation by air in the presence of light
- 2The product, carbonyl chloride (phosgene), is an extremely poisonous gas.
- 3So chloroform is kept in closed, dark coloured bottles, filled to the brim, so that light and air are kept out.
★ key step
Freons and the ozone layer
- Freons are the chlorofluorocarbons of methane and ethane. They are stable, unreactive, non-toxic, non-corrosive and easily liquefied, which made them ideal for fridges, air conditioners and spray cans.
- Freon-12 (CCl₂F₂) is made from CCl₄ by the Swarts reaction.
- Because they are so stable, freons drift unchanged up into the stratosphere.
- There, UV light breaks them and releases chlorine radicals, which start a chain reaction that destroys ozone.
- Less ozone lets more UV reach the Earth, raising the risk of skin cancer, eye disease and weaker immunity.
The DDT story
- DDT was first made in 1873. In 1939 Paul Müller found that it kills insects, and he received the Nobel Prize in Medicine in 1948.
- After the Second World War it was sprayed worldwide against the mosquitoes that spread malaria and the lice that carry typhus.
- By the late 1940s problems appeared: many insects became resistant, and DDT proved highly toxic to fish.
- It is chemically stable and fat soluble. Animals do not break it down quickly, so it is stored in fatty tissue and builds up over time.
- Its use was banned in the USA in 1973, though it is still used in some other parts of the world.
DDT was once sprayed widely in India to control malaria mosquitoes. Because it builds up in fat and harms fish and birds, India has banned its use in farming.
2 marks: “Why is chloroform stored in closed, dark coloured bottles completely filled?” In light, air oxidises it: 2CHCl₃ + O₂ → 2COCl₂ + 2HCl. Phosgene is extremely poisonous, so light and air are kept out.
2 marks: “Give two uses and two harmful effects of freons.” Uses: aerosol propellants, refrigeration and air conditioning. Harm: they reach the stratosphere and destroy ozone; more UV then raises skin cancer and eye disease.
3 marks: “What is DDT? Why was its use restricted even though it saved many lives?” p,p'-Dichlorodiphenyltrichloroethane, a chlorinated insecticide used against malaria mosquitoes. Insects became resistant, it is toxic to fish, and it is stable and fat soluble, so it builds up in animals.
Chloroform: dark, full, shut. Freon: up, up, ozone down. DDT: fat keeps it, fish feel it.