Polyhalogen compounds

Carbon compounds with more than one halogen atom are polyhalogen compounds; many are used in industry and agriculture.

Dichloromethane (methylene chloride) is a paint remover, aerosol propellant, process solvent for making drugs, and metal cleaning and finishing solvent. It harms the central nervous system: low levels in air slightly impair hearing and vision; higher levels bring dizziness, nausea, tingling and numbness in fingers and toes. On skin it burns and reddens; in the eye it can burn the cornea.

Chloroform (trichloromethane) dissolves fats, alkaloids and iodine; its main use today is making the refrigerant freon R-22. Once a general anaesthetic, it has been replaced by safer ones such as ether.

Breathing about 900 ppm chloroform briefly causes dizziness, fatigue and headache; long exposure can damage the liver (where it is metabolised to phosgene) and kidneys. In light, air slowly oxidises it to the very poisonous carbonyl chloride (phosgene), so it is kept in closed, dark bottles filled to the top.

Iodoform (triiodomethane) was used as an antiseptic, but the action came from free iodine it released, not from iodoform itself; its unpleasant smell led to iodine-containing replacements.

Carbon tetrachloride (tetrachloromethane) is used to make refrigerants and aerosol propellants, as a feedstock for chlorofluorocarbons, in pharmaceutical manufacturing and as a solvent; until the mid 1960s it was also a cleaning fluid, spot remover and fire extinguisher.

CCl₄ exposure can cause dizziness, nausea and vomiting with lasting nerve damage and, in severe cases, stupor, coma or death; it can make the heart beat irregularly or stop, and some evidence links it to liver cancer. Released into air, it rises and depletes the ozone layer, which raises UV exposure (skin cancer, eye disease, possible immune disruption).

Freons are the chlorofluorocarbons of methane and ethane: extremely stable, unreactive, non-toxic, non-corrosive, easily liquefied gases. Freon 12 (CCl₂F₂), a common industrial one, is made from CCl₄ by the Swarts reaction. By 1974 world output was about 2 billion pounds a year.

Freons used in aerosols, refrigeration and air conditioning eventually escape and diffuse unchanged into the stratosphere, where they start radical chain reactions that disturb the natural ozone balance.

DDT (p,p′-dichlorodiphenyltrichloroethane), a chlorinated insecticide, was made in 1873; Paul Muller of Geigy found its insecticidal power in 1939 and received the 1948 Nobel Prize in Physiology or Medicine. It was widely used against malaria mosquitoes and typhus-carrying lice after World War II.

Trouble with DDT: insects became resistant and it is highly toxic to fish. It is chemically stable and fat-soluble, is metabolised slowly and is stored in fatty tissue, so it builds up over time. The USA banned it in 1973, though some other countries still use it.