Polyhalogen compounds

The essentials

Compounds with more than one halogen atom are called polyhalogen compounds. Many have been very useful as solvents, refrigerants or insecticides, but several are harmful to health or the environment.

Key points

• Dichloromethane (CH₂Cl₂): solvent, paint remover, propellant in aerosols, and process solvent in drug making. Exposure can harm the central nervous system; skin contact causes burning.
• Trichloromethane (chloroform, CHCl₃): solvent for fats, alkaloids and iodine; once used as an anaesthetic. In air and light it is slowly oxidised to the very poisonous phosgene (carbonyl chloride, COCl₂), so it is stored in closed dark-coloured bottles filled to the top.
• Triiodomethane (iodoform, CHI₃): used earlier as an antiseptic because of the free iodine it releases; replaced due to its objectionable smell.
• Tetrachloromethane (CCl₄): used in making refrigerants and propellants, as a solvent, and formerly in fire extinguishers. When released into the air it reaches the upper atmosphere and depletes the ozone layer.
• Freons (chlorofluorocarbons, e.g. Freon-12, CCl₂F₂): stable, non-toxic, non-corrosive and easily liquefiable; used in refrigeration, air conditioning and aerosols. In the stratosphere they break down to radicals that destroy ozone.
• DDT (p,p′-dichlorodiphenyltrichloroethane): the first chlorinated organic insecticide, used against malaria-carrying mosquitoes. It is not metabolised quickly, is stored in fatty tissue, and builds up along food chains; many countries have banned it.

Common mistakes

• Confusing chloroform (CHCl₃) with carbon tetrachloride (CCl₄).