Alkyl Halides

The alkyl halides have the general form

where the R in the general form is typically an alkyl group with a halogen replacing one of the hydrogens. There can be multiple substitutions of halogens for hydrogens, and also variations where alkenes or alkynes are involved.

The simplest common alkyl halides are chloroform and carbon tetrachloride.

Chloroform was used as a medical anesthetic in the past, but has been found to be carcinogic. Carbon tetrachloride also has a long history of use in fire extinguishers, as a fabric cleaner, and as a solvent, but it causes liver damage and now is used much less.

An important class of alkyl halides are the chlorofluorocarbons (CFCs) which have been used in air conditioners, etc., as refrigerants. They face sharply curtailed use now because of danger to the ozone layer.

Chlorofluorocarbons (CFCs)

A group of alkyl halides which have found wide use as refrigerants are the CFCs. One of the simplest of them is marketed under the name Freon-12:

An agressive campaign is now being waged to remove CFCs from use as refrigerants because of danger to the ozone layer above the earth. The Earth's ozone layer is formed by the action of sunlight on ordinary oxygen in the atmosphere, and that ozone is a strong absorber of ultraviolet light from the sun. The major problem with CFCs is that they tend to be unreactive near the surface, and being lighter than air, travel to the upper atmosphere if released. There ultraviolet radiation can break off a chlorine, and that chlorine atom can react with an ozone (O₃) molecule to form chlorine monoxide (ClO). The ClO can react with oxygen to release the Cl to react again, destroying another ozone molecule. It is estimated that a chlorine can remain in the upper atmosphere for years, destroying thousands of O₃ molecules, degrading our shield against harmful ultraviolet radiation.

There is also concern about CFCs in the upper atmosphere as contributers to the greenhouse effect.