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8.1: Alkyl Halides

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  • A class of simple alkyl halides called chlorofluorocarbons (CFC's) was once very widely used in aerosol sprays and as refrigerants. One such example is \(\ce{CCl_3F}\), or trichlorofluoromethane. Unfortunately, CFC's are harmful to the ozone layer of our upper atmosphere. Ozone is critical in limiting the amount of damaging ultraviolet radiation that reaches the Earth. CFC's react with the ozone and damage it, leaving the earth less protected.

    Beginning in the late 1970's, ozone depletion was recognized as a significant environmental issue. The most dramatic decrease in ozone occurs seasonally over the continent of Antarctica. The size and duration of the ozone hole steadily increased, with the largest hole recorded in 2006. Fortunately, most countries have recognized the danger of CFC's and dramatically curtailed their use in recent years. It is hoped that ozone depletion will slow and that the ozone layer may eventually be restored to its earlier levels.

    Alkyl Halides

    An alkyl halide is an organic compound in which one or more halogen atoms are substituted for one or more hydrogen atoms in a hydrocarbon. The general formulas for organic molecules with functional groups use the letter \(\ce{R}\) to stand for the rest of the molecule outside of the functional group. Because there are four possible halogen atoms (fluorine, chlorine, bromine, or iodine) that can act as the functional group, we use the general formula \(\ce{R-X}\) to represent an alkyl halide. The rules for naming simple alkyl halides are listed below.

    1. Name the parent compound by finding the longest continuous carbon atom chain that also contains the halogen. Add a prefix for the particular halogen atom. The prefixes for each of the four halogens are fluoro-, chloro-, bromo-, and iodo-. If more than one kind of halogen atom is present, put them in alphabetical order. If there is more than one of the same halogen on a given carbon atom, use the prefixes di-, tri-, or tetra- before the prefix for the halogen.
    2. As with hydrocarbons, number the carbon chain in a way that makes the sum of halogen numbers as low as possible. If different halogens are in equivalent positions, give the lower number to the one that comes first in alphabetical order.
    3. Add the numerical prefix into the name before the halogen prefix.
    4. Separate numbers with commas and separate numbers from names or prefixes with a hyphen. There are no spaces in the name.

    Listed below are some examples of names and structural formulas of a few alkyl halides.

    Note that for the structure based on methane, no number needs to be used since there is only one carbon atom. In the third example, the chloro- is listed first alphabetically and the chain is numbered so that the sum of the numbers is as low as possible.

    Uses of Alkyl Halides

    Alkyl halides are often used as synthetic intermediates in the laboratory. Although these compounds were once widely used as dry cleaning solvents, coolants in refrigerators and air conditioners, and propellants in hairsprays and deodorants, increasing awareness of their toxicity has led to a widespread decrease in applications for these materials. Some specific compounds are still used. Halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) is still used in some situations as an inhalation anesthetic. The compound DDT is a very effective pesticide, but is only used when nothing else works because of its harmful effects on the environment.

    Contributors and Attributions

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.