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9.4: Halogenated Hydrocarbons: Many Uses, Some Hazards

  • Page ID
    177589
  • Learning Objectives

    • Identify haloalkanes (alkyl halides) and know their occurrence, uses, and properties.
    • Learn how to name simple haloalkanes.

    Haloalkanes also known as alkyl halides or halogenoalkanes, are a group of organic compounds in which one or more halogen atoms are substituted for one or more hydrogen atoms in a hydrocarbon. Haloalkanes are widely used commercially and, consequently, are known under many chemical and commercial names. 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 and impact on the environment has led to a widespread decrease in applications for these materials.

    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.

    Haloalkanes have higher boiling points than alkanes containing the same number of carbons. They are at best only slightly soluble in water but tend to dissolve in organic solvents.

    Methyl chloride (chloromethane, CH3Cl) was a widely used , but its use has been discontinued due to its toxicity and flammability. Methyl chloride was also once used for producing lead-based gasoline additives (tetramethyllead).

    The most important use of methyl chloride today is as a in the production of polymers. Smaller quantities are used as a solvent in the manufacture of and in .

    Large amounts of methyl chloride are produced naturally in the by the action of sunlight on and in sea foam. However, all methyl chloride that is used in industry is produced synthetically. Most methyl chloride is prepared by reacting with , according to the

    CH3OH + HCl → CH3Cl + H2O

    This can be carried out either by bubbling hydrogen chloride gas through boiling methanol with or without a , or by passing combined methanol and hydrogen chloride vapors over an catalyst at 350 °C (662 °F).

    A smaller amount of chloromethane is produced by heating a mixture of and to over 400 °C (752 °F). However, this method also results in more highly chlorinated compounds such as , , and and is usually only used when these other products are also desired.

    Short chain haloalkanes such as (CH2Cl2), (chloroform, CHCl3) and (carbon tetrachloride, CCl4) are commonly used as hydrophobic solvents in chemistry. They were formerly very common in industry; however, their use has been greatly curtailed due to their toxicity and harmful environmental effects.

    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.

    Naming Alkyl Halides

    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.

    Chlorofluorocarbons and Fluorocarbons

    Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain only (C), (H), (Cl), and (F), produced as derivative of , , and . They are also commonly known by the brand name . The most common representative is (R-12 or Freon-12). Many CFCs have been widely used as , propellants (in aerosol applications), and solvents. Because CFCs contribute to in the upper , the manufacture of such compounds has been phased out under the , and they are being replaced with other products such as hydrofluorocarbons (HFCs)

    Chlorinated or fluorinated alkenes undergo polymerization. Important include (PVC), and (PTFE, or Teflon). Billions of kilograms of chlorodifluoromethane are produced annually as precursor to , the monomer that is converted into . The production of these materials releases substantial amounts of wastes. PVC and PTFE will be discussed in more detail in Chapter 10.

    Summary

    • Haloalkanes also known as alkyl halides or halogenoalkanes, are a group of organic compounds in which one or more halogen atoms are substituted for one or more hydrogen atoms in a hydrocarbon.
    • These compounds were once widely used as dry cleaning solvents, coolants in refrigerators and air conditioners, and propellants in hairsprays and deodorants
    • The use of haloalkanes has been greatly curtailed due to their toxicity and harmful environmental effects.

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