14.12: Halogenated Hydrocarbons
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Learning Objectives
- To name halogenated hydrocarbons given formulas and write formulas for these compounds given names.
Many organic compounds are closely related to the alkanes. As we noted previously, alkanes react with halogens to produce halogenated hydrocarbons, the simplest of which have a single halogen atom substituted for a hydrogen atom of the alkane. Even more closely related are the cycloalkanes, compounds in which the carbon atoms are joined in a ring, or cyclic fashion.
The reactions of alkanes with halogens produce halogenated hydrocarbons, compounds in which one or more hydrogen atoms of a hydrocarbon have been replaced by halogen atoms:
The replacement of only one hydrogen atom gives an alkyl halide (or haloalkane). The common names of alkyl halides consist of two parts: the name of the alkyl group plus the stem of the name of the halogen, with the ending -ide . The IUPAC system uses the name of the parent alkane with a prefix indicating the halogen substituents, preceded by number indicating the substituent’s location. The prefixes are fluoro -, chloro -, bromo -, and iodo -. Thus CH 3 CH 2 Cl has the common name ethyl chloride and the IUPAC name chloroethane. Alkyl halides with simple alkyl groups (one to four carbon atoms) are often called by common names. Those with a larger number of carbon atoms are usually given IUPAC names.
Example \(\PageIndex{1}\)
Give the common and IUPAC names for each compound.
- CH 3 CH 2 CH 2 Br
- (CH 3 ) 2 CHCl
Solution
- The alkyl group (CH 3 CH 2 CH 2 –) is a propyl group, and the halogen is bromine (Br). The common name is therefore propyl bromide. For the IUPAC name, the prefix for bromine (bromo) is combined with the name for a three-carbon chain (propane), preceded by a number identifying the carbon atom to which the Br atom is attached, so the IUPAC name is 1-bromopropane.
- The alkyl group [(CH 3 ) 2 CH–] has three carbon atoms, with a chlorine (Cl) atom attached to the middle carbon atom. The alkyl group is therefore isopropyl, and the common name of the compound is isopropyl chloride. For the IUPAC name, the Cl atom (prefix chloro -) attached to the middle (second) carbon atom of a propane chain results in 2-chloropropane.
Exercise \(\PageIndex{1}\)
Give common and IUPAC names for each compound.
- CH 3 CH 2 I
- CH 3 CH 2 CH 2 CH 2 F
Example \(\PageIndex{2}\)
Give the IUPAC name for each compound.
Solution
- The parent alkane has five carbon atoms in the longest continuous chain; it is pentane. A bromo (Br) group is attached to the second carbon atom of the chain. The IUPAC name is 2-bromopentane.
- The parent alkane is hexane. Methyl (CH 3 ) and bromo (Br) groups are attached to the second and fourth carbon atoms, respectively. Listing the substituents in alphabetical order gives the name 4-bromo-2-methylhexane.
Exercise \(\PageIndex{2}\)
Give the IUPAC name for each compound.
A wide variety of interesting and often useful compounds have one or more halogen atoms per molecule. For example, methane (CH 4 ) can react with chlorine (Cl 2 ), replacing one, two, three, or all four hydrogen atoms with Cl atoms. Several halogenated products derived from methane and ethane (CH 3 CH 3 ) are listed in Table \(\PageIndex{1}\), along with some of their uses.
| Formula | Common Name | IUPAC Name | Some Important Uses |
|---|---|---|---|
| Derived from CH 4 | |||
| CH 3 Cl | methyl chloride | chloromethane | refrigerant; the manufacture of silicones, methyl cellulose, and synthetic rubber |
| CH 2 Cl 2 | methylene chloride | dichloromethane | laboratory and industrial solvent |
| CHCl 3 | chloroform | trichloromethane | industrial solvent |
| CCl 4 | carbon tetrachloride | tetrachloromethane | dry-cleaning solvent and fire extinguishers (but no longer recommended for use) |
| CBrF 3 | halon-1301 | bromotrifluoromethane | fire extinguisher systems |
| CCl 3 F | chlorofluorocarbon-11 (CFC-11) | trichlorofluoromethane | foaming plastics |
| CCl 2 F 2 | chlorofluorocarbon-12 (CFC-12) | dichlorodifluoromethane | refrigerant |
| Derived from CH 3 CH 3 | |||
| CH 3 CH 2 Cl | ethyl chloride | chloroethane | local anesthetic |
| ClCH 2 CH 2 Cl | ethylene dichloride | 1,2-dichloroethane | solvent for rubber |
| CCl 3 CH 3 | methylchloroform | 1,1,1-trichloroethane | solvent for cleaning computer chips and molds for shaping plastics |
To Your Health: Halogenated Hydrocarbons
Once widely used in consumer products, many chlorinated hydrocarbons are suspected carcinogens (cancer-causing substances) and also are known to cause severe liver damage. An example is carbon tetrachloride (CCl 4 ), once used as a dry-cleaning solvent and in fire extinguishers but no longer recommended for either use. Even in small amounts, its vapor can cause serious illness if exposure is prolonged. Moreover, it reacts with water at high temperatures to form deadly phosgene (COCl 2 ) gas, which makes the use of CCl 4 in fire extinguishers particularly dangerous.
Ethyl chloride, in contrast, is used as an external local anesthetic. When sprayed on the skin, it evaporates quickly, cooling the area enough to make it insensitive to pain. It can also be used as an emergency general anesthetic.
Bromine-containing compounds are widely used in fire extinguishers and as fire retardants on clothing and other materials. Because they too are toxic and have adverse effects on the environment, scientists are engaged in designing safer substitutes for them, as for many other halogenated compounds.
To Your Health: Chlorofluorocarbons and the Ozone Layer
Alkanes substituted with both fluorine (F) and chlorine (Cl) atoms have been used as the dispersing gases in aerosol cans, as foaming agents for plastics, and as refrigerants. Two of the best known of these chlorofluorocarbons (CFCs) are listed in Table \(\PageIndex{1}\).
Chlorofluorocarbons contribute to the greenhouse effect in the lower atmosphere. They also diffuse into the stratosphere, where they are broken down by ultraviolet (UV) radiation to release Cl atoms. These in turn break down the ozone (O 3 ) molecules that protect Earth from harmful UV radiation. Worldwide action has reduced the use of CFCs and related compounds. The CFCs and other Cl- or bromine (Br)-containing ozone-destroying compounds are being replaced with more benign substances. Hydrofluorocarbons (HFCs), such as CH 2 FCF 3 , which have no Cl or Br to form radicals, are one alternative. Another is hydrochlorofluorocarbons (HCFCs), such as CHCl 2 CF 3 . HCFC molecules break down more readily in the troposphere, and fewer ozone-destroying molecules reach the stratosphere.
Key Takeaway
- The replacement of an hydrogen atom on an alkane by a halogen atom—F, Cl, Br, or I—forms a halogenated compound.