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7.2: Nomenclature of Alkyl Halides

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    28165
  • e Learning Objective is 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 in Section 12.7, 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with halogens produce halogenated hydrocarbons, compounds in which one or more hydrogen atoms of a hydrocarbon have been replaced by halogen atoms:

    halogenated hydrocarbons.jpg

    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 CH3CH2Cl 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 3

    Give the common and IUPAC names for each compound.

    1. CH3CH2CH2Br
    2. (CH3)2CHCl

    SOLUTION

    1. The alkyl group (CH3CH2CH2–) 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.
    2. The alkyl group [(CH3)2CH–] 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.

    Skill-Building Exercise

    Give common and IUPAC names for each compound.

    1. CH3CH2I

    2. CH3CH2CH2CH2F

    Example 4

    Give the IUPAC name for each compound.

    1. Ex4 1.jpg
    2. Ex4 2.jpg

    SOLUTION

    1. 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.
    2. The parent alkane is hexane. Methyl (CH3) 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.

    Skill-Building Exercise

    Give the IUPAC name for each compound.

    1. SB 1.jpg
    2. SB 2.jpg

    A wide variety of interesting and often useful compounds have one or more halogen atoms per molecule. For example, methane (CH4) can react with chlorine (Cl2), replacing one, two, three, or all four hydrogen atoms with Cl atoms. Several halogenated products derived from methane and ethane (CH3CH3) are listed in Table 12.6 "Some Halogenated Hydrocarbons", along with some of their uses.

    Table 12.6 Some Halogenated Hydrocarbons

    Formula Common Name IUPAC Name Some Important Uses
    Derived from CH4
    CH3Cl methyl chloride chloromethane refrigerant; the manufacture of silicones, methyl cellulose, and synthetic rubber
    CH2Cl2 methylene chloride dichloromethane laboratory and industrial solvent
    CHCl3 chloroform trichloromethane industrial solvent
    CCl4 carbon tetrachloride tetrachloromethane dry-cleaning solvent and fire extinguishers (but no longer recommended for use)
    CBrF3 halon-1301 bromotrifluoromethane fire extinguisher systems
    CCl3F chlorofluorocarbon-11 (CFC-11) trichlorofluoromethane foaming plastics
    CCl2F2 chlorofluorocarbon-12 (CFC-12) dichlorodifluoromethane refrigerant
    Derived from CH3CH3
    CH3CH2Cl ethyl chloride chloroethane local anesthetic
    ClCH2CH2Cl ethylene dichloride 1,2-dichloroethane solvent for rubber
    CCl3CH3 methylchloroform 1,1,1-trichloroethane solvent for cleaning computer chips and molds for shaping plastics

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