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Introduction to the Chemistry of Alkyl Halides

Nomenclature of Alkyl Halides

Alkyl halides are formally derived from alkanes by exchanging hydrogen for halogen (fluorine, chlorine, bromine, and iodine). Depending on the degree of substitution at the carbon atom carrying the halogen, alkyl halides are classified into primary, secondary and tertiary alkyl halides.

Fig.1
Primary, secondary and tertiary alkyl halides (X = F, Cl, Br, or I)

Vinyl halides are often classified as the fourth type of alkyl halides.

Fig.2
Different representations of vinyl iodide

In the generally accepted nomenclature of alkyl halides, the name of the alkyl residue is followed by the name of the halide, e.g. methyl iodide and ethyl chloride. The IUPAC nomenclature (shown in brackets in the illustration below), considers an alkyl halide a substituted alkane, i.e. halogens are treated the same way as alkyl groups. The name of the halogen is followed by the name of the alkane, for example, iodomethane and chloromethane. If an alkyl halide contains more than one halogen, the halogen names are noted in alphabetical order, such as 1-chloro-2-iodobutane.

Fig.3
Examples of alkyl halide nomenclature

In the nomenclature of unsaturated alkyl halides, unsaturated bonds have a higher priority than halogen substituents. This requires special attention when numbering the carbon chain.

Fig.4
IUPAC nomenclature of some additional alkyl halides (haloalkanes)

Some commonly used chlorinated organic solvents, such as carbon tetrachloride (tetrachloromethane), chloroform (trichloromethane) and methylene chloride (dichloromethane), are known more by their trivial name than by their IUPAC name.

Exercises

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