alkylhalid

An alkyl halide, also called a haloalkane, is an organic compound in which one or more hydrogen atoms on an alkyl group have been replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). They have the general formula R–X, where R is an alkyl group and X is a halogen. They are commonly named by prefixing the alkyl name with the halogen, for example chloromethane, bromobutane, or tert-butyl iodide.

Alkyl halides are classified by the halogen (fluoro-, chloro-, bromo-, iodo-) and by the degree of substitution of the carbon bearing the halogen (primary, secondary, tertiary). They can also be polyhalogenated. The leaving-group ability of the halogen follows I > Br > Cl > F, and the C–X bond strength generally decreases from C–F to C–I, influencing reactivity in substitution and elimination reactions. Physical properties depend on molecular size and halogen type, with higher molecular weight and heavier halogens usually raising boiling points.

Key reactions involve the polarized C–X bond. Nucleophilic substitution can proceed via SN2 (favored by methyl and primary halides) or SN1 (favored by tertiary halides). Elimination can occur as E2 or, for some substrates, E1, forming alkenes. Common nucleophiles include OH–, CN–, OR–, and PR3 derivatives (in specialized methods). Alkyl halides are versatile building blocks in organic synthesis, enabling formation of alcohols, nitriles, ethers, and hydrocarbons, among others.

Preparation methods include converting alcohols to alkyl halides with thionyl chloride (SOCl2) or phosphorus trichloride (PCl3/PCl5), or via the Appel reaction (CCl4 with PPh3). They can also be formed by hydrohalogenation of alkenes or by radical halogenation of alkanes under appropriate conditions. Safety concerns include toxicity and potential carcinogenicity of some halides, as well as respiratory or eye irritation for certain compounds.