Haloalkyl

Haloalkyl refers to an alkyl substituent in which one or more hydrogen atoms have been replaced by halogen atoms such as fluorine, chlorine, bromine, or iodine. The resulting substituent is attached to the rest of the molecule at the carbon bearing the halogen. Common mono-haloalkyl groups include chloromethyl (-CH2Cl) and bromomethyl (-CH2Br). Haloalkyl groups can also be formed at other positions on the chain, giving substituents such as 2-chloroethyl (-CH2-CH2Cl) or 3-chloropropyl (-CH2-CH2-CH2Cl). More heavily halogenated variants, such as dichloro- or difluoroalkyl groups, also occur.

Haloalkyl groups are reactive centers in organic synthesis. The carbon–halogen bond can act as a leaving group in substitution or elimination reactions, and the halogen can influence the reactivity of the neighboring carbon. Iodo- and bromo-containing haloalkyls generally undergo nucleophilic substitution more readily than their chloro- or fluoro-containing counterparts. These groups are frequently used to activate substrates for nucleophilic attack, to introduce functional handles for further derivatization, or to form carbon–carbon or carbon–heteroatom bonds.

In synthesis, haloalkyl substituents are introduced by halogenation of appropriate precursors or by halomethylation/haloethylation strategies. Safety and handling considerations are important, as haloalkyl compounds can be irritants, toxic, or environmentally persistent. Some haloalkyls are carcinogenic or acutely toxic, and many are regulated substances. Proper storage, handling, and disposal are essential.