An allylic halide is an alkyl halide in which there is one or more halogen atoms on an allylic carbon. Allylic carbons are carbons bonded to carbon atoms that are doubly bonded to other carbon atoms.
Allylic halides are classified in the same way that other organic halides are. They undergo nucleophilic substitution reactions. The carbon bonded to the halogen contains three bulky groups. This contributes to steric hindrance. This prevents tertiary allylic halides from undergoing SN2 mechanisms as they can only react with nucleophiles via SN1 mechanisms. Primary and secondary allylic halides can react with nucleophiles via either SN1 or SN2 mechanisms.