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An allylic halide has a halogen atom bonded to a carbon adjacent to a carbon-carbon double bond. The halogen-bearing carbon is not itself one of the two alkene carbons.

The adjacent π system can stabilize developing charge or participate in delocalized intermediates. Allylic halides can therefore undergo substitution with patterns different from an otherwise comparable saturated alkyl halide.

An organic compound containing a carbon-halogen bond. The bonding environment matters: a vinylic halide has the halogen directly attached to a double-bonded carbon, whereas an allylic halide has it attached to the adjacent saturated carbon.

A vinylic halide has a halogen atom bonded directly to a carbon of a carbon-carbon double bond. Vinyl chloride is the simplest common example.

Ordinary substitution at a saturated carbon cannot be transferred unchanged to this geometry and bonding. The distinction between a vinylic halide and an allylic halide is therefore mechanistically useful, even when both molecules contain an alkene and a halogen.