alkyl halide

Take an alkane — a plain chain of carbons and hydrogens — and swap one hydrogen for a halogen atom (fluorine, chlorine, bromine, or iodine). What you get is an alkyl halide: a carbon framework with a halogen handle bolted onto it. Chloroethane (CH3-CH2-Cl) and bromocyclohexane are everyday examples.

The general formula is R-X, where R is the alkyl (carbon) part and X is the halogen. The carbon–halogen bond is polar: the halogen is more electronegative, so it pulls the shared electrons toward itself, leaving the carbon with a partial positive charge (written C(delta+)). That slightly positive carbon is exactly the electron-poor spot a nucleophile is looking for, which is why alkyl halides are the classic substrate for nucleophilic substitution. Alkyl halides are also classified as primary, secondary, or tertiary depending on how many other carbons are attached to the carbon bearing the halogen — and that classification largely decides which substitution mechanism runs.

Beware a common point of confusion: an alkyl halide has the halogen on an sp3 carbon. If the halogen sits directly on a benzene ring (an aryl halide) or on a carbon double-bonded carbon (a vinyl halide), it does NOT undergo ordinary SN1/SN2 substitution — those bonds are stronger and the geometry blocks the usual pathways. The reactivity discussed here is specifically for halogens on saturated carbon.