In dehydrohalogenation reactions, the halogen and an adjacent proton are removed from halocarbons, thus forming an alkene.  For example, with bromoethane and sodium hydroxide (NaOH) in ethanol, the hydroxide ion HO abstracts a hydrogen atom. A Bromide ion is then lost, resulting in ethene, HO and NaBr. Thus, haloalkanes can be converted to alkenes. Similarly, dihaloalkanes can be converted to alkynes.
In related reactions, 1,2-dibromocompounds are debrominated by zinc dust to give alkenes and geminal dihalides can react with strong bases to give carbenes.