haloalkenes

Haloalkenes are alkenes that bear one or more halogen substituents on the carbon framework. The halogen atoms are fluorine, chlorine, bromine, or iodine. When the halogen is directly attached to a double-bond carbon, the compound is typically called a vinyl halide. Haloalkenes can be mono- or polyhalogenated and may occur as vinyl halides or as haloalkenes with halogens on adjacent carbons.

Preparation of haloalkenes typically involves hydrohalogenation of alkynes to yield vinyl halides and dehydrohalogenation of vicinal dihalides to form vinyl halides with defined substitution. In synthetic sequences, haloalkenes serve as versatile building blocks because the carbon–halogen bond participates in cross-coupling reactions and other transformations. The C–X bond reactivity follows the general trend I > Br > Cl, with fluorine often being less reactive in many cross-coupling conditions.

Reactions and applications: Vinyl halides undergo palladium- and nickel-catalyzed cross-coupling with organometallic partners (such as in Suzuki, Negishi, and related reactions), enabling formation of new carbon–carbon bonds. They can also be converted to alkynes via dehydrohalogenation or reduced to alkanes. In industry, vinyl chloride is the major monomer for polyvinyl chloride (PVC); other haloalkenes are valuable intermediates in the synthesis of pharmaceuticals, agrochemicals, and fine chemicals. Safety: Haloalkenes, especially vinyl chloride and certain di- and polyhalogenated derivatives, can be toxic and carcinogenic; handling requires appropriate containment, ventilation, and protective equipment.