vicdihalides

Vicinal dihalides, or vic-dihalides, are organic compounds in which two halogen atoms are attached to adjacent carbon atoms in a saturated or unsaturated hydrocarbon framework. The term distinguishes them from geminal dihalides, where both halogens reside on the same carbon.

Most commonly, vicinal dihalides are prepared by halogenation of alkenes: addition of a diatomic halogen (such as Br2, Cl2, or I2) across a carbon–carbon double bond gives 1,2-dihaloalkanes. The reaction generally proceeds with anti addition, placing the two halogens on opposite faces of the former double bond. A related route converts vicinal diols to the corresponding dihalides using reagents such as thionyl chloride (SOCl2) or phosphorus tribromide in the presence of a halide source.

A key property of vicinal dihalides is their tendency to undergo double dehydrohalogenation upon treatment with strong bases. This sequence removes two equivalents of hydrogen halide and yields an alkyne, first giving a vinyl halide and then the terminal or internal alkyne, depending on substrate. Vicinal dihalides can also participate in various substitution or cross-coupling reactions under catalytic conditions to assemble more complex molecules.

Examples and applications include 1,2-dibromoethane, derived from ethene, which can be converted to acetylene via double dehydrohalogenation. 1,2-dichloroalkanes are common vicinal dihalides used in synthesis and industry, though some, such as 1,2-dichloroethane, are hazardous. Vicinal dihalides thus serve as versatile intermediates in organic synthesis, particularly as precursors to alkynes.