Preparation of Alkynes

Alkynes are prepared by the dehydrohalogenation of a gem- or vic-dihalide. A dehydrohalogenation reaction is an elimination reaction in which a hydrogen and halogen are removed by a base, forming a $\pi$ bond. A gem-dihalide (or geminal dihalide) is an organic compound with two halides bonded to the same carbon. A vic-dihalide (or vicinal dihalide) is an organic compound with two halides bonded to adjacent carbons. Vic-dihalides are prepared by an addition reaction of Br₂ to an alkene. An addition reaction is a reaction in which two or more reactants combine to form products. It adds atoms or groups to the starting material. An example is an addition of Br₂ to an alkene.

Alkynes can be prepared from alkanes through a sequence of reactions. A radical halogenation reaction will add a halogen to an alkane, creating a haloalkane (halide attached to an alkyl chain). An elimination reaction converts the haloalkane to an alkene. The addition reaction of Br₂ to the alkene will create a vic-dihalide. Finally, the vic-dihalide undergoes a double dehydrohalogenation E2 elimination to create an alkyne. The double dehydrohalogenation E2 elimination reaction is a multistep mechanism that first creates a double bond when one halide and hydrogen are removed, and then a second halide and hydrogen are removed, creating a triple bond. Alkynes are also prepared by the double dehydrohalogenation E2 elimination of gem-dihalides. Gem-dihalides are prepared from the reaction of PCl₅ with a ketone. This double dehydrohalogenation E2 elimination reaction is also a multistep mechanism that first creates a double bond from removing one halide and hydrogen and then creates a triple bond from removing a second halide and hydrogen pair.