Alkynes can be prepared by the double E2 elimination of vic-dihalides (two halogens on neighboring carbons) or by the double E2 elimination of gem-dihalides (two halogens on the same carbon).
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 π 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.
Conversion of an Alkane to 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.