Bereiding van alkynen: dehydrohalogenering

In addition to the alkylation pathway, alkynes can also be prepared by dehydrohalogenation of vicinal or geminal dihalides.

Vicinal dihalides are compounds with halogens on adjacent carbons, whereas compounds with halogens on the same carbon are called geminal dihalides.

For example, a vicinal dichloride can be synthesized from an alkene by the addition of chlorine in the presence of an inert solvent like dichloromethane. While a geminal dichloride can be prepared by treating a ketone with phosphorous pentachloride.

In the presence of a strong base like sodium amide in liquid ammonia, the dihalides lose two equivalents of hydrogen halide through two successive E2 elimination reactions. Hence the name double dehydrohalogenation.

The first elimination reaction proceeds with the abstraction of a proton by sodium amide and simultaneous departure of the halide leaving group to form a haloalkene.

In the second elimination reaction, another equivalent of the base reacts with the haloalkene to yield the desired alkyne. Thus, at least two equivalents of sodium amide are required for the reaction to go to completion.

Similarly, treatment of geminal dihalides with sodium amide gives alkynes t