Alkynes can be prepared by double dehydrohalogenation of vicinal or geminal dihaloalkanes.
Double elimination from geminal dihaloalkanes
![Alkyne synthesis - Double elimination Sintesis alquinos - Doble eliminación](/images/stories/organica-i/alkynes/doble-eliminacion/doble-eliminacion01.png)
The mechanism of this reaction consists of two successive E2 deletions. In the first elimination the disposition of the leaving group with respect to the subtracted hydrogen must be anti.
![Alkyne synthesis by double elimination Síntesis de alquinos mediante doble eliminación](/images/stories/organica-i/alkynes/doble-eliminacion/doble-eliminacion02.png)
Double elimination from vicinal dihaloalkanes
![Alkyne synthesis by double elimination Síntesis de alquinos mediante doble eliminación](/images/stories/organica-i/alkynes/doble-eliminacion/doble-eliminacion03.png)
By means of double elimination, the triple bond between the carbons that have the bromines attached will be obtained.
![Alkyne synthesis by double elimination Síntesis de alquinos mediante doble eliminación](/images/stories/organica-i/alkynes/doble-eliminacion/doble-eliminacion04.png)
To obtain terminal alkynes from vicinal or geminal dihaloalkanes it is necessary to use 3 equivalents of sodium amide, due to the presence of acidic hydrogen in the alkyne.
![Alkyne synthesis by double elimination Síntesis de alquinos mediante doble eliminación](/images/stories/organica-i/alkynes/doble-eliminacion/doble-eliminacion05.png)
Double elimination to form terminal alkynes can also be done with potassium tert-butoxide in DMSO.