Alkyne Halogenation

Alkynes contain two π-bonds, so when they are exposed to electrophiles, they do exactly what you would expect them to do: they react TWICE with them. 

Halogens Add Twice to Alkynes

Keep in mind that for some of these reactions, vinyl carbocations will be created. While unstable, these cannot rearrage, so don’t worry about carbocation rearrangements when reacting with alkynes!

General Reaction:

  • Product: gem-dihalides

General Reaction:

  • Product: tetrahalides

Hydration of Alkynes Yield Tautomers

Vinyl alcohols (alcohols directly on a double bond) undergo a process called tautomerization. We will devote an entire chapter to this process next semester, so you aren’t expected to fully understand it yet.

For now, just memorize what the enol and keto forms look like, so you can predict the products that form when you add alcohol to an alkyne. 

Both acid-catalyzed hydration and oxymercuration-reduction of any alkyne leads to formation of a ketone. These reactions both yield a Markovnikov vinyl alcohol, which then tautomerizes.  

Concept: Anti-Markovnikov addition of alcohols to terminal alkynes yields aldehydes.   

3m

Hydroboration-oxidation of terminal alkynes leads to formation of aldehydes. (If the alkyne is not terminal, it will just yield a ketone). This reaction yields an anti-Markovnikov vinyl alcohol, which will tautomerize into a carbonyl on the terminal position, which is the definition of an aldehyde.