Alkynes undergo addition reactions in a way similar to alkenes, but since they have a triple bond (one sigma and two pi bonds), they can add more atoms across the bond.
Here’s how it happens, step by step in simple words:
- Nature of the triple bond:
- The triple bond has lots of electron density, so it attracts electrophiles (electron-loving species).
- One pi bond is broken first, and the alkyne becomes an alkene.
- If more reagent is available, the second pi bond can also break, turning the alkyne into an alkane.
- Types of addition reactions:
- Hydrogenation: Alkynes add hydrogen in the presence of a metal catalyst. Partial hydrogenation gives an alkene, and full hydrogenation gives an alkane.
- Halogenation: Alkynes react with halogens (like chlorine or bromine) to form dihalo-alkenes first, and then tetrahalo-alkanes if excess halogen is added.
- Hydrohalogenation: Alkynes add hydrogen halides (like HCl, HBr). Depending on conditions, Markovnikov or anti-Markovnikov products form.
- Hydration: Alkynes add water in the presence of an acid catalyst to give carbonyl compounds (aldehydes or ketones).
- Control of product:
- With one equivalent of reagent, the reaction usually stops at the alkene stage.
- With excess reagent, it goes further to the alkane stage.
In short: Alkynes add reagents stepwise — first forming an alkene, then an alkane (if more reagent is present).