Reactions of Alkynes: MDCAT Chemistry notes
Reactions of Alkynes for MDCAT: hydrogenation, halogen and HX addition by Markovnikov rule, hydration to aldehyde or ketone, acidity and oxidation.
Why alkynes react
The triple bond contains one $\sigma$ and two $\pi$ bonds. The $\pi$ electrons are exposed, so alkynes undergo electrophilic addition, usually in two stages: alkyne $\rightarrow$ alkene derivative $\rightarrow$ alkane derivative. Alkynes are generally less reactive than alkenes towards electrophilic addition.
Addition reactions
- Hydrogen: with Ni or Pt, ethyne takes up two $\mathrm{H_2}$ to give ethane. With a poisoned (Lindlar's) catalyst the reaction stops at the alkene.
- Halogens: bromine in $\mathrm{CCl_4}$ is decolourised. Ethyne gives 1,2-dibromoethene and then 1,1,2,2-tetrabromoethane.
- Hydrogen halides: both HX molecules add by Markovnikov's rule. After the first addition the halogen already on carbon directs the second H to the other carbon, so both halogens end up on the same carbon, giving a geminal dihalide: $$\mathrm{HC\equiv CH \xrightarrow{HBr} CH_2{=}CHBr \xrightarrow{HBr} CH_3{-}CHBr_2}$$ The product is 1,1-dibromoethane, not 1,2-dibromoethane.
Hydration (Kucherov reaction)
Alkynes add water when warmed with dilute $\mathrm{H_2SO_4}$ and $\mathrm{HgSO_4}$ (mercuric ion catalyst). An unstable enol forms first and tautomerises to a carbonyl compound.
| Alkyne | Enol | Final product |
|---|---|---|
| Ethyne, $\mathrm{HC\equiv CH}$ | Vinyl alcohol | Ethanal (acetaldehyde) |
| Propyne, $\mathrm{CH_3C\equiv CH}$ | Prop-1-en-2-ol (OH on middle C, Markovnikov) | Propanone (acetone) |
Ethyne is the only alkyne that gives an aldehyde; all others give ketones.
Acidity of terminal alkynes
The hydrogen on an sp carbon ($\mathrm{\equiv C{-}H}$) is weakly acidic because sp carbon has 50% s-character. Terminal alkynes therefore form metal acetylides:
- With sodium: $\mathrm{2HC\equiv CH + 2Na \rightarrow 2HC\equiv CNa + H_2}$.
- With ammoniacal $\mathrm{AgNO_3}$: white precipitate of silver acetylide.
- With ammoniacal $\mathrm{Cu_2Cl_2}$: red precipitate of copper acetylide.
These tests distinguish a terminal alkyne from alkenes and from internal alkynes such as but-2-yne.
Oxidation and polymerisation
- Cold dilute alkaline $\mathrm{KMnO_4}$ (Baeyer's reagent) is decolourised; ethyne is oxidised to oxalic acid.
- Ethyne passed through a red-hot tube trimerises to benzene.
Common MDCAT traps
- Two HBr add to the same carbon: 1,1-dibromoethane. "2,2-dibromoethane" is not a valid name.
- Hydration needs $\mathrm{HgSO_4/H_2SO_4}$, not a magnesium or zinc chromate catalyst.
- Propyne gives propanone (a ketone), not propanal, because of Markovnikov addition.
- Iodine with NaOH is the iodoform test, not a way to make ketones from alkynes.
Quick revision
- Alkynes add two molecules of reagent across the triple bond.
- HX adds by Markovnikov's rule to give a geminal dihalide.
- Ethyne + water ($\mathrm{HgSO_4}$) gives ethanal; propyne gives propanone.
- Terminal alkynes are acidic and give Ag and Cu acetylides.
- Three ethyne molecules can join to form benzene.