6. Alkynes
Alkynes carry two π bonds and a rare property for a hydrocarbon: a terminal alkyne is acidic enough to be deprotonated, turning it into a carbon nucleophile for building longer chains.
Acidity and Alkylation
A terminal alkyne has pKa ≈ 25 — far more acidic than an alkane — because the sp-hybridized carbon holds the lone pair close. A strong base like NaNH2 deprotonates it to an acetylide ion, which does SN2 with alkyl halides.
Example: Propyne + NaNH2, then CH3I: $$\text{CH}_3\text{–C≡CH} \xrightarrow[\text{2. CH}_3\text{I}]{\text{1. NaNH}_2} \text{CH}_3\text{–C≡C–CH}_3 \quad \text{(2-butyne)}$$
Addition Reactions
- HBr (1 equiv): adds Markovnikov to give a vinyl bromide; excess HBr gives a geminal dibromide.
- Hydration (HgSO4/H2SO4/H2O): Markovnikov addition of water gives an enol that tautomerizes to a ketone. 1-Butyne → 2-butanone.
- Hydroboration–oxidation: anti-Markovnikov → aldehyde (from terminal alkynes).
Reduction: Cis vs Trans Alkenes
Lindlar's catalyst (H2/Pd, poisoned) reduces an alkyne to a cis alkene; Na/NH3 (dissolving metal) gives the trans alkene.
Example: 2-Butyne + H2/Lindlar → cis-2-butene; 2-butyne + Na/NH3 → trans-2-butene.
Practice Problems
- What is the product of propyne + NaNH2, then CH3I?
- Predict the product of HBr (1 equiv) adding to propyne.
- What alkene does Lindlar reduction of 3-hexyne give?
- Why are terminal alkynes acidic when alkanes are not?
Answers: 1. 2-butyne 2. 2-bromopropene 3. cis-3-hexene 4. The acetylide anion sits in an sp orbital — high s-character holds the negative charge close to the nucleus, stabilizing it