DAT Review · Organic Chemistry
Alkyne Reactions: Acetylide Chemistry and Addition Reactions
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In 30 seconds
Scope: Terminal alkyne acidity and acetylide alkylation, hydrogenation to cis/trans alkenes, electrophilic addition (HX, X₂), hydration (Markovnikov and anti-Markovnikov), and distinguishing alkynes from alkenes. Expect 2–4 questions. The Lindlar vs. dissolving metal reduction is a classic favorite.
The college version
Core Review
Terminal Alkyne Acidity
Terminal alkynes (RC≡CH) have a pKa of ~25 — uniquely acidic among hydrocarbons. By comparison: alkane ~50, alkene ~44, ammonia ~38, alcohol ~16.
Why acidic? The conjugate base (acetylide ion, RC≡C⁻) has the negative charge in an sp orbital (50% s-character). More s-character = electrons held closer to the nucleus = more stable anion (ARIO: Orbital effect).
Deprotonation: Requires a strong base. NaNH₂ (pKa of NH₃ ~38) works; NaOH (pKa of H₂O ~15.7) does NOT.
RC≡CH + NaNH₂ → RC≡C⁻ Na⁺ + NH₃
Acetylide Ion as Nucleophile (SN2)
Acetylide ions are strong nucleophiles that undergo SN2 reactions with primary alkyl halides (and methyl halides), forming new C-C bonds:
RC≡C⁻ + R'-CH₂-X → RC≡C-CH₂-R' + X⁻
Substrate limitation: ONLY methyl and 1° alkyl halides. With 2° and 3° alkyl halides, the acetylide acts as a base, causing E2 elimination instead.
This is a powerful carbon-carbon bond-forming reaction — it extends the carbon skeleton by the alkyne unit.
Alkyne Hydrogenation
| Reagent | Product | Stereochemistry |
|---|---|---|
| H₂, Pd/C (or Pt, Ni) | Alkane (complete reduction) | — |
| H₂, Lindlar catalyst (Pd/CaCO₃, poisoned with Pb) | cis-alkene | Syn addition (H₂ adds to same face) |
| Na (or Li) in NH₃(l) | trans-alkene | Anti addition (radical anion mechanism) |
Lindlar catalyst: The "poisoned" palladium stops at the alkene stage. Both hydrogens add from the same face → cis (Z) alkene.
Dissolving metal reduction: One-electron transfers via a radical anion intermediate. The more stable trans (E) alkene forms after protonation. This is a dissolving metal (Birch-like) reduction.
Mnemonics: Lindlar = "L"ess substituted gets "L"ocked as cis. Na/NH₃ goes "N"aturally trans.
Electrophilic Addition to Alkynes
HX addition (Markovnikov, twice):
RC≡CH + 2 HX → RCX₂-CH₃ (geminal dihalide) Both halogens end up on the SAME carbon (the more substituted one via Markovnikov addition). The intermediate is a vinyl halide.
X₂ addition (twice):
RC≡CH + 2 X₂ → RCX₂-CHX₂ (tetrahalide) When controlled to one equivalent: trans addition gives the (E)-1,2-dihaloalkene.
Alkyne Hydration
| Reaction | Reagents | Regiochemistry | Product |
|---|---|---|---|
| Markovnikov hydration | HgSO₄, H₂SO₄, H₂O | Markovnikov (OH on more substituted C) | Ketone (via enol tautomerization) |
| Hydroboration-oxidation | 1. (Sia)₂BH 2. H₂O₂, NaOH | Anti-Markovnikov (OH on less substituted C) | Aldehyde (terminal alkynes only) |
Tautomerization: The initial enol product is unstable and rapidly tautomerizes to the carbonyl:
- Enol from Markovnikov hydration → methyl ketone (with terminal alkynes) or more substituted ketone.
- Enol from hydroboration-oxidation of terminal alkynes → aldehyde.
Why dialkylborane (Sia₂BH)? BH₃ would add twice to alkynes. A bulky dialkylborane adds only once, stopping at the vinylborane stage. (Sia = siamyl = 1,2-dimethylpropyl).
Internal vs. Terminal Alkynes
Internal alkynes (RC≡CR') have no acidic proton. They undergo the same addition reactions but give different hydration products (ketones only, mixtures if unsymmetrical).
Common Traps
- Treating all alkynes like terminal alkynes: Only terminal alkynes (RC≡CH) can be deprotonated and used as nucleophiles. Internal alkynes have no acidic proton.
- Confusing Lindlar and Na/NH₃ products: Lindlar = cis, Na/NH₃ = trans. This is one of the most common DAT mistakes.
- Expecting Markovnikov hydration to give an alcohol: The initial enol tautomerizes to a ketone or aldehyde. You never isolate the enol.
- Forgetting SN2 substrate restrictions for acetylide alkylation: Must be methyl or 1° alkyl halide.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Alkynes are alkenes with an extra π bond, and they have one special trick: if there's a hydrogen on the triple bond, you can pull it off with a super-strong base, making a "carbon hook" (acetylide) that grabs onto other carbon chains. Hydrogenating an alkyne is like stopping a movie at different frames: full blast (Pd/C) goes all the way to alkane; "chill mode" (Lindlar) stops at cis-alkene; and a totally different method (Na/NH₃) somehow flips to trans.
Key takeaways
- Acetylide + 1° alkyl halide = new C-C bond (SN2). Does NOT work with 2° or 3° (elimination).
- Lindlar → cis (Z) alkene. Na/NH₃ → trans (E) alkene.
- Hydration of terminal alkynes: HgSO₄/H₂SO₄ → methyl ketone; hydroboration-oxidation → aldehyde.
- HX adds twice following Markovnikov's rule both times → geminal dihalide.
- Terminal alkynes are acidic (pKa ~25) because the conjugate base has sp character — only NaNH₂ or stronger can deprotonate.
- Propyne + NaNH₂, then CH₃CH₂Br → ? Answer: 2-pentyne (CH₃C≡CCH₂CH₃). NaNH₂ deprotonates the terminal alkyne to form CH₃C≡C⁻. This acetylide does SN2 on ethyl bromide (1° alkyl halide), extending the chain by two carbons.
- 2-butyne (CH₃C≡CCH₃) + H₂, Lindlar catalyst → ? Answer: cis-2-butene. Lindlar catalyst stops at the alkene; both H's add syn → cis (Z) double bond.
- 1-butyne + HgSO₄, H₂SO₄, H₂O → ? Answer: 2-butanone (CH₃CH₂COCH₃). Markovnikov hydration: OH adds to the more substituted alkyne carbon (C-2), giving an enol that tautomerizes to the methyl ketone.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Deprotonate terminal alkynes and use acetylide ions in SN2 alkylations
- Predict products of alkyne hydrogenation with Lindlar catalyst vs. Na/NH₃
- Apply Markovnikov and anti-Markovnikov hydration to alkynes
- Write mechanisms for double addition of HX and X₂ to alkynes
- Design syntheses using acetylide chemistry for carbon-carbon bond formation
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