Alkynes

Structure

Alkynes have the general formula \mathrm{C_nH_{2n-2}} (for one triple bond). Each triple-bonded carbon is sp hybridised: two sp orbitals point 180° apart, forming a σ bond to the partner carbon and one to H or C, while two unhybridised p orbitals on each carbon overlap sideways to give two π bonds at right angles. The result is a linear C–C≡C–C unit. The C≡C bond (0.120 nm) is shorter and stronger than C=C (0.134 nm), but each π bond is weaker than the σ bond.

Naming: ethyne, propyne, but-1-yne, but-2-yne. There is no E/Z isomerism about a triple bond, because each carbon has only one substituent.

Reactions

Like alkenes, alkynes undergo electrophilic addition, but they can add twice:

• Hydrogen (Ni or Pt catalyst): alkyne → alkene → alkane. A deactivated ("poisoned") palladium catalyst, Lindlar's catalyst, stops at the Z (cis) alkene.
• Bromine: decolourises bromine water; adds once to give a dibromoalkene, then again to give a tetrabromoalkane.
• Hydrogen halides: follow Markovnikov's rule; two equivalents give a geminal dihalide (both halogens on the same carbon).
• Water (with H_2SO_4 and a Hg^{2+} catalyst): gives an enol that rearranges to a ketone (or ethanal from ethyne).

Alkynes are often a little less reactive than alkenes toward electrophiles, because the vinyl cation intermediate is less stable.

Acidity of terminal alkynes

The C–H on a terminal alkyne (pKa about 25) is far more acidic than in alkenes or alkanes, because the conjugate base's lone pair sits in an sp orbital with 50% s character, held close to the nucleus. Strong bases such as sodium amide (NaNH_2) form alkynide ions, which are good nucleophiles: they attack primary halogenoalkanes to extend the carbon chain, e.g. \mathrm{HC{\equiv}C^- + CH_3CH_2Br \rightarrow HC{\equiv}CCH_2CH_3 + Br^-}.

Acidity and alkynide chemistry

Terminal alkynes (pKa ~25) are deprotonated by NaNH_2, nBuLi, LDA or Grignard reagents (alkyl Grignards with pKa ~50 conjugate acids). Alkynides are versatile carbon nucleophiles:

• S_\mathrm{N}2 with primary halides/sulfonates (secondary and tertiary eliminate, as alkynides are also strong bases).
• Addition to aldehydes and ketones (propargylic alcohols), epoxides (at the less hindered carbon) and, with Lewis acid, to other electrophiles.

Ethyne can be alkylated twice, so it is a two-carbon linchpin for joining fragments. Alkynes are made from vicinal or geminal dihalides by double E2 with NaNH_2, or from aldehydes by the Corey–Fuchs or Seyferth–Gilbert (Ohira–Bestmann) homologations.

Controlled reduction

Stereoselective reduction is a key reason to use alkynes:

• Lindlar (Pd/CaCO_3, Pb, quinoline) + H_2: syn addition, Z-alkene.
• Na or Li in liquid NH_3 (dissolving metal): electron transfer gives a radical anion, protonation and a second electron transfer; the vinyl anion adopts the less hindered trans geometry, giving the E-alkene.
• Hydroalumination or hydrozirconation (Schwartz's reagent) gives vinyl metals that can be trapped with electrophiles.

Hydration and hydroboration

• Markovnikov hydration (Hg^{2+} or Au(I) catalysts) gives the enol, which tautomerises to a methyl ketone.
• Anti-Markovnikov hydroboration with a bulky borane (disiamylborane, 9-BBN, to prevent double addition), then H_2O_2/NaOH, gives the enol at the terminal carbon, which tautomerises to an aldehyde.

Choosing between these converts one terminal alkyne into either a ketone or an aldehyde.

Halogen and hydrogen halide addition

Br_2 adds anti to give the E-dibromoalkene first. HX gives the Markovnikov vinyl halide; radical HBr with peroxides gives the anti-Markovnikov product.

Metal-catalysed couplings

• Sonogashira coupling: Pd(0)/Cu(I) couples terminal alkynes with aryl or vinyl halides, via a copper acetylide that transmetallates to Pd.
• Glaser/Hay coupling: Cu-catalysed oxidative homocoupling to diynes.
• CuAAC "click" reaction: Cu(I) catalyses the 1,3-dipolar cycloaddition of terminal alkynes with azides to give 1,4-disubstituted 1,2,3-triazoles regioselectively, in water and at room temperature. Strain-promoted versions with cyclooctynes need no copper and are used in living systems.
• Alkyne metathesis with Mo or W alkylidyne catalysts forms macrocycles.

Retrosynthetic value

An alkyne in a target is often a disguised Z- or E-alkene, ketone, aldehyde or 1,2-diketone. Disconnect C–C bonds next to the triple bond to an alkynide plus an electrophile, then decide on the final functional group transformation.