Alkyne Structure and Reactions

Core Idea

Alkynes contain a C≡C triple bond — one sigma and two mutually perpendicular pi bonds — with sp-hybridized carbons and linear geometry (180°). Terminal alkynes (R–C≡C–H) are weakly acidic (pKa ≈ 25) because the sp carbon's high s-character keeps bonding electrons close to the nucleus, stabilizing the conjugate base alkynide anion. Alkynes undergo electrophilic addition similar to alkenes but can add two equivalents of reagent. Selective partial reduction to cis-alkenes uses Lindlar's catalyst; dissolving metal (Na/NH₃) reduction gives the trans-alkene.

How It's Best Learned

Compare acidity: water (pKa 16) > terminal alkyne (25) > alkene vinyl H (44) > alkane (50) and explain each trend. Practice predicting the product of one vs two equivalents of HBr addition to a terminal alkyne, applying Markovnikov's rule at each step.

Common Misconceptions

• Terminal alkyne protons are not strongly acidic — deprotonation requires bases like NaNH₂ or n-BuLi, not NaOH.
• Partial reduction selectivity (cis vs trans) is determined entirely by the choice of reducing agent, not by any property of the alkyne.
• Alkyne addition of two equivalents does not give the same stereochemistry as two sequential alkene additions.

Explainer

You already know that alkenes feature a C=C double bond with sp² hybridization and trigonal planar geometry. Alkynes take this one step further: a C≡C triple bond consists of one sigma bond and two pi bonds, with the two pi bonds oriented perpendicular to each other. The carbon atoms are sp-hybridized, meaning each uses two hybrid orbitals (one for the sigma bond to the other triple-bond carbon, one for the bond to the substituent) and two unhybridized p orbitals for the pi bonds. This gives alkynes a distinctive linear geometry with 180° bond angles — the region around the triple bond is a rod, not a plane.

This linear geometry has a surprising chemical consequence: terminal alkynes are weakly acidic. The C–H bond on a terminal alkyne (R–C≡C–H) has a pKa of about 25, which is dramatically more acidic than an alkene vinyl C–H (~44) or an alkane C–H (~50). The explanation connects directly to hybridization. An sp orbital has 50% s-character, compared to 33% for sp² and 25% for sp³. Since s orbitals hold electrons closer to the nucleus, the sp orbital stabilizes the negative charge on the resulting alkynide anion (R–C≡C⁻) more effectively. This acidity is synthetically powerful: bases like NaNH₂ can deprotonate terminal alkynes to generate nucleophilic alkynide ions, which then react with electrophiles to form new carbon-carbon bonds.

Alkynes undergo electrophilic addition reactions similar to alkenes, but with a key difference: since there are two pi bonds available, alkynes can react with one *or* two equivalents of reagent. Adding one equivalent of HBr to a terminal alkyne follows Markovnikov's rule, placing the bromine on the more substituted carbon to give a vinyl halide. Adding a second equivalent yields a geminal dihalide. Controlling the stoichiometry — stopping at one equivalent — is an important synthetic skill.

Perhaps the most useful alkyne reaction is partial reduction to an alkene, where the choice of reagent controls stereochemistry completely. Lindlar's catalyst (a poisoned palladium catalyst) delivers hydrogen to the same face of the triple bond, producing the cis-alkene exclusively. Dissolving metal reduction (sodium in liquid ammonia) proceeds through a radical anion mechanism that delivers hydrogen to opposite faces, giving the trans-alkene. This stereochemical control makes alkynes valuable synthetic intermediates: you can build a triple bond, then selectively reduce it to whichever alkene geometry you need.