Alkynes: Hydrocarbons with Triple Bonds

What makes an alkyne

Alkynes are hydrocarbons containing at least one carbon–carbon triple bond. They follow the general formula CnH2n-2 — four fewer hydrogens than the corresponding alkane — because each triple bond replaces four C–H bonds' worth of hydrogen.

They are the most unsaturated hydrocarbon family: with two pi bonds per triple bond, they have the greatest capacity for addition reactions. Each pi bond can accept one addition in sequence.

The simplest alkyne is ethyne (HC≡CH), universally known as acetylene, followed by propyne (CH3C≡CH). Alkynes are less common in nature than alkenes but very important industrially.

The linear triple bond

Each triple-bond carbon is sp hybridized. Its two hybrid orbitals point 180° apart: one forms the sigma bond to the neighboring carbon, the other forms a sigma bond to its substituent (H or alkyl group).

The two unhybridized p orbitals on each carbon overlap sideways with their counterparts to form two pi bonds at right angles to each other, creating a cylindrical sheath of electron density around the bond axis.

The geometry is strictly linear — H–C≡C–H is a straight line — and the bond is short (about 120 pm) and strong. The cylindrical symmetry means there are no distinct 'sides', which is why alkynes cannot show cis/trans isomerism.

Naming alkynes

Alkynes take the suffix -yne, with the longest chain containing the triple bond as the parent. Numbering gives the triple bond the lowest possible locant.

• HC≡CH → ethyne (acetylene)
• CH3C≡CH → propyne
• CH≡CCH2CH3 → but-1-yne
• CH3C≡CCH3 → but-2-yne

But-1-yne is a terminal alkyne (triple bond at the chain end, with an acidic hydrogen); but-2-yne is an internal alkyne. The distinction matters for acidity and for some reactions.

Ethyne (acetylene)

Ethyne, as acetylene, is best known as a fuel gas. Burned in pure oxygen, it produces a flame reaching about 3000°C — hot enough to melt steel — and oxy-acetylene torches are standard equipment for welding and cutting metal in construction and repair work.

The extreme temperature comes from ethyne's high energy content: the triple bond stores a great deal of energy, and complete combustion in pure oxygen releases it rapidly.

Beyond welding, ethyne is a major industrial feedstock. It is converted into vinyl chloride (for PVC), synthetic rubber components, and a range of solvents and chemicals — a reminder that a simple molecule can anchor an entire branch of industry.

The acidity of terminal alkynes

Terminal alkynes (RC≡CH) are unusual among hydrocarbons: their C–H bond is weakly acidic, with a pKa around 25. That is still an extremely weak acid — far weaker than water — but it is about 10^20 times more acidic than an alkane C–H bond.

The reason is hybridization. The sp carbon has 50% s-character, and s orbitals hold electrons closer to the nucleus than p orbitals do. The carbon is effectively more electronegative, stabilizing the negative charge left behind when the proton departs.

Only very strong bases can remove this proton, forming an acetylide anion (RC≡C−) that is a powerful nucleophile used to extend carbon chains in synthesis. Internal alkynes, with no hydrogen on the triple bond, show none of this chemistry.

Reactivity: addition in two stages

Alkynes undergo addition reactions much like alkenes, but in two stages — one per pi bond. Ethyne adds one molecule of HBr to give bromoethene, and a second to give 1,1-dibromoethane.

Catalytic hydrogenation with a nickel catalyst reduces alkynes all the way to alkanes: ethyne + 2H2 → ethane. Special poisoned catalysts can stop the reduction at the alkene stage, a useful selectivity in synthesis.

Markovnikov's rule governs the first addition exactly as for alkenes. The second addition follows the same logic applied to the resulting alkene, so the two new atoms usually end up on the same carbon.

Common mistakes

Real-world applications

• Oxy-acetylene torches weld and cut steel in construction, shipbuilding, and repair work.

• Ethyne is an industrial feedstock for making plastics, solvents, and synthetic chemicals.

• Terminal alkyne anions are useful building blocks in organic synthesis for extending carbon chains.

• Alkyne-based reactions are used to attach labels to biomolecules in chemical biology research.