Acetylene

Ball-and-stick Stick Space-filling

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4 atoms, 3 bonds.

IUPAC name: acetylene
Molar mass: 26.0380(44) g/mol
Atoms: 4
SMILES: C#C
InChIKey: HSFWRNGVRCDJHI-UHFFFAOYSA-N
PubChem CID: 6326

Made of

Structural formula of acetylene. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Torches that weld and cut metal burn acetylene in oxygen. According to the Hazardous Substances Data Bank (HSDB), a flame of equal volumes of the two gases runs hotter than any other fuel gas pairing on the commercial market. The fuel is a small hydrocarbon of carbon and hydrogen whose two carbons share a triple bond. MeSH, the US National Library of Medicine’s subject vocabulary, calls it the simplest two-carbon alkyne, a hydrocarbon family defined by that triple bond. Its IUPAC name is simply acetylene, and a mole weighs 26.04 g.

At room temperature acetylene is a gas that CAMEO Chemicals describes as lighter than air and burning with a sooty flame. It barely dissolves in water: HSDB gives 1200 mg/L at 25 °C. Acetone is far more accommodating, taking up 25 times its own volume of the gas at 15 °C.

Structure and bonding

All four atoms of acetylene sit on one straight line: a hydrogen, two carbons, then the second hydrogen. The Labels button prints H, C, C and H along that axis, and the angle at each carbon in the model is 180°. Three sticks join the carbons as a triple bond; each hydrogen hangs on by a single bond. Chem1 explains the straight line through hybridisation, the mixing of an atom’s orbitals into new ones of matching shape. Each carbon forms two equivalent bonds, so it is taken to be sp-hybridised, and its two sp orbitals point 180° apart.

The three sticks are not three identical bonds. In Chem1’s orbital picture one is a sigma (σ) bond lying along the axis, and two are pi (π) bonds. Those form from two leftover p orbitals on each carbon, each pair perpendicular both to the other and to the axis. In the Space-filling view the spheres fuse into a short rod with no bend anywhere, unlike ethylene, whose carbons Chem1 gives 120° angles. PubChem’s computed model puts the carbon–carbon distance at 1.200 Å and each C–H at 1.065 Å.

Where it turns up

In a welding shop acetylene arrives as a compressed gas in cylinders. NIOSH, the US National Institute for Occupational Safety and Health, describes it as a compressed gas for welding and cutting metals, shipped under pressure dissolved in acetone. HSDB adds brazing, joining metal parts with a melted filler metal, as well as hardening and local heating of metal, and the glass industry works with the flame too. Los Alamos National Laboratory counts oxy-acetylene welding among the large uses of oxygen.

The gas also turns up somewhere less obvious: Haz-Map, a US occupational health database, lists smoking cigarettes as an activity that carries exposure to it. HSDB says the major US producers make acetylene in two ways: by partially oxidising natural gas, or alongside ethylene in steam cracking. Smaller producers react calcium carbide with water, and gas made that way carries phosphine and some arsine.

Why it matters

Most acetylene never reaches a torch. HSDB reports that in 2000 about 80 per cent of US production served as a closed-system intermediate for making other chemicals. The remaining 20 per cent went mainly into oxyacetylene torches. Its products as a feedstock include vinyl chloride, vinyl acetate, acrylonitrile, trichloroethylene, 1,4-butanediol and synthetic rubber. HSDB also lists making acetaldehyde and acetic acid, use as an illuminant, signalling, and precipitating metals, copper in particular.

Inside the body the gas is mostly a passenger. HSDB states that it is rapidly absorbed and leaves unchanged, and that the lungs excrete it quickly. It tends to be removed from the blood rather than retained there. Haz-Map, citing the American Conference of Governmental Industrial Hygienists (ACGIH), records its use as a general anaesthetic in the 1920s.

At the level of single enzymes it is not inert. HSDB describes acetylene alkylating, or attaching a carbon group to, the haem that cytochrome P450 enzymes work with. That reaction runs during the enzyme’s own catalytic turnover and inactivates it, a process called suicide inactivation.

Properties

Properties of acetylene Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 6326.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₂H₂
Molar mass	26.0380(44) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	2
Hydrogen atoms	2
Bonds	3
Single bonds	2
Triple bonds	1
PubChem molecular weight	26.04 g/mol	g/mol	PubChem

Safety

Acetylene is an extremely flammable gas under the Globally Harmonized System (GHS), with the signal word Danger. It is also a simple asphyxiant: it dilutes the oxygen in air, and in closed or confined spaces it can cause dizziness or asphyxiation without warning. NIOSH lists headache and dizziness among its symptoms, and frostbite from the liquid. Workplace guidance for acetylene is set out in the NIOSH Pocket Guide card and the PubChem Laboratory Chemical Safety Summary.

Questions

Why is acetylene a linear molecule?

Each carbon in acetylene forms two equivalent bonds, one to hydrogen and one to the other carbon. Chem1 takes each carbon to be sp-hybridised on that basis, and the two sp orbitals point 180° apart, so the molecule is linear. The Ball-and-stick view shows the result: H, C, C and H threaded on one straight axis, with no bend at either carbon.

How many sigma and pi bonds are in acetylene?

C₂H₂ has three sigma bonds and two pi bonds. Each carbon uses one sp hybrid orbital for its bond to hydrogen and the other for the carbon–carbon sigma bond. Two p orbitals left on each carbon, at right angles to each other, pair up across the molecule into the two pi bonds. Together with the sigma bond they make up the triple bond, HC≡CH.

What is acetylene used for besides welding?

HSDB consumption figures for 1974 give 26.5 per cent of acetylene to vinyl chloride, 21 per cent to welding and metal cutting, and 20 per cent to acrylates and acrylic acid. A 1982 figure gives metal cutting and welding 30 per cent. The US Environmental Protection Agency lists it as an intermediate, a fuel, a heat-transfer agent and a monomer.

Why does acetylene smell like garlic?

NIOSH, the US National Institute for Occupational Safety and Health, describes acetylene as a colourless gas with a faint, ethereal odour and says the commercial grade smells of garlic. HSDB calls the odour not unpleasant unless the gas is impure, naming phosphine as the impurity responsible. CAMEO Chemicals, a hazard database, gives the gas a faint garlic-like odour. Acetylene made from calcium carbide and water carries phosphine and some arsine, according to HSDB.

Made of the same elements

C₂H₄ Ethylene 6 atoms, 5 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.

All 18 compounds made of carbon and hydrogen

C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.