Propane

Ball-and-stick Stick Space-filling

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11 atoms, 10 single bonds.

IUPAC name: propane
Molar mass: 44.0970(76) g/mol
Atoms: 11
SMILES: CCC
InChIKey: ATUOYWHBWRKTHZ-UHFFFAOYSA-N
PubChem CID: 6334

Made of

Structural formula of propane. Each unlabelled corner is a carbon atom, and the two hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Homes beyond the reach of natural gas mains can run on propane. The US Energy Information Administration (EIA) ties its use to places where natural gas supply is limited or unavailable. At ordinary pressure propane boils at −42.1 °C, and HSDB (the Hazardous Substances Data Bank) notes that moderately increased pressure turns it liquid. The EIA adds that it becomes a gas again once a valve releases it from the pressurised container.

Chemically it is an alkane, one of the hydrocarbons with the general formula C_nH_2n+2, built from carbon and hydrogen alone. With three carbons it is C₃H₈, 44.1 g/mol, and its IUPAC name is propane. Its four-carbon neighbour in the series is butane.

Walter Snelling, a US scientist, identified propane and butane in gasoline in 1912, according to the EIA. He found that cooling and pressurising the two gases changed them to liquids that could be stored and shipped in pressurised containers.

Structure and bonding

The readout counts 11 atoms and 10 single bonds. With Labels on, three C atoms make the backbone: the middle carbon carries two H, and each end carbon carries three. In the Ball-and-stick view the backbone kinks at the middle carbon instead of running straight. PubChem’s computed model puts that C–C–C angle at 111.7°, with both C–C bonds 1.519 Å long. Chem1 describes alkane chains as tetrahedra joined end-to-end, and here each carbon’s four bonds point toward the corners of its own tetrahedron.

Every bond in the drawing is single, and the drawing matches the real bonding. CAMEO Chemicals, a hazard database, files propane among the saturated hydrocarbons, whose carbons share only single bonds. The drawing has no rings or charges. In Chem1’s hybrid orbital account, each carbon uses four sp³ hybrid orbitals, mixed orbitals aimed at the corners of a tetrahedron. One of them overlaps a neighbour’s to form each C–C bond. The Stick view reduces the molecule to that zig-zag of bonds, while the Space-filling view buries the kink under the hydrogen spheres and leaves a short, knobbly rod.

Where it turns up

Propane is a constituent of natural gas and crude petroleum, making up 0.1 to 3 per cent of natural gas by HSDB’s figures. It arrives as a by-product of processing natural gas and refining petroleum. Gas plants separate it by fractional distillation under pressure, partly to stop liquids condensing and causing operational problems in pipelines (HSDB). Pipeline operators also set strict heat-content specifications for the gas they accept, which limits how much of the hydrocarbon gas liquids it may carry, according to the EIA.

In homes, the EIA lists space and water heating, cooking, clothes drying, gas fireplaces, barbecue grills and backup generators. Farms burn it to heat livestock housing and greenhouses and to dry crops, and HSDB records its use in flaming weeds among corn, soybeans, cotton, tobacco and strawberries. It also serves as an aerosol, cosmetic and food propellant.

Why it matters

As a liquid, propane takes up 270 times less room than as a gas. That, the EIA explains, is why it is stored and transported in liquid form. Returned to atmospheric pressure, it becomes a gas that stoves and heaters can burn. Demand is highly seasonal, peaking in autumn and winter, and the same fuel runs cars, school buses, delivery vans, tractors and lawn mowers.

According to HSDB, the petrochemical industry takes about 44 per cent of the propane used in the US. Residential and commercial use, grills excluded, accounts for 43 per cent. In petrochemical crackers, plants that break hydrocarbons into smaller ones, it is a feedstock for ethylene, propylene and other olefins, the EIA reports. Olefins are hydrocarbons with a carbon–carbon double bond. Those olefins end up mostly as plastics and resins, and also as glues, solvents and coatings.

Properties

Properties of propane Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 6334.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₃H₈
Molar mass	44.0970(76) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	3
Hydrogen atoms	8
Bonds	10
PubChem molecular weight	44.1 g/mol	g/mol	PubChem

Safety

Propane is an extremely flammable gas held under pressure, and its GHS (Globally Harmonized System) classification carries the signal word Danger. The vapour is heavier than air, collects in low spaces and can displace the oxygen there, while evaporating liquid can cause frostbite. NIOSH, the US National Institute for Occupational Safety and Health, lists dizziness, confusion and asphyxia among exposure symptoms and sets workplace limits on its Pocket Guide card. Its authoritative safety sheet is the PubChem Laboratory Chemical Safety Summary for propane.

Questions

What shape is a propane molecule?

No single shape word fits it, since propane has no one central atom. Its three carbons form a short chain, and each carbon sits at the middle of its own tetrahedron of four bonds. Chem1 calls such chains tetrahedra joined end-to-end and says the backbone is not really straight. In the viewer the chain bends at the middle carbon, where PubChem's computed model puts the C–C–C angle at 111.7°.

Is propane the same as LPG?

Not exactly. Liquefied petroleum gas (LPG) is a mixture whose make-up depends on the grade. HSDB describes one LPG as formed mainly of butane, about 62 per cent, with about 36 per cent propane. LPG sold as a transport fuel under the Gas Processors Association's HD-5 specification must be at least 90 per cent propane and no more than 5 per cent propylene.

Does propane have a smell?

Pure propane is odourless, according to HSDB. Fuel propane often carries an added odorant with a foul smell, and ethyl mercaptan is one compound used that way for both natural gas and propane. CAMEO Chemicals, a hazard database, describes propane as having a faint petroleum-like odour and says that for transport it may be stenched, meaning treated with such a smelling agent.

What happens to propane in the body?

Breathing it in is the main way propane enters the body, according to HSDB, and very little passes through the skin. In mice, inhaled propane was converted to isopropyl alcohol and then to acetone. That second step, the oxidation of isopropyl alcohol to acetone, took place with the enzyme alcohol dehydrogenase. Microorganisms break propane down by another route, the malonyl succinate pathway.

Made of the same elements

C₃H₆ Propylene 9 atoms, 8 bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.

All 18 compounds made of carbon and hydrogen

C₈H₈ Styrene 16 atoms, 16 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₂H₂ Acetylene 4 atoms, 3 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.