Propylene

Ball-and-stick Stick Space-filling

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9 atoms, 8 bonds.

IUPAC name: prop-1-ene
Molar mass: 42.0810(72) g/mol
Atoms: 9
SMILES: CC=C
InChIKey: QQONPFPTGQHPMA-UHFFFAOYSA-N
PubChem CID: 8252

Made of

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

Propylene and propene are two names for one gas, and its IUPAC name is prop-1-ene. ChEBI (Chemical Entities of Biological Interest) describes propene as an alkene, propane with one double bond, at position 1. The formula is C₃H₆, three atoms of carbon and six of hydrogen, and a mole of it weighs 42.08 g. Two of the hydrogens sit on the end carbon of the double bond, one on the middle carbon and three on the methyl group, CH₃, at the other end.

The gas has no colour and, on the CAMEO Chemicals hazard record, a faint odour like petroleum. It boils at −47.68 °C, according to HSDB (the Hazardous Substances Data Bank). Liquid propylene spilled on water floats and boils away. Chemical plants work with two grades of propene, HSDB records: a less pure chemical grade and a highly pure polymer grade.

Structure and bonding

Propylene’s three carbons make a shallow bend, with a double bond on one side of the middle carbon and a single bond on the other. The Labels button prints C on those three atoms and H on the six hydrogens around them. The computed model makes the double bond 1.337 Å long and the single bond 1.492 Å.

Turned edge-on, the model shows seven atoms in one flat sheet: the carbons, the three hydrogens on the double-bonded pair and one methyl hydrogen. The other two methyl hydrogens stand out of it, one above and one below. The model catches the methyl group in one position, but that group can turn about its single bond. Rotation around a carbon–carbon double bond is not possible, Stephen Lower’s Chem1 text states.

The Stick view strips the spheres away, leaving the double and single lines side by side on a bare skeleton. In the Space-filling view the nine atoms merge into one compact body, thicker at the methyl end than at the flat CH₂ end. What the double stick stands for is set out on the ethylene page.

Where it turns up

Refineries make propylene in their cracking units, which break long, heavy hydrocarbons into lighter molecules. Every refinery olefin, or alkene, comes from those units, according to the US Energy Information Administration (EIA). Propane and propylene are the main purity products refiners sell, streams at least 90 per cent one compound. Refinery-grade propene is 50 to 70 per cent propene in propane, according to HSDB. It goes mainly into liquefied petroleum gas for heating or into motor gasoline to raise its octane. In California the accepted standard for propane fuel is HD-10, which contains up to 10 per cent propylene.

Polypropylene, the plastic made from the gas, goes into rope, binder covers, plastic bottles, non-woven fabrics and electric kettles, according to Chem1, and into carpet fibres, according to HSDB. Its resistance to fatigue suits it to food containers, their lids and the flip-top lids on bottles of ketchup. Left uncoloured, it is translucent but not clear.

Why it matters

Almost two thirds of all propene becomes polypropylene, a plastic that makes up 25 per cent of all plastic products on HSDB’s figure. The gas polymerises only when heated under pressure with a catalyst present.

Any catalytic or thermal cracking of hydrocarbons yields propylene, and in refineries fluid catalytic cracking (FCC) is by far the most important route, HSDB notes. A conventional FCC unit typically yields 3 per cent propene; adding a material called ZSM-5 zeolite to its catalyst can lift that to 20 per cent. Stripping hydrogen from propane over catalysts such as platinum or chromium also makes it, according to HSDB, and the EIA says propane serves as a dedicated feedstock for on-purpose propylene.

About 3 per cent comes from metathesis, a reaction in which two olefins become two others as their double bonds shift. Phillips developed it in the 1960s as the Triolefin process. That process ran the other way, converting propene into ethylene and 2-butene. Growing demand for propene stopped it in 1972, and when propene prices rose, running the reaction backwards, towards propene, became attractive.

Properties

Properties of propylene Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 8252.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₃H₆
Molar mass	42.0810(72) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	3
Hydrogen atoms	6
Bonds	8
Single bonds	7
Double bonds	1
PubChem molecular weight	42.08 g/mol	g/mol	PubChem

Safety

Propylene is an extremely flammable gas stored under pressure, with the GHS (Globally Harmonized System) signal word Danger and a warning that a heated container may explode. Heavier than air, the gas can pool in low places and leave them short of oxygen, the International Chemical Safety Card warns. Haz-Map lists it as a simple asphyxiant, and liquid propylene can cause frostbite as it evaporates. The PubChem Laboratory Chemical Safety Summary for propylene sets out the full classification.

Questions

Is propylene the same as propene?

Yes, the two names belong to one compound, the gas C₃H₆. Its IUPAC name, prop-1-ene, adds a number: the 1 marks the first carbon of the double bond, which lies at one end of the three-carbon chain. The plastic made from it is called polypropylene.

What shape is a propylene molecule?

It takes no single shape word. Names such as trigonal planar describe the atoms arranged around one central atom, and each of propylene's three carbons is a centre in its own right. The computed model gives 124.0° for the C–C=C angle along the chain, 118.0° between the two hydrogens on the double-bonded end and 108.2° to 108.3° between hydrogens of the methyl group.

What is propylene used for besides polypropylene?

Propylene oxide, acrylic acid, acrylonitrile and cumene are its other important products, HSDB reports, and the gas is also an intermediate for acetone and isopropanol. In 1974 half of it went to organic chemicals, polypropylene's 11 per cent among them, and 46 per cent to gasolines. By 1984 polypropylene took 34 per cent, acrylonitrile 17, propylene oxide 11 and cumene 9.

Does inhaled propylene build up in the body?

Hardly at all, on HSDB's account. In rats, most propylene breathed into the lungs is breathed out again without reaching the blood, and HSDB states that the gas leaves the body rapidly, unchanged, through the lungs. A model of how the gas enters and leaves the human body predicts that it is eliminated too quickly to accumulate. The first step of its metabolism yields propylene oxide.

Made of the same elements

C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 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.