CID 6325

C₂H₄

Ethylene

Ball-and-stick Stick Space-filling

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6 atoms, 5 bonds.

IUPAC name: ethene
Molar mass: 28.0540(48) g/mol
Atoms: 6
SMILES: C=C
InChIKey: VGGSQFUCUMXWEO-UHFFFAOYSA-N
PubChem CID: 6325

Made of

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

Plastic bags, milk jugs and antifreeze all start from ethylene, a gas made of carbon and hydrogen. Its IUPAC name is ethene, and a mole of it weighs 28.05 g. ChEBI, a database of biologically relevant molecules, classes it as an alkene, a hydrocarbon whose carbons share a double bond. The US Energy Information Administration (EIA) lists it among the alkenes, or olefins, beside propylene and the butylenes.

CAMEO Chemicals, a hazard database, describes a colourless gas with a sweet odour that is lighter than air. The Hazardous Substances Data Bank (HSDB) gives its boiling point as −103.8 °C, far below any room temperature. Chilled below 50 °F, about 10 °C, it travels as a pressurised liquid. In water it dissolves only slightly, while ethanol and ether take it up very readily.

Structure and bonding

Ethylene’s two carbons sit at the centre, held by a double bond that shows as a double stick. Two hydrogens hang from each on single sticks, splayed like the arms of a flattened X. The Labels button prints C on the middle pair and H on the outer four. Turned edge-on, the model thins almost to a line, since all six atoms lie in one plane. The Stick view keeps only that flat skeleton.

Chem1 gives each carbon a flat trigonal arrangement with 120° between its bonds; the computed model shows 118.5° for H–C–H and 120.7° for H–C–C.

In Chem1’s hybrid picture, two of each carbon’s p orbitals mix with its 2s orbital into three sp² hybrids. These form sigma bonds, whose electron density lies along the line between the two nuclei. The fourth orbital on each carbon, a p orbital upright to the plane, overlaps its twin in a pi bond whose electrons lie above and below the sheet of atoms. The double stick thus stands for two unlike bonds, and the viewer draws no pi cloud. A bent-bond model with no hybrids at all, Chem1 adds, describes the bonding equally well.

Where it turns up

Chem1 places low-density polyethylene (LDPE) in ordinary plastic bags, food containers and six-pack can rings, and high-density polyethylene (HDPE) in milk jugs and waste bins. The gas itself works directly on fruit. HSDB lists it as a plant growth regulator applied commercially to speed the ripening of various fruits. Under 0.5 million kg a year goes on fruit and vegetables, by the same account.

Metalworkers meet it in oxyethylene welding, which Haz-Map records among its uses. Traces leave the human body too: HSDB reports ethylene in the exhaled breath of two out of eight people tested. Haz-Map records its past use as a general anaesthetic, and ChEBI lists a second role for it as a refrigerant.

Why it matters

Most ethylene ends up as plastic. HSDB traces its production to steam cracking of petroleum hydrocarbons, fed by ethane, propane, butanes, naphthas and gas oils. Ethane is the main feedstock in the USA, while naphthas supply over 80 per cent in western Europe and Japan. The EIA says ethane goes mainly to ethylene, whose products mostly become plastics. HSDB’s consumption profile gives low-density polyethylene 28 per cent, high-density 24 per cent and ethylene oxide 14.8 per cent. The EPA’s Chemical Data Reporting puts the aggregated US volume for 2023 at 75 billion to just under 80 billion pounds.

Polyethylene, in Chem1’s words the simplest polymer, is a long chain of two-carbon units. A free-radical initiator lends its unpaired electron to one ethylene molecule, which can then bond to the next. LDPE went commercial in 1933. In the early 1950s Karl Ziegler found catalysts that link ethylene efficiently, and he shared the 1963 Nobel Prize in Chemistry with Giulio Natta.

Properties

Properties of ethylene Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 6325.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₂H₄
Molar mass	28.0540(48) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	2
Hydrogen atoms	4
Bonds	5
Single bonds	4
Double bonds	1
PubChem molecular weight	28.05 g/mol	g/mol	PubChem

Safety

Pure ethylene is an extremely flammable gas, and its harmonised EU classification carries the signal word Danger with the statements “extremely flammable gas” and “may cause drowsiness or dizziness”. CAMEO warns that a flame can flash back to a leak. Escaping gas can suffocate by lowering the oxygen in a confined space, the International Chemical Safety Card notes, and contact with the cold liquid can cause frostbite. The PubChem Laboratory Chemical Safety Summary for ethylene is the authoritative sheet.

Questions

Is ethylene a flat molecule?

Yes. Chem1 places each carbon at the centre of a flat trigonal arrangement of three partners, one carbon and two hydrogens, with angles of 120° around it. The two arrangements share one plane, and the computed model puts every atom within a tenth of an ångström of it. The pi electrons sit above and below the sheet, where the viewer draws nothing.

What is the double bond in ethylene made of?

Two bonds of different kinds join the carbons. In Chem1's hybrid picture, sp² orbitals make a sigma bond whose electron density peaks on the line between the nuclei. A leftover p orbital on each carbon, standing upright to the plane, merges with its partner into a pi bond. That pair of pi electrons forms the fourth bond to each carbon, the other half of the double bond.

Why is ethylene used to ripen fruit?

ChEBI, a database of biologically relevant molecules, lists ethylene as a plant hormone. HSDB records that it induces ripening, makes leaves droop and fall, and promotes senescence, the ageing of plant tissue, and that growers use it commercially to hasten the ripening of various fruits. MeSH, the US medical subject index, files it among the plant growth regulators, hormones made naturally in plants that control growth and other functions.

What happens to ethylene in the body?

Very little of it stays. HSDB reports that 94.4 per cent of the ethylene reaching the lungs is breathed out again without entering the wider circulation, because it dissolves poorly in blood, and gives its biological half-life in humans as 0.65 hours. In mice and rats some of it is converted into ethylene oxide, in rats by enzymes in the liver. The ACGIH, a US body of workplace hygienists, rates ethylene not classifiable as a human carcinogen.

Made of the same elements

CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 bonds.
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.

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