CID 8003

C₅H₁₂

Pentane

C 5
H 12

Ball-and-stick Stick Space-filling

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17 atoms, 16 single bonds.

IUPAC name: pentane
Molar mass: 72.151(12) g/mol
Atoms: 17
Polarity: nonpolar
SMILES: CCCCC
InChIKey: OFBQJSOFQDEBGM-UHFFFAOYSA-N
PubChem CID: 8003

Made of

C H

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

Pentane’s vapour is 2.48 times as dense as air, on the figure from the Hazardous Substances Data Bank (HSDB). For that reason the International Chemical Safety Card warns that it can pool in low-lying spaces and leave too little oxygen there. At room temperature the vapour comes off a clear, colourless liquid, with an odour that CAMEO Chemicals, a hazard database, calls petroleum-like.

From five carbons on, Chem1 notes, alkanes, the hydrocarbons of general formula C_nH_2n+2, take Greek number prefixes in their names, and pent- stands for five. Pentane’s formula is C₅H₁₂, five carbon and twelve hydrogen atoms, with a molar mass of 72.15 g/mol. ChEBI (Chemical Entities of Biological Interest) classes it as a straight-chain alkane, its five carbons joined in one unbranched row. Its boiling point, 36.06 °C on HSDB’s figure, is one step in a climb along the straight chains that the octane page follows to the forces between molecules.

Structure and bonding

Seventeen atoms make up the model, five carbons and twelve hydrogens, joined by sixteen single bonds. In Ball-and-stick the carbons trace a zig-zag, with an angle of 111.3° or 111.5° at each of the three inner carbons in PubChem’s computed model. Why such a chain bends is set out under molecular shapes. Viewed along its edge, the chain is flat. All five carbons lie within 0.04 Å of one plane, and the end carbons sit 5.04 Å apart, in one fully extended arrangement.

In Stick, with no spheres in the way, the three bends stand out. Labels marks each atom C or H, showing a CH₃ group at each end and three CH₂ groups between. That is Chem1’s H₃C–(CH₂)_n–CH₃ pattern for alkane chains, with n equal to 3. Space-filling swells the atoms to full size, and hydrogen then makes up most of an elongated, knobbly surface.

What the model draws is what pentane has: single bonds only, with no ring, double bond or charge. CAMEO Chemicals files it among the saturated aliphatic hydrocarbons, and ChEBI assigns it the role of a non-polar solvent.

Where it turns up

Pentane comes from petroleum, where HSDB records it in light gasoline fractions, in motor and aviation fuel and in petroleum ether. Fractional distillation separates it, rectification (a further distillation) purifies it, and it is sold in pure, technical and commercial grades.

The US Energy Information Administration (EIA) counts natural gasoline, also called pentanes plus, among the hydrocarbon gas liquids, a group that also holds ethane, propane and the butanes. That product is C₅H₁₂ together with heavier alkanes, a mixture in which pentane is only one part. For 2023, the US Environmental Protection Agency’s Chemical Data Reporting puts US production of pentane itself at 2.5 billion to under 4 billion pounds.

Pentane also has natural sources. LOTUS, a natural-products database, reports n-pentane in Calendula officinalis and Allium ampeloprasum. The human body makes it too, by lipid peroxidation, the oxidative breakdown of polyunsaturated fatty acids, and breathes it out, HSDB explains.

Why it matters

Pentane is a blowing agent for plastics. Haz-Map, a US occupational health database, names it as a foaming agent for expanded polystyrene (EPS) and for polyurethane. It is also a carrier in polymerisation processes and a solvent in adhesives. Beyond foam, it fuels lighters and blowtorches and serves as a laboratory solvent and a gasoline additive. HSDB also lists it as a raw material for other industrial chemicals.

As natural gasoline, pentane and its heavier companions go to petrochemical plants as feedstock, the raw material they turn into other chemicals, EIA reports. Motor gasoline also takes them as an additive. In the United States, natural gasoline can be blended into fuel ethanol as a denaturant, making the alcohol undrinkable as the law there requires.

Even in healthy people a low level of lipid peroxidation goes on, and HSDB notes that measuring the pentane they breathe out detects it. In 47 healthy people aged 21 to 79, the pentane they breathed out rose with age, a statistically significant trend. That rise was independent of the antioxidant levels in their diet.

Properties

Properties of pentane Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 8003.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₅H₁₂
Molar mass	72.151(12) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	5
Hydrogen atoms	12
Bonds	16
PubChem molecular weight	72.15 g/mol	g/mol	PubChem

Safety

Pentane is a highly flammable liquid, and its EU classification carries the signal word Danger. Swallowed, it may be fatal if it enters the airways, and its vapour may cause drowsiness or dizziness. The liquid also strips oils from the skin, which can leave it dry or cracked. The PubChem Laboratory Chemical Safety Summary gathers these classifications, and the NIOSH (US National Institute for Occupational Safety and Health) card for n-pentane holds the workplace limits.

Questions

How long is a pentane molecule?

About 5 Å from one end carbon to the other: 5.04 Å in PubChem's computed model, which shows one fully extended arrangement with all five carbons in a plane. Each of its four C–C bonds measures 1.52 to 1.53 Å, so laid out in a line they would span about 6.1 Å. The angle of about 111° at each inner carbon shortens the chain to 5.04 Å.

Why does pentane float on water instead of mixing with it?

Being less dense than water and nearly insoluble in it, pentane floats, CAMEO Chemicals explains. HSDB puts its density at 0.6262 g/cm³ at 20 °C, and a litre of water at 25 °C dissolves only 38 mg of it. With several organic liquids it is the opposite: pentane is miscible, mixing in any proportion, with ethanol, acetone, chloroform, benzene and ethyl ether.

Is pentane a liquid or a gas?

A liquid at room temperature, though it boils at 36.06 °C on HSDB's figure. NIOSH, the US National Institute for Occupational Safety and Health, notes that pentane is a gas above 97 °F, which is about 36 °C. At 25 °C its vapour pressure, the pressure of pentane vapour over the liquid in a closed container, is already 514 mm Hg, HSDB records.

What happens to pentane in the body?

Little liquid pentane is expected to cross the skin, HSDB notes, because it evaporates from the surface first. In blood, it dissolves less readily than either n-hexane or n-heptane. The body converts what it does take up mainly by hydroxylation, adding an OH group to the chain. In rat and rabbit liver microsomes, fragments of liver cells, 2-pentanol made up 83 to 89 per cent of the products, with 3-pentanol a minor one.

Made of the same elements

C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.

All 18 compounds made of carbon and hydrogen

C₈H₈ Styrene 16 atoms, 16 bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 bonds.