Isobutane

C 4
H 10

Ball-and-stick Stick Space-filling

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14 atoms, 13 single bonds.

IUPAC name: 2-methylpropane
Molar mass: 58.124(10) g/mol
Atoms: 14
SMILES: CC(C)C
InChIKey: NNPPMTNAJDCUHE-UHFFFAOYSA-N
PubChem CID: 6360

Made of

C H

Structural formula of isobutane. 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.

One extra carbon on the middle of propane turns it into isobutane. ChEBI (Chemical Entities of Biological Interest) calls it an alkane, a hydrocarbon whose formula fits C_nH_2n+2. It is propane with a methyl group substituted at position 2, ChEBI adds, as the systematic name 2-methylpropane records. Propane’s chain keeps both ends, and its middle carbon trades one hydrogen for a methyl group, a carbon carrying three hydrogens. Four carbon and ten hydrogen atoms make C₄H₁₀, 58.12 g/mol, a formula butane is built from as well.

At room temperature isobutane is a gas, colourless and faintly smelling of petroleum according to CAMEO Chemicals, a hazardous-materials database. It boils at −11.7 °C and straight-chain butane at −0.50 °C, on figures from the Hazardous Substances Data Bank (HSDB). At 70 °F, about 21 °C, its vapour pressure is 3.1 atmospheres on the card from NIOSH, the US National Institute for Occupational Safety and Health. That is about three times sea-level air pressure.

Structure and bonding

All three carbon–carbon bonds in the model leave from one atom. With Labels on, that central carbon shows three C neighbours and a single H. Each of the other carbons carries three H symbols, giving CH₃CH(CH₃)₂, as NIOSH writes the formula. Fourteen atoms and thirteen single bonds make up the whole model, three C–C and ten C–H. In Ball-and-stick, PubChem’s computed model spreads the three C–C bonds 110.6° apart, each 1.527 Å long, near the 109.5° of a tetrahedral carbon in Chem1’s account.

Turned so that the central C–H bond points at the viewer, the model shows three identical CH₃ arms spaced evenly around it. In Stick view the three C–C bonds form a low tripod with the central carbon at its peak. Space-filling does the opposite: the ten hydrogens swell until they cover most of the carbon, and the molecule becomes a compact, lumpy ball. The drawing matches the real bonding, single bonds only, with no double bond, ring or charge.

Where it turns up

Raw natural gas carries a small share of isobutane, about 0.35 per cent of its molecules on HSDB’s figure, and gas plants recover it from that stream. It is also recovered from liquefied petroleum gas (LPG) and from refinery streams left by catalytic cracking and catalytic reforming (HSDB). Refinery gases, wet natural gas and natural gasoline all hold it as an important component, according to HSDB.

In Haz-Map, an occupational health database, isobutane appears as a fuel source and as an aerosol propellant, the gas that drives a spray out of its can. In cosmetics it is classed as a propellant, and hairsprays are among the products that carry it. ChEBI gives it the role of food propellant as well. Away from spray cans, high-purity isobutane can serve as a refrigerant, according to the US Energy Information Administration (EIA).

Why it matters

On EIA’s account, isobutane from gas plants, from refineries or rearranged from normal butane is made into alkylates. These raise the octane rating of gasoline and help control its volatility, the ease with which it evaporates. Industry also strips hydrogen from isobutane with a catalyst, a step called dehydrogenation, to supply more raw material for high-octane fuel (HSDB).

Fuel took almost all of it in 1975, by HSDB’s breakdown. About 94 per cent was converted into alkylate for gasoline, and about 4 per cent was blended in to control volatility. The remaining 2 per cent or so became propylene oxide and tert-butyl alcohol. By 1987 motor-fuel alkylate still took about 89 per cent of US isobutane consumption.

Outside fuel, liquid isobutane reacts with oxygen in a process called autoxidation to give tert-butyl hydroperoxide. Synthetic rubber, organic synthesis and a fluid for calibrating instruments are among its other uses on HSDB’s list. US production reported to the Environmental Protection Agency for 2023 lay between 40 billion and 45 billion pounds.

Properties

Properties of isobutane Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 6360.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₄H₁₀
Molar mass	58.124(10) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	4
Hydrogen atoms	10
Bonds	13
PubChem molecular weight	58.12 g/mol	g/mol	PubChem

Safety

Isobutane catches fire readily and is classed as an extremely flammable gas. EU rules add cancer and genetic-defect classifications for grades that contain butadiene. Breathed in, it slows the central nervous system, and the rapidly evaporating liquid can cause frostbite. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card on isobutane, which gives a recommended workplace exposure limit, are the reference sheets.

Questions

What shape is an isobutane molecule?

No single shape word covers isobutane: each of its four carbons is a centre with four bonds of its own. The central carbon bonds to the other three carbons and to one hydrogen, so the frame branches from one point. Every carbon points its bonds towards the corners of a tetrahedron, close to the 109.5° tetrahedral angle Chem1 gives. In PubChem's computed model the central carbon's C–C–C angles are 110.6° and its H–C–C angles 108.3°.

Is isobutane carcinogenic?

Isobutane on its own carries one hazard statement in EU regulation: extremely flammable gas. Isobutane that contains butadiene has a separate EU entry, which adds may cause genetic defects and may cause cancer, and the cancer classification attaches to those grades only. Japan's national classification also lists may cause drowsiness or dizziness and causes damage to organs, with no cancer statement. The PubChem Laboratory Chemical Safety Summary gathers these classifications.

Is isobutane used in food?

Yes, as a propellant, the gas that pushes a product out of a pressurised container. ChEBI gives isobutane the role of food propellant, and the EU's register of food improvement agents places it among food additives. Isobutane is also affirmed as generally recognised as safe (GRAS) for adding directly to human food, a US Food and Drug Administration status that HSDB records.

What happens to isobutane in the body?

Inhaled isobutane depresses the central nervous system. The effects run from dizziness and poor coordination through to anaesthesia and stopped breathing, on CAMEO Chemicals' description, with an irregular heartbeat as a rare but dangerous complication. Mice that inhaled isobutane formed tert-butanol (tert-butyl alcohol) from it, and in laboratory tests liver microsomes, enzyme-bearing fragments of liver cells, made the same alcohol from it. It also appears on a United Nations list of volatile substances abused by inhalation.

Made of the same elements

C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.

All 18 compounds made of carbon and hydrogen

C₂H₆ Ethane 8 atoms, 7 single bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.