Isooctane

C 8
H 18

Ball-and-stick Stick Space-filling

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26 atoms, 25 single bonds.

IUPAC name: 2,2,4-trimethylpentane
Molar mass: 114.232(20) g/mol
Atoms: 26
Polarity: nonpolar
SMILES: CC(C)CC(C)(C)C
InChIKey: NHTMVDHEPJAVLT-UHFFFAOYSA-N
PubChem CID: 10907

Made of

C H

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

The octane numbers on fuel pumps sit on a scale that one liquid fixes at 100. That liquid is isooctane, and 100 is the antiknock octane number the Hazardous Substances Data Bank (HSDB) lists for it. Its systematic name, 2,2,4-trimethylpentane, reads as a plan of the molecule.

ChEBI, the Chemical Entities of Biological Interest database, describes a five-carbon pentane chain with two methyl groups on its second carbon and one on its fourth. A methyl group is CH₃, a carbon carrying three hydrogens. Those three branches bring the count to eight carbon and eighteen hydrogen atoms, C₈H₁₈, with a molar mass of 114.23 g/mol. Straight-chain octane has exactly the same formula. The name isooctane has also served for 2-methylheptane, HSDB notes; the model shows 2,2,4-trimethylpentane alone.

Structure and bonding

One carbon in the model has no hydrogen at all: carbon 2 of the name, bonded to four other carbons. Three of them are methyl groups; the fourth is the chain’s lone CH₂, leading on to a CH that holds the last two methyls. Labels puts C or H on all 26 atoms, so the eight carbons can be counted off along the chain and its branches.

The Stick view reduces all this to a bare branched frame. In Ball-and-stick, each carbon’s four bonds point roughly to the corners of a tetrahedron, the tetrahedral coordination Chem1 describes for carbon in most of its compounds. In this computed structure the angles around isooctane’s carbons run from 104 to 117°. In Space-filling the methyls close round carbon 2 until almost none of it shows.

CAMEO Chemicals, an emergency-response database, files isooctane among the saturated hydrocarbons, whose carbons carry only single bonds. All 25 bonds in the model are single, so what it draws is the molecule’s real bonding. Its one simplification is the pose, a single computed arrangement of the atoms.

Where it turns up

Octane testing is where isooctane serves as a yardstick. Blended with heptane and standards such as toluene, it makes reference fuels of precisely known octane, as the US Energy Information Administration (EIA) describes them. It comes in pure, research and spectrophotometric grades, the last made for spectrophotometry, the measurement of how much light a sample absorbs. The US Environmental Protection Agency (EPA) lists it as a solvent and thinner, a reagent for organic synthesis and a medium for spectrophotometric analysis. In cosmetics it works as a solvent.

It is produced by refining petroleum and by reacting butylenes with isobutane, according to HSDB. For 2023 the US volume of 2,2,4-trimethylpentane reported to EPA’s Chemical Data Reporting was 250 million to under 400 million pounds. Releases come from products of the petroleum and gasoline industry, and EPA includes isooctane among its hazardous air pollutants. It has also been reported from Zingiber officinale, in the LOTUS database of natural products.

Why it matters

Knock, the engine fault behind the octane scale, begins inside the cylinder once the fuel is already burning. Heat and pressure from the main combustion can set unburned fuel alight on its own, EIA explains. That second, uncontrolled burn makes the cylinder pressure spike and sets off the knock. The fuel’s energy is then released unevenly, which can strain and damage the piston, and the fault was common before computer-controlled ignition spread.

The octane rating, which EIA calls the antiknock property of gasoline, rests on the pressure at which a fuel ignites by itself in a test engine. Isooctane is highly resistant to that auto-ignition and is assigned 100, while normal heptane is the 0 reference fuel. A sample is rated by adjusting the engine’s cylinder height, which changes its compression ratio, how hard it squeezes the fuel. The aim is the pressure at which the sample and reference blends above and below it knock with similar intensity. The rating takes its very name from this compound, according to EIA.

Properties

Properties of isooctane Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 10907.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₈H₁₈
Molar mass	114.232(20) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	8
Hydrogen atoms	18
Bonds	25
PubChem molecular weight	114.23 g/mol	g/mol	PubChem

Safety

Isooctane catches fire easily: its GHS (Globally Harmonized System) classification calls the liquid and its vapour highly flammable, under the signal word Danger. The vapour is heavier than air, the International Chemical Safety Cards note, and may travel along the ground to be ignited far from the liquid. It may be fatal if swallowed and it enters the airways, causes skin irritation, may cause drowsiness or dizziness and is very toxic to aquatic life, with lasting effects. Workplace exposure limits exist for it, and the PubChem Laboratory Chemical Safety Summary gathers them with the full classification.

Questions

What does 87 octane mean?

It places a fuel on a scale fixed by isooctane at 100 and normal heptane at 0. The higher the number, the more pressure the fuel withstands before it self-ignites in a test engine. At US stations, EIA lists regular as generally 87, midgrade as 89 to 90 and premium as 91 to 94. When knock sensors detect knocking, the engine's computer delays the spark, moving combustion away from peak compression at some cost in efficiency.

Does isooctane have any double bonds?

No. Isooctane is an alkane, a hydrocarbon of general formula C_nH_2n+2 in Chem1's account, so its eight carbons carry eighteen hydrogens. Every carbon must have four bonds, Chem1 adds, which gives eight carbons 32 bond positions. The eighteen C–H bonds fill eighteen of them and the seven C–C bonds the other fourteen, one at each end. None is left for a double bond, and CAMEO Chemicals files isooctane among the saturated hydrocarbons.

Is isooctane the same as octane?

No, although both have the formula C₈H₁₈. Straight-chain octane strings its eight carbons end to end, while isooctane is a five-carbon chain carrying three methyl branches. Rats also handle them differently. In rat studies reported by HSDB, isooctane left almost entirely through the kidneys, while octane was split roughly evenly between urine and carbon dioxide breathed out, a difference that may be one factor in how each affects rat kidneys.

Does isooctane damage the kidneys?

In male rats it does. Repeated exposure leaves protein droplets in their kidney cells, HSDB reports, and evidence suggests the droplets hold alpha 2u-globulin, a small protein specific to male rat urine. Male kidneys, not female ones, retained one metabolite, a compound the body makes from it: 2,4,4-trimethyl-2-pentanol. One proposal has aldehyde metabolites binding the protein and hampering its processing; another study found no such bond. In people, its long-term effects are unknown, on EPA's account.

Made of the same elements

C₈H₁₈ Octane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.

All 18 compounds made of carbon and hydrogen

C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 bonds.