Octane

Not to be confused with octene or octyne.

For the gasoline rating system, see Octane rating. For other uses, see Octane (disambiguation).

Octane is a hydrocarbon and also an alkane with the chemical formula C₈H₁₈, and the condensed structural formula CH₃(CH₂)₆CH₃. Octane has many structural isomers that differ by the location of branching in the carbon chain. One of these isomers, 2,2,4-trimethylpentane (commonly called iso-octane), is used as one of the standard values in the octane rating scale.

Octane

Names
Systematic IUPAC name Octane[1]
Other names n-Octane
Identifiers
CAS Number	111-65-9 Y
3D model (JSmol)	Interactive image
Beilstein Reference	1696875
ChEBI	CHEBI:17590 N
ChEMBL	ChEMBL134886 Y
ChemSpider	349 Y
DrugBank	DB02440 N
ECHA InfoCard	100.003.539
EC Number	203-892-1
Gmelin Reference	82412
KEGG	C01387 Y
MeSH	octane
PubChem CID	356
RTECS number	RG8400000
UNII	X1RV0B2FJV Y
UN number	1262
CompTox Dashboard (EPA)	DTXSID0026882
InChI InChI=1S/C8H18/c1-3-5-7-8-6-4-2/h3-8H2,1-2H3 Y Key: TVMXDCGIABBOFY-UHFFFAOYSA-N Y
SMILES CCCCCCCC
Properties
Chemical formula	CH3(CH2)6CH3
Molar mass	114.232 g·mol−1
Appearance	Colourless liquid
Odor	Gasoline-like[2]
Density	0.703 g/cm3
Melting point	−57.1 to −56.6 °C; −70.9 to −69.8 °F; 216.0 to 216.6 K
Boiling point	125.1 to 126.1 °C; 257.1 to 258.9 °F; 398.2 to 399.2 K
Solubility in water	0.007 mg/dm3 (at 20 °C)
log P	4.783
Vapor pressure	1.47 kPa (at 20.0 °C)
Henry's law constant (kH)	29 nmol/(Pa·kg)
Conjugate acid	Octonium
Magnetic susceptibility (χ)	−96.63·10−6 cm3/mol
Refractive index (nD)	1.398
Viscosity	0.509 mPa·s (25 °C)[3] 0.542 mPa·s (20 °C)
Thermochemistry
Heat capacity (C)	255.68 J/(K·mol)
Std molar entropy (S⦵298)	361.20 J/(K·mol)
Std enthalpy of formation (ΔfH⦵298)	−252.1 to −248.5 kJ/mol
Std enthalpy of combustion (ΔcH⦵298)	−5.53 to −5.33 MJ/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H225, H304, H315, H336, H410
Precautionary statements	P210, P261, P273, P301+P310, P331
NFPA 704 (fire diamond)	1 3 0
Flash point	13.0 °C (55.4 °F; 286.1 K)
Autoignition temperature	220.0 °C (428.0 °F; 493.1 K)
Explosive limits	0.96 – 6.5%
Lethal dose or concentration (LD, LC):
LDLo (lowest published)	428 mg/kg (mouse, intravenous)[4]
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 500 ppm (2350 mg/m3)[2]
REL (Recommended)	TWA 75 ppm (350 mg/m3) C 385 ppm (1800 mg/m3) [15-minute][2]
IDLH (Immediate danger)	1000 ppm[2]
Related compounds
Related alkanes	Methane Ethane Propane Butane Pentane Hexane Heptane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Octane is a component of gasoline and petroleum. Under standard temperature and pressure, octane is an odorless, colorless liquid. Like other short-chained alkanes with a low molecular weight, it is volatile, flammable, and toxic. Octane is 1.2 to 2 times more toxic than heptane.^[5]

Isomers

N-octane has 23 constitutional isomers. 8 of these isomers have one stereocenter; 3 of them have two stereocenters.

(3S,4S)-3,4-Dimethylhexane (top left) and (3R,4R)-3,4-Dimethylhexane (top right) are non-superimposable mirror images, so they are chiral enantiomers. (meso)-3,4-Dimethylhexane (bottom) has a superimposable mirror image, so it is an achiral meso compound.

Achiral isomers:

• 2-Methylheptane
• 4-Methylheptane
• 3-Ethylhexane
• 2,2-Dimethylhexane
• 2,5-Dimethylhexane
• (meso)-3,4-Dimethylhexane
• 3,3-Dimethylhexane
• 3-Ethyl-2-methylpentane
• 3-Ethyl-3-methylpentane
• 2,2,4-Trimethylpentane (i.e. iso-octane)
• 2,3,3-Trimethylpentane
• 2,3,4-Trimethylpentane
• 2,2,3,3-Tetramethylbutane

Chiral isomers:

• (3R)-3-Methylheptane
• (3S)-3-Methylheptane
• (3R)-2,3-Dimethylhexane
• (3S)-2,3-Dimethylhexane
• (4R)-2,4-Dimethylhexane
• (4S)-2,4-Dimethylhexane
• (3R,4R)-3,4-Dimethylhexane
• (3S,4S)-3,4-Dimethylhexane
• (3R)-2,2,3-Trimethylpentane
• (3S)-2,2,3-Trimethylpentane

Production and use

In petrochemistry, octanes are not typically differentiated or purified as specific compounds. Octanes are components of particular boiling fractions.^[6]

A common route to such fractions is the alkylation reaction between iso-butane and 1-butene, which forms iso-octane.^[7]

Octane is commonly used as a solvent in paints and adhesives.

N-octane is the octane isomer that has the longest carbon skeleton. Unlike its constitutional isomers, it has a very low knock resistance.

The octane isomer, iso-octane, is used as one of the standards for octane ratings. It has a rating of 100 by definition.

The octane isomer 2,3,3-Trimethylpentane has an octane rating exceeding 100.