Octane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH3(CH2)6CH3. Octane has many structural isomers that differ by the amount and 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.

Systematic IUPAC name
Other names
3D model (JSmol)
ECHA InfoCard 100.003.539
EC Number
  • 203-892-1
MeSH octane
RTECS number
  • RG8400000
UN number 1262
  • InChI=1S/C8H18/c1-3-5-7-8-6-4-2/h3-8H2,1-2H3 Y
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
0.007 mg/dm3 (at 20 °C)
log P 4.783
Vapor pressure 1.47 kPa (at 20.0 °C)
29 nmol/(Pa·kg)
Conjugate acid Octonium
−96.63·10−6 cm3/mol
  • 0.509 mPa·s (25 °C)[3]
  • 0.542 mPa·s (20 °C)
255.68 J/(K·mol)
361.20 J/(K·mol)
−252.1 to −248.5 kJ/mol
−5.53 to −5.33 MJ/mol
GHS labelling:
H225, H304, H315, H336, H410
P210, P261, P273, P301+P310, P331
NFPA 704 (fire diamond)
Flash point 13.0 °C (55.4 °F; 286.1 K)
220.0 °C (428.0 °F; 493.1 K)
Explosive limits 0.96 – 6.5%
Lethal dose or concentration (LD, LC):
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
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 (petrol). As with all low-molecular-weight hydrocarbons, octane is volatile and very flammable.

Use of the term in gasoline

"Octane" is colloquially used as a short form of "octane rating", particularly in the expression "high octane". "Octane rating" is an index of a fuel's ability to resist engine knock in engines having different compression ratios, which is a characteristic of octane's branched-chain isomers, especially iso-octane. The octane rating of gasoline is not directly related to the power output of an engine. Using gasoline of a higher octane than an engine is designed for cannot increase power output.

The octane rating was originally determined by mixing fuels from only normal heptane and iso-octane (2,2,4-trimethylpentane, a highly branched octane), and assigning anti-knock ratings of zero for normal heptane and 100 for pure iso-octane. The anti-knock rating of this mixture would be the same as the percentage of iso-octane in the mix. Different isomers of octane can contribute to a lower or higher octane rating. For example, n-octane (the straight chain of 8 carbon atoms with no branching) has a -20 (negative) Research Octane Rating, whereas pure iso-octane has an RON rating of 100. Some fuels have an octane rating higher than 100, notably those containing methanol or ethanol.

Metaphorical use

Octane became well known in American popular culture in the mid- and late 1960s, when gasoline companies boasted of "high octane" levels in their gasoline advertisements.

The compound adjective "high-octane", meaning powerful or dynamic, is recorded in a figurative sense from 1944.[5] By the mid-1990s, the phrase was commonly being used as an intensifier and it has found a place in modern English vernacular.


Octane has 18 structural isomers (24 including stereoisomers):

  • Octane (n-octane)
  • 2-Methylheptane
  • 3-Methylheptane (2 enantiomers)
  • 4-Methylheptane
  • 3-Ethylhexane
  • 2,2-Dimethylhexane
  • 2,3-Dimethylhexane (2 enantiomers)
  • 2,4-Dimethylhexane (2 enantiomers)
  • 2,5-Dimethylhexane
  • 3,3-Dimethylhexane
  • 3,4-Dimethylhexane (2 enantiomers + 1 meso compound)
  • 3-Ethyl-2-methylpentane
  • 3-Ethyl-3-methylpentane
  • 2,2,3-Trimethylpentane (2 enantiomers)
  • 2,2,4-Trimethylpentane (isooctane)
  • 2,3,3-Trimethylpentane
  • 2,3,4-Trimethylpentane
  • 2,2,3,3-Tetramethylbutane


  1. "octane - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification and Related Records. Retrieved 6 January 2012.
  2. NIOSH Pocket Guide to Chemical Hazards. "#0470". National Institute for Occupational Safety and Health (NIOSH).
  3. Dymond, J. H.; Oye, H. A. (1994). "Viscosity of Selected Liquid n‐Alkanes". Journal of Physical and Chemical Reference Data. 23 (1): 41–53. Bibcode:1994JPCRD..23...41D. doi:10.1063/1.555943. ISSN 0047-2689.
  4. "Octane". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  5. Oxford English Dictionary.
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