Octane

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.

Octane
Names
Systematic IUPAC name
Octane[1]
Other names
n-Octane
Octacarbane
Identifiers
CAS Number
3D model (JSmol)
3DMet
Beilstein Reference
 1696875
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.003.539
EC Number
  • 203-892-1
Gmelin Reference
 82412
KEGG
MeSH octane
PubChem CID
RTECS number
  • RG8400000
UNII
UN number 1262
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):
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.

Isomers

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

References
  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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