Octane is a
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
and an
alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in wh ...
with the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
, and the condensed structural formula . Octane has many
structural isomer
In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term meta ...
s 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 is a component of
gasoline
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic c ...
(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
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic c ...
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.
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
In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term meta ...
(24 including
stereoisomers):
*Octane (''n''-octane)
*
2-Methylheptane
*
3-Methylheptane (2
enantiomers)
*
4-Methylheptane
*
3-Ethylhexane
*
2,2-Dimethylhexane
The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline o ...
*
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
External links
*
*
* Dr. Duke's Phytochemical and Ethnobotanical Databases, Octane
{{Hydrides by group
Alkanes
Hydrocarbons