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 ex ...

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 which ...

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 chemical compound, compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct chemical bond, b ...

s that differ by the amount and location of branching in the

carbon chain In chemistry, catenation is the bonding of atoms of the same element into a series, called a ''chain''. A chain or a ring shape may be ''open'' if its ends are not bonded to each other (an open-chain compound), or ''closed'' if they are bonded ...

. One of these isomers, 2,2,4-trimethylpentane (commonly called iso-octane) is used as one of the standard values in the

octane rating An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating ...

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 co ...

(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 "

," particularly in the expression "high octane". "Octane rating" is an index of a fuel's ability to resist

engine knock In spark ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging or pinking) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignite ...

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

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 In linguistics, an intensifier (abbreviated ) is a lexical category (but ''not'' a traditional part of speech) for a modifier that makes no contribution to the propositional meaning of a clause but serves to enhance and give additional emotional co ...

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

stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...

s): *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 3,3-Dimethylhexane is a colourless, odourless liquid, chemical compound in the family of hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 ...

* 3,4-Dimethylhexane (2 enantiomers + 1

meso compound A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more stereocenters, the molecule is not chiral. A meso compound is "sup ...

) * 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