Octane is a hydrocarbon and also an alkane with the chemical formula C<sub>8</sub>H<sub>18</sub>, and the condensed structural formula CH<sub>3</sub>(CH<sub>2</sub>)<sub>6</sub>CH<sub>3</sub>. 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 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.

Isomers

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

thumb|437x437px|[[3,4-Dimethylhexane|(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.

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

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

{| class="wikitable sortable mw-collapsible"

|none|thumb|338x338px|N-octane is the octane isomer that has the longest carbon skeleton. Unlike its constitutional isomers, it has a very low knock resistance.

|none|thumb|271x271px|The octane isomer, iso-octane, is used as one of the standards for octane ratings. It has a rating of 100 by definition.

|none|thumb|224x224px|The octane isomer 2,3,3-Trimethylpentane has an octane rating exceeding 100.

|}

See also

  • Octane rating, measure of a fuel's ability to withstand compression in an internal combustion engine without causing engine knocking

References