Octane

Octane is a

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. For examples, as a neurotoxin with narcotic effects, n-octane is almost twice times as toxic as n-heptane.^[1]

Isomers

N-octane has 23

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

Octane in Culture

"Octane" is colloquially used in the expression "high-octane".^[2] The term is used to describe a powerful action because of the association with the concept of "octane rating". This is a misleading term, because 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 its power output.

Octane became well known in American popular culture in the 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. By the 1990s, the phrase was commonly being used as a word intensifier, and it has found a place in modern English slang.

Difference between Octane and Octane Rating

Common Misconceptions on Octane Rating

Due to its name, the chemical octane is often misunderstood as the only substance that determines the octane rating (or octane number) of a fuel. This is an inaccurate description. In reality, the octane rating is defined as a number describing the stability and ability of a fuel to prevent an engine from unwanted combustions^[3] that occur spontaneously in the other regions within a cylinder (i.e. delocalized explosions from the spark plug). This phenomenon of combustion is more commonly known as engine knocking or self-ignition, which causes damage to pistons over time and reduces the lifespan of engines.

In 1927, Graham Edgar^[4] devised the method of using iso-octane and n-heptane as reference chemicals, in order to rate the knock resistance of a fuel with respect to this isomer of octane,^[5] thus the name "octane rating". By definition, the isomers iso-octane and n-heptane have an octane rating of 100 and 0, respectively.^[6] Because of its more volatile nature, n-heptane ignites and knocks readily, which gives it a relatively low octane rating^[7]; the isomer iso-octane causes less knocking because it is more branched and combusts more smoothly. In general, branched compounds with a higher intermolecular force (e.g. London dispersion force for iso-octane) will have a higher octane rating, because they are harder to ignite.^[8]

Examples of misconceptions

There are numerous articles circulating the internet that propagate this misunderstanding. Below are some factually inaccurate statements.

Statement 1

When faced with the removal of lead as the primary octane provider in gasoline, refiners had two available alternatives, BTEX and ethanol....[5]

This statement is inaccurate, even though the alternatives indeed have a high octane rating. Alternative fuels do not contain octane, and cannot "provide" octane.

Statement 2

So, octane does not enhance the explosion in the cylinder like most people think... It just prevents the air-fuel mixture from igniting before the spark plug does it.[9]

While the first part of this statement is factual, the complete statement implies that octane is the only chemical contributing to octane rating, which is false.

Statement 3

You may be surprised by how much octane is in E85. Ethanol naturally has a high octane rating...[10]

This is a misleading statement. Octane rating is not necessarily (in fact, rarely) due to the presence of octane. The chemical ethanol itself has a high octane rating, resulting in high knock resistance in E85 fuels.

Octane Ratings of Octane Isomers

Octane isomers such as n-octane and 2,3,3-trimethylpentane have an octane rating of -20 and 106.1, respectively (RON measurement).^[11] The large differences between the octane ratings for the isomers show that the compound octane itself is clearly not the only factor that determines octane ratings, especially for commercial fuels consist of a wide variety of compounds.

Octane

Names
Systematic IUPAC name Octane[12]
Other names n-Octane
Identifiers
CAS Number	111-65-9 Y
3D model ( JSmol )	Interactive image
3DMet	B00281
Beilstein Reference	1696875
ChEBI	CHEBI:17590 N
ChEMBL	ChEMBL134886 Y
ChemSpider	349 Y
DrugBank	DB02440 N
ECHA InfoCard	100.003.539
EC Number	203-892-1
Gmelin Reference	82412
KEGG	C01387 Y
MeSH	octane
PubChem CID	356
RTECS number	RG8400000
UNII	X1RV0B2FJV Y
UN number	1262
CompTox Dashboard (EPA)	DTXSID0026882
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[13]
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)[14] 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):
LDLo (lowest published)	428 mg/kg (mouse, intravenous)[15]
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 500 ppm (2350 mg/m3)[13]
REL (Recommended)	TWA 75 ppm (350 mg/m3) C 385 ppm (1800 mg/m3) [15-minute][13]
IDLH (Immediate danger)	1000 ppm[13]
Related compounds
Related alkanes	Methane Ethane Propane Butane Pentane Hexane Heptane
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

Production and Use

Industrial Production

Like other alkanes, octane is commonly synthesised by industrial plants in large scales, which can be broken down into 3 steps.

Natural Gas Liquid (NGL) fractionation^[16]

From a petroleum or natural gas source, different alkanes are extracted at specific temperature and pressure, due to their different physical properties.

Alkylation Units

Shorter hydrocarbons (e.g. ethane, butane, iso-butene) can be combined to form longer hydrocarbons like octane. A common example is the alkylation reaction between iso-butane and 1-butene, which forms iso-octane.^[17]

Catalytic Reforming

Through isomerisation, different isomers of octane can be produced.

Laboratory Use

Octane is not only found in gasoline, but it is also commonly used as a solvent in paints and adhesives. Moreover, it can be used an organic, non-polar solvent for various reactions, such as selective alkylations and hydrogenations.^[18]

Because of octane's role in metabolism^[19], the mechanism of its breakdown and oxidation is studied in biochemical labs, especially in organisms such as rats^[20] and E. Coli^[21].

References