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Title: Isooctane  
Author: World Heritage Encyclopedia
Language: English
Subject: Octane rating, List of gasoline additives, Antiknock agent
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CAS number 540-84-1 YesY
PubChem 10907 YesY
ChemSpider 10445 YesY
EC number 208-759-1
UN number 1262
MeSH 2,2,4-trimethylpentane
ChEBI CHEBI:62805 YesY
RTECS number SA3320000
Beilstein Reference 1696876
Jmol-3D images Image 1
Molecular formula C8H18
Molar mass 114.23 g mol−1
Appearance Colorless liquid
Odor Odorless
Density 692 mg mL−1
Melting point

-107--107 °C, 165.71-165.83 K, -161--161 °F

Boiling point

99-99 °C, 372.2-372.6 K, 210-211 °F

log P 4.373
Vapor pressure 5.5 kPa (at 21 °C)
kH 3.0 nmol Pa−1 kg−1
λmax 210 nm
Refractive index (nD) 1.391
Std enthalpy of
−260.6–−258.0 kJ mol−1
Std enthalpy of
−5462.6–−5460.0 kJ mol−1
Standard molar
328.03 J K−1 mol−1
Specific heat capacity, C 242.49 J K−1 mol−1
GHS pictograms
GHS signal word DANGER
GHS hazard statements H225, H304, H315, H336, H410
GHS precautionary statements P210, P261, P273, P301+310, P331
EU Index 601-009-00-8
EU classification N
R-phrases R11, R38, R50/53, R65, R67
S-phrases (S2), S16, S29, S33
NFPA 704
Flash point −12 °C
396 °C
Explosive limits 1.1–6.0%
Related compounds
Related alkanes
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

2,2,4-Trimethylpentane, also known as isooctane, iso-octane, is an organic compound with the formula (CH3)3CCH2CH(CH3)2. It is one of several isomers of octane (C8H18). This particular isomer is the standard 100 point on the octane rating scale (the zero point is n-heptane). It is an important component of gasoline, frequently used in relatively large proportions to increase the knock resistance of the fuel.[2]

Strictly speaking, if we follow the standard meaning for ‘iso’, the name isooctane should be reserved for the isomer 2-methylheptane. However, 2,2,4-trimethylpentane is by far the most important isomer of octane and so, historically, it has ended up with this name.[3]


Isooctane is produced on a massive scale in the petroleum industry by distillation of petroleum. It can also be produced from isobutylene by dimerization (a variant of alkylation) using an Amberlyst catalyst to produce a mixture of iso-octenes. Hydrogenation of this mixture produces 2,2,4-trimethylpentane.[4]


Engine knocking is an unwanted process that can occur during combustion in internal combustion engines. Graham Edgar in 1926 added different amounts of n-heptane and 2,2,4-trimethylpentane to gasoline, and discovered that the knocking stopped when 2,2,4-trimethylpentane was added. This was the origin of the octane rating scale.[5] Test motors, using 2,2,4-trimethylpentane gave a certain performance which was standardized as 100 octane. The same test motors, run in the same fashion, using heptane, gave a performance which was standardized as 0 octane. All other compounds and blends of compounds then were graded against these two standards and assigned octane numbers.


In common with all hydrocarbons, inhalation or ingestion of large quantities of iso-octane is harmful. In rare cases a stronger reaction can occur.[6] It is also flammable (as one would expect for a fuel).

See also


External links

  • International Chemical Safety Card 0496
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