2,2,4-Trimethylpentane, also known as isooctane or 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.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||114.232 g·mol−1|
|Density||0.692 g cm−3|
|Melting point||−107.38 °C; −161.28 °F; 165.77 K|
|Boiling point||99.30 °C; 210.74 °F; 372.45 K|
|Vapor pressure||5.5 kPa (at 21 °C)|
|3.0 nmol Pa−1 kg−1|
|UV-vis (λmax)||210 nm|
Refractive index (nD)
Heat capacity (C)
|242.49 J K−1 mol−1|
|328.03 J K−1 mol−1|
Std enthalpy of
|−260.6 to −258.0 kJ mol−1|
Std enthalpy of
|−5462.6 to −5460.0 kJ mol−1|
|GHS signal word||DANGER|
|H225, H304, H315, H336, H410|
|P210, P261, P273, P301+310, P331|
|Flash point||−12 °C (10 °F; 261 K)|
|396 °C (745 °F; 669 K)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Strictly speaking, if the standard meaning of ‘iso’ is followed, 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 been assigned this name.
Engine knocking is an unwanted process that can occur during high compression ratios 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 work was the origin of the octane rating scale. Test motors using 2,2,4-trimethylpentane gave a certain performance that 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, 2,2,4-trimethylpentane is flammable.
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- Dimerization of isobutylene, Amberlyst.com
- Fuels and lubricants handbook, Volume 1, George E. Totten, Steven R. Westbrook, Rajesh J. Shah, page 62
- 2,2,4-Trimethylpentane, Integrated Risk Information System, United States Environmental Protection Agency