Isooctane

isooctane [¦ī·sō′äk‚tān]

(organic chemistry)

(CH₃)₂CHCH₂C(CH₃)₃Flammable, colorless liquid boiling at 99°C; slightly soluble in alcohol and ether, insoluble in water; used in motor fuels and as a chemical intermediate.

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Isooctane 

2,2,4-trimethylpentane, the saturated aliphatic hydrocarbon C(CH₃)₃—CCH₂—CH(CH₃)₂.

Isooctane is a transparent colorless liquid with a gasoline odor. Melting point, - 107.38°C; boiling point, 99.24°C; density, 0.69192 g/cm³ (20°C); n_D²⁰, 1.39145; heat of combustion, 5.463 megajoules per mole, or 1,305.29 kilocalories per mole (25°C; p = const); heat of vaporization, 307.63 joules per g, or 73.50 calories per g (25°C). Isooctane is insoluble in water but soluble in ordinary organic solvents. It forms azeotropes (for example, with benzene, methanol, and ethanol). The knock rating of isooctane is assigned an octane number of 100.

Straight-run gasolines contain small amounts of isooctane along with other octane isomers. Industrially, isooctane is made by hydrogenating diisobutylene over a catalyst (for example, copper-chromium) or by alkylating isobutane with isobutylene in the presence of concentrated H₂S0₄, A1C1₃, BF₃, or other catalysts. Isooctane is used (as an additive) in making aviation gasolines, which must have excellent antiknock properties.