Alkylation (petroleum) In the petroleum industry, a chemical process in which an alkene (ethylene, propylene, and so forth) and a hydrocarbon, usually 2-methylpropane, are combined to produce a higher-molecular-weight and higher­carbon-number product. The product has a higher octane rating and is used to improve the quality of gasoline-range fuels. The process was originally developed during World War II to produce high-octane aviation gasoline. Its current main application is in the production of unleaded automotive gasoline. See ALKENE; GASOLINE.

The alkylation reaction is initiated by the addition of a proton (H+) to the olefin [reaction (l)J. The protonated olefin (carbonium ion) then reacts with the isobutane by abstraction of a proton from the isobutane to produce the t-butyl carbonium ion [reaction (2)]. Reaction of this tertiary carbonium ion with the olefin proceeds by combination of the two species [reaction (3)] to produce a more complex six-carbon carbonium ion which yields a stabilized product by abstraction of a proton from an­other molecule of isobutane [reaction (4)]. The reaction progresses to more product by reaction of the t-butyl and carbonium ion.

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