A process in which an alkyl group is added to another organic molecule such as by removing a hydrogen atom from an alkane and adding a methyl group. In the refining of crude oil, it is used to upgrade petroleum through the alkylation of isobutane with alkenes (olefins) such as propene, in the presence of either sulphuric or hydrofluoric acid as a catalyst. The reaction takes place as a two-phase reaction at ambient temperature. The reaction products are a mixture of branched hydrocarbons with a high octane rating. The octane number of the mixture depends mainly on the kind of alkenes used. Iso-octane has an octane rating of 100 and is the result of reacting isobutane with butene (butylene).