These include hydrocracking, hydrogenation, alkylation (S2), oxidation, metalation and nucleophilic aromatic substitution. It further consists of alkylation and acylation. Uracil readily undergoes regular reactions including oxidation, nitration, and alkylation. In addition to protonation, pyridine undergoes N-centered alkylation, acylation, and "N"-oxidation. Quaternary ammonium compounds are prepared by the alkylation of tertiary amines with a halocarbon. It is used as a catalyst in reactions such as alkylation, dealkylation, hydrogenation and oxidation. In oil refining contexts, alkylation refers to a particular alkylation of isobutane with olefins. Major chemical intermediates from the alkylation with ethylene is ethylbenzene, precursor to styrene. More complex alkylation of a alcohols and phenols involve ethoxylation. Sulfides are typically prepared by alkylation of thiols. Alkylphenols are a family of organic compounds obtained by the alkylation of phenols. HF serves as a catalyst in alkylation processes in refineries. Primary phosphines are typically prepared by alkylation of phosphine. Electrophilic alkylation of simple pyrrole is uncommon. Isobutane is the principal feedstock in alkylation units of refineries.