Since the discovery by Friedel and Crafts that aluminum chloride catalyzes the condensation of alkyl and acyl halides with various aromatic compounds to effect substitution of an alkyl or acyl group for more or more hydrogen atoms of the aromatic compound, this reaction has been greatly extended in scope with respect to alkylating or acylating agents and catalysts. The use of aluminum chloride as a catalyst was studied by Thomas. Aspects of this study appeared in an earlier volume of this series. The present discussion is limited to the direct alkyl, cycloalky, or aralkyl residues containing no functional groups into various aromatic compounds under various catalysts, such as e.g., aluminum trichloride, zinc dichloride, and hydrogen fluoride. The alkylating agents include olefins, highly strained cycloparaffins, polyalkylbenzenes, alkyl halides, alcohols, esters, of inorganic and organic acids, and ethers. The aromatic compound may be e.g., a hydrocarbon, an aryl chloride, or bromide, a mono-or polyhdrydric phenol or its ether. The Friedel-Crafts process is frequently the most useful method for the introduction of an alkyl group.