Friedel-Crafts catalysts, hydrogen fluoride

Hydrogen fluoride acts both as a Friedel-Crafts catalyst and a fluorinating agent in a one-step preparation of trifluoromethylated aromatics [10] (Figure 2.3). 

Since the Friedel-Crafts catalysts are usually thought of as halides of a few metals, a natural question to raise might be If these halides are really acids differing in no fundamental manner from H-acids in their behavior, why do not H-acids catalyze reactions of the Friedel-Crafts type also The answer is that they do. Hydrogen fluoride,phosphoric acid, and sulfuric acid have... 

Other catalysts which may be used in the Friedel - Crafts alkylation reaction include ferric chloride, antimony pentachloride, zirconium tetrachloride, boron trifluoride, zinc chloride and hydrogen fluoride but these are generally not so effective in academic laboratories. The alkylating agents include alkyl halides, alcohols and olefines. 

Polyphosphoric acid is a commonly used catalyst for this reaction however, in some cases a mixture of hydrogen bromide/acetic acid gives better results. Acylation of the S-phenyl-, V-(4-tolyl)- or S-(l-naphthyl)-substituted thiobenzenepyruvic acids 3a-c affords the corresponding dibenzo[A,/]thiepins in satisfactory yields, while reaction of the S-(4-methoxyphenyl) or S-(2-naphthyl) derivatives fails to provide any thiepin.60 The intramolecular Friedel-Crafts acylation of 2-(arylsulfanyl)benzeneacetic acids also yields the corrresponding dibenzothiepins in this case the use of hydrogen fluoride sometimes results in purer products.38 The applicability of this method is restricted to the synthesis of stable bisannulated thiepins. 

With halide catalysts of the Friedel-Crafts type (e.g., aluminum chloride or boron fluoride) in the presence of hydrogen halide the formation of the carbonium ion results in the addition of the proton from the promoter to the pi electrons ... 

Linear alkylbenzenes are made from -paraffins (C10 to C14) by either partial dehydrogenation to olefins and addition to benzene with hydrogen fluoride (HF) as catalyst or by chlorination of the paraffins and Friedel-Crafts reaction with benzene and an aluminum chloride catalyst. 

The alkylation of paraSins with olefins to yield higher molecular weight branched-chain paraffins may be carried out thermally or catalyt-ically. The catalysts for the reaction fall into two principal classes, both of which may be referred to as acid-acting catalysts (1) anhydrous halides of the Friedel-Crafts type and (2) acids. Representatives of the first type are aluminum chloride, aluminum bromide, zirconium chloride, and boron fluoride gaseous hydrogen halides serve as promoters for these catalysts. The chief acid catalysts are concentrated sulfuric acid and liquid hydrogen fluoride. Catalytic alkylations are carried out under sufficient pressure to keep at least part of the reactants in the liquid phase. 

Halide catalysts of the Friedel-Crafts type. The most important catalysts of this class are anhydrous aluminum chloride and boron fluoride promoted by small amounts of hydrogen chloride and hydrogen fluoride, respectively. It has been suggested that a function of the hydrogen halide is to convert the aluminum or boron halide to a more active acidic form. For example ... 

In Friedel-Crafts reactions, ortho substitution of the polymer is more likely to occur if the reaction is conducted at elevated temperatures and for a relatively long reaction time. To overcome these problems, it has been proposed to use a boron trifluoride catalyst in anhydrous hydrogen fluoride. Another procedure uses lithium chloride and aluminum chloride to polymerize p-phenoxybenzoyl chloride as the ketone monomer and p-phenoxybenzenesulfonyl chloride as the sulfone monomer. ... 

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