Hydrogen fluoride with acyl halides

Crafts process is to generate carbonium ions from the alkyl or acyl halides. It would be expected, then, that a number of other combinations of starting materials and reagents which lead to carbonium ions should be capable of effecting acylation or alkylation. Indeed we find that olefins (p. 35), alcohols (p. 36), ethers (p. 36), and esters (p. 37) can be used as starting materials for aromatic alkylation reactions in the presence of such catalysts as boron trifluoride, sulfuric acid, or anhydrous hydrogen fluoride.69 Acylations can be carried out with acids (p. 37),64 acid halides (p. 230), and acid anhydrides (p. 37). The Fries reaction65 (in which phenolic esters are converted to hydroxy aromatic ketones by means of aluminum chloride) appears to be an example of a typical acylation reaction in which the ester itself acts as the source of an acyl carbonium ion ... 

After the reaction is completed the products can be poured into water and ice and the aqueous hydrofluoric acid disposed of down the drain. Care must again be exercised as the mixing of hydrogen fluoride and water generates considerable heat. No hazards or disposal difficulties are incurred with hydrofluoric acid in the usual drain lines as these are made of iron pipe and the hydrogen fluoride is soon absorbed as firm complexes with the iron. It is not detectable at any great distance from the source. For some reactions this simple form of disposal is not satisfactory, for example, in the preparation of an acyl halide or other product which reacts with water. Here distillation of the hydrogen fluoride... 

As hydrogen fluoride functions with equal ease in alkylation with olefins, alkyl halides, or alcohols, and in acylation with acids, acid anhydrides as well as acyl halides, a wide choice of reagents is possible and a separate operation of the reconversion of them is often saved. With aluminum chloride the alkyl halides and acyl halides are the preferred reagents and frequently must be made from more plentiful, cheaper, and readily available substances. 

A very reactive halogen atom, such as that of mi acyl or sulfonyl halide, is replaced by fluorine by the action of almost any inorganic fluoride. The most convenient method consists in heating gently a mixture of an acyl or sulfonyl chloride with zinc or antimony fluoride in an apparatus which permits the acyl fluoride to distil as it is formed. The acyl fluoride usually boils about 40° lower than the chloride, and its removal from the reaction mixture results in quantitative yields. Com- plete interchange also can be effected with hydrogen fluoride, but more elaborate equipment is required. Good results have been reported for the synthesis of formyl and acetyl fluorides from mixtures of fonnic or... 

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