Hydrogen halides from Friedel-Crafts reactions

The opposite of the stabilisation of an ester is its activation. If we include in the concept ester the alkyl halides, their Friedel-Crafts reactions provide familiar examples of this phenomenon. An unusual example especially relevant to our present considerations is provided by some results made available to me in advance of publication by Giusti and Andruzzi. Their results [38] on the polymerisation of styrene by iodine and hydrogen iodide can be interpreted in terms of an organic iodide derived from styrene, probably 1-phenylethyl iodide, being activated by the co-ordination of one or two molecules of iodine. This process appears to polarise the C—I bond to such an extent that the normally stable ester becomes activated to a chain-propagating species and induces a pseudocationic polymerisation ... 

Friedel-Crafts aromatic substitution reactions have been widely explored in polymer chemistry [29,30] and generally proceed with Lewis acids such as AICI3 with elimination of hydrogen halides. In superacid solutions, however, the Friedel-Crafts reactions take place with dehydration from the oxygen of the carbonyl group and the proton of aromatics. The reactivity of the pro-tonated carbonyl group in the superacid can be further increased by the... 

Acyl halides bearing a-hydrogen atoms, and acetyl chloride in particular, have limited stability in the presence of strong Lewis acids. Elimination of proton from the acylium ion leads to the ketene. This intermediate undergoes ready Friedel-Crafts acylation, acetyl chloride eventually forming the diacetylace-tylium ion, ° which is a poor acylating agent. For this reason, excessive reaction temperatures and times should be avoided in Friedel-Crafts acetylations. 

In spite of the impression that may have been gained from the earlier part of this chapter, reactions involving the use of aluminum halides, and especially aluminum chloride, constitute the majority of Friedel-Crafts acylation reactions. The interaction of the three components, i.e. the acyl halide, the aromatic substrate and the aluminum halide, results in the formation of hydrogen halide and a complex of the aromatic ketone with aluminum halide from which the ketone is literated after hydrolysis. There are a number of sequences in which the three components can be mixed. 

This is a typical electrophilic aronatic substitution in which the aromatic substrate is attacked by some form of the arylsulphonylium cation and hydrogen displaced as a proton. The attacking reagent is formed by interaction of the sulphonyl chloride with a Friedel Crafts catalyst the catalyst should be selected from Fed, SbCl or Ind, since with these halides only catalytic quantities are requiredf in contrast to the equimolar quantities usually recommended for Friedel Crafts condensations %d.th acid halides (9). This choice of catalyst is important as it eliminates side reactions and helps considerably with the problem of catalyst removal from the polymers. 

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... 

---