Antimony pentachloride Friedel-Crafts reaction

Antimony pentachloride is a reactive Lewis acid that can be used for Friedel-Crafts reactions and some other Lewis-acid-catalyzed reactions. The HF-SbF5 system is known as magic acid, and carbocations are stabilized in this medium.353 By using the HF-SbF5 system, alkylation of acetophenone (a relatively unreactive aromatic compound) has been achieved (Scheme 87). 

Aromatic and polycyclic hydrocarbons give characteristic colors with antimony pentachloride or aluminum chloride in chloroform solution. A Friedel-Crafts reaction probably takes place, giving rise to colored triphenylmethane derivatives of the type... 

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. 

Oki and his co-workers (177) also found that these halogenated compounds (107) exhibited enormous differences in reactivity when they were treated with Lewis acids. The sc form undergoes a Friedel-Crafts type cyclization in the presence of titanium tetrachloride, which is a weak Lewis acid, whereas the ap form survives these conditions. The latter reacts in the presence of the stronger Lewis acid antimony pentachloride. This difference is apparently caused by a chloro group in proximity to the site where a cationic center develops during the reaction (Scheme 12). 

The initiation of tetrahydrofuran polymerization by direct addition of oxonium salts is of interest because it reveals a good deal about the mechanism, but for practical purposes the salts may be formed in the reaction mixture. The obvious method is, of course, to add a little epichlorohydrin to the mixture of monomer and Friedel Crafts reagent for only antimony pentachloride is sufficiently active to start the reaction with monomer alone, but other reactions which accomplish the same purpose are ... 

The importance of acyl cations in acetylation is indicated in several studies (Gore, 1955). Friedel-Crafts catalysts alter the rate of acylation but do not modify the selectivity of the reaction. For example, the antimony pentachloride-catalyzed benzoylation of toluene proceeds 1.3 x 103 more rapidly than the aluminum chloride-catalyzed reaction. 

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