Ethyl halides Friedel-Crafts reaction

Friedel-Crafts reaction. A type of reaction involving anhydrous aluminum chloride and similar metallic halides as catalysts, discovered in 1877 by Charles Friedel, a French chemist (1832-1899), and James Mason Crafts, an American chemist (1830-1917), during joint research in France it has been developed since then for many important industrial uses, exemplified by the condensation of ethyl chloride and benzene to form ethylbenzene and the manufacture of acetophenone from acetyl chloride and benzene. The name is now applied to a wide variety of acid-catalyzed organic reactions. 

The Friedel-Crafts reactions succeed because relatively stable carbocations can be formed by the reaction of aluminum chloride (an exceptionally strong Lews acid with which an alkyl halide will react) in solvents that are nonnucleophilic. The strongest nucleophile available in the reaction vessel is benzene, which attacks the cation to start the Friedel-Crafts reaction. If stable carbocations cannot be formed, as with methyl chloride or ethyl chloride, the AICI3 complexes can also alkylate benzenes. 

Alkylations of arenes by ethyl halides under Friedel-Crafts conditions have been carried out numerous workers. Excellent accounts of the developments made in this area are available from a review, as well as a recent book. Equations (9) to (11) illustrate some ethylation reactions. " ... 

Whether or not the complex in Figure 14.35 can actually form a free carbocation depends on the alkyl halide used in the reaction. Friedel-Crafts alkylation succeeds with a variety of alkyl halides, including methyl-, ethyl-, isopropyl-, and /erf-butyl chlorides and bromides (Fig. 14.36). 

We would not expect the methyl or ethyl cation to be formed in these reactions, and surely these alkylations take place by attack of benzene on the complexed halide. / r/-Butyl chloride, on the other hand, can produce the relatively stable r/-butyl cation, not an unlikely intermediate at all. So, the detailed mechanism of the Friedel-Crafts alkylation depends on the nature of the R group. 

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