Boron trichloride Friedel-Crafts reaction

The ammonia is the base, while the boron trichloride is the acid, because it accepts the lone pair from the ammonia. This definition is of great use to the organic chemist in, for example, the Friedel-Crafts reaction. Here, the reagent, an alkyl halide, reacts with an aromatic compound such as toluene, in the presence of a catalyst such as aluminium trichloride. There must be approximately the same amount of the catalyst as there is alkyl halide, and not just trace amounts of the catalyst, i.e. the aluminium trichloride is acting in this reaction as a stoichiometric catalyst. The reason becomes apparent when... 

Lewis acids may also act as catalysts with an efficiency roughly parallel to that found in Friedel-Crafts and related reactions These rearrangements are first-order in acid and a substrate-acid adduct is believed to be rapidly formed which rearranges in a slow step with concomitant release of catalyst. Adducts of benzoyl azide with boron trichloride and trifluoride have been isolated at low temperatures... 

The reaction was found particularly useful as a relatively selective and mild nitration method, for example allowing mononitration of durene and other highly alkylated benzenes, which with mixed acid usually undergo dinitration. (Table XIl). Methyl nitrate-boron trifluoride can also be used to achieve dinitration of tet-ramethylbenzenes by using two and three molar excess of methyl nitrate, respectively. Relative yields of mono- and dinitro product compositions are shown in Table XIII. Other Friedel-Crafts type catalysts can also be used, but boron trifluoride was found to be the most suitable. Aluminum trichloride and titanium (IV) chloride in the nitration of pentamethylbenzene caused formation of significant amounts of chlorinated derivatives, whereas sulfuric acid led to nitrodemethylation products. 

Compared to the relatively young history of the pure metal, aluminium compounds have been known for ages from the above-cited alum class to the more exclusive transition metal-doped aluminium oxides like ruby and sapphire (corundum varieties with chromium for the former and titanium and iron impurities for the latter) or aluminosilicate-like emeralds (a beryl type with chromium and vanadium impurities). However, to the synthetic chemist, aluminium chloride, is de facto one of the first jewels of the aluminium family. Aluminium trichloride (together with titanium tetrachloride, tin tetrachloride and boron trifluoride) is an exemplary Lewis acid that finds many applications in organic synthesis It is extensively used for instance in Friedel-Crafts alkylations and acylations, in Diels-Alder-type cycloadditions and polymerisation reactions. Its involvement in a wide range of reactions has been documented in many reviews and book chapters. ... 

Cationic polymerization of unsaturated compounds proceeds through the stage of carbanion cations, called also carbocations. Typical catalysts for this reaction are strong protic acids such as sulfuric acid, perchloric and trifluoroctane or the Lewis acids, which include halides of elements III, IV and V groups of the periodic table (Friedel-Crafts catalysts), such as boron trifluoride, aluminum trichloride, tin tetrachloride and titanium tetrachloride. The activity of Friedel-Crafts catalysts increases significantly the presence of small quantities of cocatalysts, that is, ihe compounds which most often are the source of protons. 

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