Aluminum Chloride-Nitrobenzene Complex

Chlorides, listing 2 C236—C241 acetyl chloride 1 A56 aluminum chloride 3 C236 aluminum chloride-nitrobenzene complex 3C236... 

Aluminum chloride dissolves readily in chlorinated solvents such as chloroform, methylene chloride, and carbon tetrachloride. In polar aprotic solvents, such as acetonitrile, ethyl ether, anisole, nitromethane, and nitrobenzene, it dissolves forming a complex with the solvent. The catalytic activity of aluminum chloride is moderated by these complexes. Anhydrous aluminum chloride reacts vigorously with most protic solvents, such as water and alcohols. The ability to catalyze alkylation reactions is lost by complexing aluminum chloride with these protic solvents. However, small amounts of these "procatalysts" can promote the formation of catalyticaHy active aluminum chloride complexes. 

Free-ion attack is more likely for sterically hindered R. ° The ion CH3CO " has been detected (by IR spectroscopy) in the liquid complex between acetyl chloride and aluminum chloride, and in polar solvents such as nitrobenzene but in nonpolar solvents such as chloroform, only the complex and not the free ion is present. In any event, 1 mol of catalyst certainly remains complexed to the product at the end of the reaction. When the reaction is performed with RCO" SbF6, no catalyst is required and the free ion (or ion pair) is undoubtedly the attacking entity. ... 

Large discrepancies were found in applying the treatment to the Friedel-Crafts benzoylation of the methylbenzenes in nitrobenzene solution (Brown et al., 1958a). It was suggested that these difficulties arose from the formation of ternary complexes of aluminum chloride, the aromatic hydrocarbon, and the solvent nitrobenzene. This notion was tested by a study of the total and positional rates of acetylation... 

The most common solvents are benzene and carbon disulfide. Others such as petroleum ether and sj/m-tetrachloroethane also have been employed successfully. Nitrobenzene has served as a unique solvent in conjunction with aluminum chloride, since a complex is formed reducing the activity of the catalyst. 

Naphthalene can be acetylated by acetyl chloride in the presence of aluminum chloride. The orientation of substitution is determined by the particular solvent used predominantly alpha in carbon disulfide or solvents like tetrachloroethane, predominantly beta in nitrobenzene. (The effect of nitrobenzene has been attributed to its forming a complex with the acid chloride and aluminum chloride which, because of its bulkiness, attacks the roomier beta position.)... 

Nitrobenzene is a preferred solvent for some Friedel-Crafts reactions both because it has high solvent power for organic compounds and because it forms a complex with, and dissolves, aluminum chloride. Thus 532 g. of aluminum chloride, added to 2.1 1. of nitrobenzene in 50-g. portions with stirring and with cooling as required, dissolves and remains in solution at room temperature. ... 

On addition of aluminum chloride to a solution of the ether in nitrobenzene at room temperature a red or violet complex is formed. After a few hours, ice and a little hydrochloric acid are added, when the complex goes into the aqueous phase. When this layer is separated and heated with dilute hydrochloric acid at 50° for 2 hrs., the demethylated product is liberated and separates on cooling. A carbomethoxy group if present remains intact. 

Nitrobenzene and nitroalkanes are good solvents for Friedel-Crafts acylation reactions. As well as being good solvents they also form addition complexes with Lewis acids such as aluminum chloride. The formation of the complex appears to reduce disproportionation and rearrangement reactions and thus allow acylation to be achieved under mild conditions. The acetylation of toluene in nitrobenzene affords more 4-methylacetophenone than when the reaction is conducted in cafbon disulfide. These results evidently reflect a lower steric demand in the reaction carried out in carbon disulfide. The reaction shown in equation (17) when carried out in nitrobenzene leads to the formation of the products (1) and (2) in good yield. However, when the solvent was changed to nitroethane, an improved yield (82%) was obtained and the ratio of (1) (2) changed from 44 1 to 61 1. ... 

Friedel-Crafts Alkylation and Acylation. Just as for nitrobenzene, these reactions cannot be accomplished Halogenation. With two moles of aluminum chloride (one that complexes on nitrogen and the other to activate the halogen) and a reaction temperature of 80-115°C, pyridine can be chlorinated at the 3-position. These conditions also give some... 

Quinolines and isoquinolines can also react with electrophiles at the pyridine side. This can be rationalized by a different reaction mechanism involving the prior introduction of a nucleophile in the heterocyclic quinoline/isoquinoline ring followed by an electrophilic substitution involving attack on the intermediate enamine. Notable is the electrophilic bromination of isoquinoline hydrobromide in a solvent like nitrobenzene that provides 81% yield of 4-bromoisoquinoline, in contrast to the bromination or chlorination of an isoquinoline-aluminum chloride complex that affords 78% of 5-bromoisoquinoline. Exhaustive bromination or chlorination under Lewis acid conditions usually yields mixtures of 5,8-halogenated isoquinolines along with 5,7,8-trisubstituted derivatives. ... 

This reaction was first reported by Baddeley in 1930s. It is the migration of alkyl groups in polyalkylbenzenes or polynuclear aromatic compounds in the presence of anhydrous aluminum chloride or the mixture of protonic acid and Lewis acid. In one of Badde-ley s experiments, when 1,3,4-tri-n-propylbenzene was warmed with AICI3 at 100°C, the 1,3,5-tri-n-propyl was formed, along with the lower and higher alkylated benzenes. After extensive studies, it was found that the amount of a-isomer can be reduced by the addition to the reaction mixture of a variety of substances (e.g., nitrobenzene and excess acid chloride) that complex strongly with aluminum chloride. Likewise, less a-isomer has been observed when this reaction is carried out in nitrobenzene. In addition, isomerization of hindered aromatic ketones occurs if the ketones are melted with an excess amount of aluminum chloride and sodium chloride. ... 

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