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AlCl 3 is a common Lewis-acid catalyst for Friedel-Crafts reactions, both acylations and alkylations. [14] Important products are detergents and ethylbenzene . These types of reactions are the major use for aluminium chloride, for example, in the preparation of anthraquinone (used in the dyestuffs industry) from benzene and phosgene . [ 12 ]
A typical example of a Lewis acid in action is in the Friedel–Crafts alkylation reaction. [5] The key step is the acceptance by AlCl 3 of a chloride ion lone-pair, forming AlCl − 4 and creating the strongly acidic, that is, electrophilic, carbonium ion. RCl +AlCl 3 → R + + AlCl − 4
A typical Lewis acid catalyst is aluminium trichloride. Because, however, the product ketone forms a rather stable complex with Lewis acids such as AlCl 3, a stoichiometric amount or more of the "catalyst" must generally be employed, unlike the case of the Friedel–Crafts alkylation, in which the catalyst is constantly regenerated. [13]
For similar reasons, anhydrous aluminium salts cannot be made by heating their "hydrates": hydrated aluminium chloride is in fact not AlCl 3 ·6H 2 O but [Al(H 2 O) 6]Cl 3, and the Al–O bonds are so strong that heating is not sufficient to break them and form Al–Cl bonds instead: [2] 2[Al(H 2 O) 6]Cl 3 Al 2 O 3 + 6 HCl + 9 H 2 O
Two common modes of Lewis acid catalysis in reactions with polar mechanisms. In reactions with polar mechanisms, Lewis acid catalysis often involves binding of the catalyst to Lewis basic heteroatoms and withdrawing electron density, which in turn facilitates heterolytic bond cleavage (in the case of Friedel-Crafts reaction) or directly activates the substrate toward nucleophilic attack (in ...
This generates a free acylium carbocation which reacts in a classical electrophilic aromatic substitution with the aromatic ring. The abstracted proton is released as hydrochloric acid where the chlorine is derived from aluminium chloride. The orientation of the substitution reaction is temperature dependent.
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A Lewis acid – such as zinc chloride (ZnCl 2), iron(III) chloride (FeCl 3), or aluminum chloride (AlCl 3) – coordinates to the halogen on the acid halide, activating the compound towards nucleophilic attack by an activated aromatic ring. For especially electron-rich aromatic rings, the reaction will proceed without a Lewis acid.