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Friedel–Crafts alkylations can be reversible. Although this is usually undesirable it can be exploited; for instance by facilitating transalkylation reactions. [10] 1,3-Diisopropylbenzene is produced via transalkylation, a special form of Friedel–Crafts alkylation. It also allows alkyl chains to be added reversibly as protecting groups.
The overall reaction mechanism, denoted by the Hughes–Ingold mechanistic symbol S E Ar, [3] begins with the aromatic ring attacking the electrophile E + (2a). This step leads to the formation of a positively charged and delocalized cyclohexadienyl cation, also known as an arenium ion, Wheland intermediate, or arene σ-complex (2b).
The Friedel-Crafts alkylation process involves chlorination of n-paraffins to monochloroparaffins followed by alkylation of benzene using aluminum chloride (AlCl 3) catalyst. This method is one of the oldest commercial routes to LABs. Each process generates LAB products with distinct features.
Friedel-Crafts alkylation: alkylbenzenes can be synthesized from olefins or alkyl halides with aromatic compounds in the presence of a catalyst such as AlCl 3, HF, or H 2 SO 4. [ 4 ] Gattermann-Koch reaction : named after German chemists Ludwig Gattermann and Julius Arnold Koch , the Gattermann-Koch reaction is a catalyzed formylation of ...
Nucleophilic alkylating agents can displace halide substituents on a carbon atom through the SN2 mechanism. With a catalyst, they also alkylate alkyl and aryl halides, as exemplified by Suzuki couplings. The Kumada coupling employs both a nucleophilic alkylation step subsequent to the oxidative addition of the aryl halide (L = Ligand, Ar = Aryl).
Clemmensen reduction conditions are particularly effective at reducing aryl [4]-alkyl ketones, [5] [6] such as those formed in a Friedel-Crafts acylation. The two-step sequence of Friedel-Crafts acylation followed by Clemmensen reduction constitutes a classical strategy for the primary alkylation of arenes.
Alkylation of various benzhydryl compounds has been demonstrated using the corresponding alkyl halides, both primary (benzyl chloride, β-phenylethyl chloride, and n-octyl bromide) and secondary (benzhydryl chloride, α-phenylethyl chloride, and isopropyl chloride), in yields between 86 and 99%.
The mechanisms are difficult to distinguish because many Lewis acids can behave as oxidants. Reactions taking place at room-temperature with well-known one-electron oxidizing agents likely proceed through a radical cation mechanism and reactions requiring elevated temperatures likely proceed through an arenium ion mechanism.