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Friedel-Crafts reactionThe Friedel-Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877.[1] There are two main types of Friedel-Crafts reactions: alkylation reactions and acylation reactions. This reaction type is part of electrophilic aromatic substitution. Several reviews have been written.[2][3][4][5] Additional recommended knowledge
Friedel-Crafts alkylationFriedel-Crafts alkylation involves the alkylation of an aromatic ring and an alkyl halide using a strong Lewis acid catalyst. With anhydrous ferric chloride as a catalyst, the alkyl group attaches at the former site of the chloride ion. This reaction has one big disadvantage, namely that the product is more nucleophilic than the reactant due to the electron donating alkyl-chain. Therefore, another hydrogen is substituted with an alkyl-chain, which leads to overalkyation of the molecule. Also, if the chlorine is not on a tertiary carbon, carbocation rearrangement reaction will occur. This is due to the relative stability of the tertiary carbocation over the secondary and primary carbocations. Steric hindrance can be exploited to limit the number of alkylations, as in the t-butylation of 1,4-dimethoxybenzene. Alkylations are not limited to alkyl halides: Friedel-Crafts reactions are possible with any carbocationic intermediate such as those derived from alkenes and a protic acid or lewis acid, enones and epoxides. In one study the electrophile is a bromonium ion derived from an alkene and NBS:[6] In this reaction samarium(III) triflate is believed to activate the NBS halogen donor in halonium ion formation. Friedel-Crafts dealkylationFriedel-Crafts alkylation is a reversible reaction. In a reversed Friedel-Crafts reaction or Friedel-Crafts dealkylation, alkyl groups can be removed in the presence of protons and a Lewis acid. For example, in a multiple addition of ethyl bromide to benzene, ortho and para substitution is expected after the first monosubstitution step because an alkyl group is an activating group. However, the actual reaction product is 1,3,5-triethylbenzene with all alkyl groups as a meta substituent.[7] Thermodynamic reaction control makes sure that thermodynamically favored meta substitution with steric hindrance minimized takes prevalence over less favorable ortho and para substitution by chemical equilibration. The ultimate reaction product is thus the result of a series of alkylations and dealkylations. Friedel-Crafts acylationFriedel-Crafts acylation is the acylation of aromatic rings with an acyl chloride using a strong Lewis acid catalyst. Friedel-Crafts acylation is also possible with acid anhydrides. Reaction conditions are similar to the Friedel-Crafts alkylation mentioned above. This reaction has several advantages over the alkylation reaction. Due to the electron-withdrawing effect of the carbonyl group, the ketone product is always less reactive than the original molecule, so multiple acylations do not occur. Also, there are no carbocation rearrangements, as the carbonium ion is stabilized by a resonance structure in which the positive charge is on the oxygen.
Reaction mechanismIn a simple mechanistic view, the first step consists of dissociation of a chlorine atom to form an acyl cation: This is followed by nucleophilic attack of the arene toward the acyl group: Finally, a chlorine atom reacts to form HCl, and the AlCl3 catalyst is regenerated: Friedel-Crafts hydroxyalkylationArenes react with certain aldehydes and ketones to the hydroxyalkylated product for example in the reaction of the mesityl derivative of glyoxal with benzene[8] to form a benzoin with an alcohol rather than a carbonyl group: Scope & variationsThis reaction is related to several classic named reactions:
DyesFriedel-Crafts reactions have been used in the synthesis of several triarylmethane and xanthene dyes [13]. Examples are the synthesis of thymolphthalein (a pH indicator) from two equivalents of thymol and phthalic anhydride: A reaction of phthalic anhydride with resorcinol in the presence of zinc chloride gives the fluorophore Fluorescein. Replacing resorcinol by N,N-diethylaminophenol in this reaction gives rhodamine B: References
FC reactions in Org. Synth.Friedel-Crafts reactions appear in Organic Syntheses:
See also
Categories: Substitution reactions | Carbon-carbon bond forming reactions |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Friedel-Crafts_reaction". A list of authors is available in Wikipedia. |