The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring.^[1] Friedel–Crafts reactions are of two main types: alkylation reactions and acylation reactions. Both proceed by electrophilic aromatic substitution.^[2][3][4][5]

Friedel–Crafts alkylation involves the alkylation of an aromatic ring with an alkyl halide using a strong Lewis acid, such as aluminium chloride, ferric chloride, or other MXn reagent, as catalyst.^[6] The general mechanism for tertiary alkyl halides is shown below.^[7]

For primary (and possibly secondary) alkyl halides, a carbocation-like complex with the Lewis acid, [R(+)---(X---MXn)(–)] is more likely to be involved, rather than a free carbocation.

This reaction suffers from the disadvantage that the product is more nucleophilic than the reactant because alkyl groups are activators for the Friedel–Crafts reaction. Consequently, overalkylation can occur. Steric hindrance can be exploited to limit the number of alkylations, as in the t-butylation of 1,4-dimethoxybenzene.^[8]

Furthermore, the reaction is only useful for primary alkyl halides in an intramolecular sense when a 5- or 6-membered ring is formed. For the intermolecular case, the reaction is limited to tertiary alkylating agents, some secondary alkylating agents (ones for which carbocation rearrangement is degenerate), or alkylating agents that yield stabilized carbocations (e.g., benzylic or allylic ones). In the case of primary alkyl halides, the carbocation-like complex (R(+)---X---Al(-)Cl3) will undergo a carbocation rearrangement reaction to give almost exclusively the rearranged product derived from a secondary or tertiary carbocation.^[7]

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, Lewis acid, enones, and epoxides. An example is the synthesis of neophyl chloride from benzene and methallyl chloride:^[9]

In one study the electrophile is a bromonium ion derived from an alkene and NBS:^[10]

In this reaction samarium(III) triflate is believed to activate the NBS halogen donor in halonium ion formation.

Friedel–Crafts alkylation has been hypothesized to be reversible. In a retro-Friedel–Crafts reaction or Friedel–Crafts dealkylation, alkyl groups are removed in the presence of protons or other 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.^[11] 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.^[12]

Friedel–Crafts acylation involves the acylation of aromatic rings. Typical acylating agents are acyl chlorides. Typical Lewis acid catalysts are acids and aluminium trichloride. However, because the product ketone forms a rather stable complex with Lewis acids such as AlCl3, 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. Friedel–Crafts acylation is also possible with acid anhydrides.^[13] Reaction conditions are similar to the Friedel–Crafts alkylation. 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 acylium ion is stabilized by a resonance structure in which the positive charge is on the oxygen.

The viability of the Friedel–Crafts acylation depends on the stability of the acyl chloride reagent. Formyl chloride, for example, is too unstable to be isolated. Thus, synthesis of benzaldehyde through the Friedel–Crafts pathway requires that formyl chloride be synthesized in situ. This is accomplished by the Gattermann-Koch reaction, accomplished by treating benzene with carbon monoxide and hydrogen chloride under high pressure, catalyzed by a mixture of aluminium chloride and cuprous chloride.

The reaction proceeds through generation of an acylium center. The reaction is completed by deprotonation of the arenium ion by AlCl4−, regenerating the AlCl3 catalyst. Nevertheless, in contrast to the alkylation reaction, the formed ketone is a moderate nucleophile, which forms a complex with aluminum trichloride. The formation of this complex is typically irreversible under reaction conditions. Thus, a stochiometric quantity of AlCl3 is needed. The complex is destroyed upon aqueous workup to give the desired ketone. For example, the classical synthesis of deoxybenzoin calls for 1.1 equivalents of AlCl3 with respect to the limiting reagent, phenylacetyl chloride.^[14]

If desired, the resulting ketone can be subsequently reduced to the corresponding alkane substituent by either Wolff–Kishner reduction or Clemmensen reduction. The net result is the same as the Friedel–Crafts alkylation except that rearrangement is not possible.^[15]

Arenes react with certain aldehydes and ketones to form the hydroxyalkylated products, for example in the reaction of the mesityl derivative of glyoxal with benzene:^[16]

As usual, the aldehyde group is more reactive electrophile than the phenone.

This reaction is related to several classic named reactions:

[[INLINE_IMAGE|//upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Bogert-Cook_Synthesis.png/500px-Bogert-Cook_Synthesis.png|//upload.wikimedia.org/wikipedia/commons/0/0d/Bogert-Cook_Synthesis.png 1.5x|Bogert-Cook Synthesis.png|h130|w500]]

Friedel–Crafts reactions have been used in the synthesis of several triarylmethane and xanthene dyes.^[32] 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:

The Haworth reaction is a classic method for the synthesis of 1-tetralone.^[33] In this reaction, benzene is reacted with succinic anhydride, the intermediate product is reduced and a second FC acylation takes place with addition of acid.^[34]

In a related reaction, phenanthrene is synthesized from naphthalene and succinic anhydride in a series of steps which begin with FC acylation.

Reaction of chloroform with aromatic compounds using an aluminium chloride catalyst gives triarylmethanes, which are often brightly colored, as is the case in triarylmethane dyes. This is a bench test for aromatic compounds.^[35]