Friedel-Crafts reaction
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The Friedel-Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. 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.
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[edit] Friedel-Crafts alkylation
Friedel-Crafts alkylation involves the alkylation of an aromatic ring and an alkyl halide using a strong Lewis acid catalyst. With anhydrous aluminium 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.
This reaction is reversible. In a reversed Friedel-Crafts reaction or Friedel-Crafts dealkylation, alkyl groups can be removed in the presence of protons and a Lewis acid.
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. The actual reaction product is 1,3,5-triethylbenzene with all alkyl groups as a meta substituent. 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.
[edit] Friedel-Crafts acylation
Friedel-Crafts acylation is the acylation of aromatic rings with an acyl chloride using a strong Lewis acid catalyst. 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 an aromatic carbocation is very stable compared to alkyl carbocations.
[edit] Scope & variations
- The acylated reaction product can be converted into the alkylated product via a Clemmensen reduction.
- A reaction modification with an aromatic phenyl ester as a reactant is called the Fries rearrangement.
- In the Scholl reaction two arenes couple directly.
- In the Zincke-Suhl reaction p-cresol is alkylated to a cyclohexadienone with tetrachloromethane
- In the Blanc chloromethylation a chloromethyl group is added to an arene with formaldehyde, hydrochloric acid and zinc chloride.
[edit] References
- ↑ Friedel, C.; Crafts, J. M. Compt. Rend. 1877, 84, 1392 & 1450.
- ↑ Price, C. C. Org. React. 1946, 3, 1. (Review)
- ↑ Groves, J. K. Chem. Soc. Rev. 1972, 1, 73. (Review)
- ↑ Eyley, S. C. Comp. Org. Syn. 1991, 2, 707-731. (Review)
- ↑ Heaney, H. Comp. Org. Syn. 1991, 2, 733-752. (Review)
- ↑ Preparation of 1,3,5-Tris(aminomethyl)-2,4,6-triethylbenzene from Two Versatile 1,3,5-Tri(halosubstituted) 2,4,6-Triethylbenzene Derivatives Karl J. Wallace, Robert Hanes, Eric Anslyn, Jeroni Morey, Kathleen V. Kilway, Jay Siegeld Synthesis 2005, 2080-2083. (DOI:10.1055/s-2005-869963)
[edit] See also
- Darzens-Nenitzescu synthesis of ketones
- Haworth phenanthrene synthesis
- Nencki reaction
