Friedel Craft acylation reaction is a type of electrophilic aromatic substitution reaction wherein the hydrogen of the aromatic ring is substituted with the acyl group (R-CO-), and a new Carbon-Carbon bond. The acylation reaction requires a Lewis acid catalyst such as AlCl3 and an acid chloride (R-CO-Cl). The byproduct formed is HCl.
Under the Friedel Craft Reaction condition, the aniline (Lewis base) binds to the electrophile AlCl3 (Lewis acid) to give a coordination complex (salt).
Acid + Base = Salt
The positive charge on the nitrogen is electron withdrawing, and it pulls the electron density of the ring by negative inductive effect (-I effect). It therefore, deactivates the ring for further acylation reactions. The complex precipitates out of the reaction mixture and the acylation reaction is not observed.
A similar case is seen for mono and disubstituted anilines.
In the case of Phenols, the acylation takes place at the oxygen (O-acylation) and not at the carbon of the ring (C-acylation) to give a phenolic ester.
Nevertheless, a method does exist to convert the O-acylated product to the C-acylated product. The O-acylated product after isolation is treated with an excess of AlCl3, a reaction known as Fries rearrangement to obtain ortho and para hydroxy acetophenone. The ortho and the para isomer can be later separated by chromatographic techniques.
The Phenol does not form a coordination complex with AlCl3 similar to the aniline as the lone pair on the OH is tightly held and is less basic than the -NH2 group.
But if the oxygen of the phenol is substituted, (example, Anisole) it then prefers to undergoes acylation reaction at the carbon of the benzene ring (C-acylation). The para isomer would be the major product.
In summary, Phenols do not undergo FC acylation reaction because of O-acylation and in the case of Anilines, it forms a salt with AlCl3 and precipitates out of the reaction mixture.
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