Fries Rearrangement

What is the Fries Rearrangement Reaction?

• Fries Rearrangement is an organic rearrangement reaction in which an aryl ester is transformed into a hydroxy aryl ketone with the help of a Lewis acid catalyst and an aqueous acid.
•  In this reaction, an acyl group belonging to the phenolic ester migrates to the aryl ring.
•  It is important to note that Fries rearrangement is ortho and para selective, i.e. the acyl group attaches itself at the ortho or para positions of the aryl ring. 
• The selectivity of the reaction can be directed by modifying the reaction conditions (such as the temperature under which the reaction is conducted, or the solvent used in the reaction).

An illustration detailing the Fries rearrangement undergone by phenyl acetate (acetoxy benzene) is provided above. Note that the products feature ortho and para migrations of the acyl group.

Fries Rearrangement Mechanism

Initially, the carbonyl oxygen belonging to the acyl group forms a complex with the Lewis acid catalyst (usually AlCl₃). The formation of the complex with the carbonyl oxygen is favoured over the complexation of the phenolic oxygen since the carbonyl oxygen is richer in electrons and is, therefore, a better Lewis base.

Now, the bond between the phenolic oxygen and the acyl complex becomes polarised, resulting in the rearrangement of the AlCl₃ bond to the phenolic oxygen. This results in the generation of an acylium carbocation.

The acylium carbocation goes on to attack the aromatic ring via an electrophilic aromatic substitution reaction. It is important to note that the orientation of this electrophilic aromatic substitution is temperature-dependent. Low reaction temperatures favour substitutions at the para position and relatively high temperatures favour ortho substitution.

The mechanism of the Fries rearrangement reaction is illustrated above. The use of a non-polar solvent in this reaction also favours the formation of ortho-substituted products. Highly polar solvents favour para substitution in this reaction.

Limitations of Fries Rearrangement

The key limitations of Fries rearrangement are listed below.

• Owing to its relatively harsh reaction conditions, only esters with relatively stable acyl components can be used in this reaction.
• Low yields are obtained when heavily substituted acyl components exist.
• The presence of deactivating or meta-directing groups on the aromatic ring results in low yields.

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