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Fries Rearrangement: Phenolic Esters to Ketones

By Chemca Editorial Team • Last Updated: January 2026 • 8 min read

The Fries Rearrangement involves the conversion of Phenolic Esters (phenyl esters) into ortho- and para-hydroxy aryl ketones. The reaction is catalyzed by Lewis acids such as anhydrous Aluminum Chloride ($AlCl_3$).

1. General Reaction

An ester of phenol undergoes an acyl migration from the phenolic oxygen to the aromatic ring carbon (ortho or para position).

$$ \underbrace{C_6H_5OCOR}_{\text{Phenyl Acetate}} \xrightarrow{\text{Anhyd. } AlCl_3, \ \Delta} \underbrace{o\text{-Hydroxyketone}}_{\text{Minor/Major}} + \underbrace{p\text{-Hydroxyketone}}_{\text{Major/Minor}} $$

Conditions:

• Substrate: Phenyl Ester ($Ph-O-CO-R$).
• Catalyst: Lewis Acid (Anhydrous $AlCl_3$, $BF_3$, $TiCl_4$).
• Solvent: Nitrobenzene, $CS_2$, or solvent-free.

2. Detailed Mechanism

The accepted mechanism is a combination of intermolecular and intramolecular pathways involving an Acylium Ion intermediate.

Step 1: Coordination and Cleavage

The carbonyl oxygen of the ester coordinates with the Lewis acid ($AlCl_3$). The $C-O$ bond weakens and breaks, generating an acylium ion ($R-C \equiv O^+$) and a phenoxide-aluminum complex.

$$ Ph-O-CO-R + AlCl_3 \rightarrow Ph-O-AlCl_2 + R-C \equiv O^+ + Cl^- $$

Step 2: Electrophilic Aromatic Substitution

The generated Acylium ion acts as an electrophile and attacks the aromatic ring of the phenoxide complex at the ortho or para position (Friedel-Crafts Acylation).

Step 3: Hydrolysis

Acidic workup hydrolyzes the aluminum complex to yield the final hydroxy ketone.

3. Temperature Effect (Crucial Concept)

The product distribution is heavily dependent on the reaction temperature.

Low Temperature ($< 60^\circ C$)

Major Product: Para-Isomer
This is the Kinetic Product. Formation at the para position is faster due to less steric hindrance.

High Temperature ($> 160^\circ C$)

Major Product: Ortho-Isomer
This is the Thermodynamic Product. The ortho isomer is more stable due to chelation (interaction between the carbonyl oxygen and aluminum atom, or intramolecular H-bonding in the final product).

4. Photo-Fries Rearrangement

The rearrangement can also be induced by UV light without a catalyst. This proceeds via a Free Radical Mechanism. The ester bond undergoes homolytic cleavage (Norrish Type I) to form phenoxy and acyl radicals, which recombine at ortho/para positions.

$$ Ph-O-CO-R \xrightarrow{h\nu} [PhO^\bullet + ^\bullet COR]_{\text{cage}} \rightarrow o/p\text{-Hydroxyketone} $$

5. Limitations

• Deactivating groups on the aromatic ring significantly decrease the yield (similar to Friedel-Crafts limitations).
• The acyl group can sometimes be lost entirely (deacylation) under harsh conditions.

Fries Rearrangement Quiz

Test your concepts on Phenolic ester rearrangement. 10 MCQs with explanations.

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