Keto-enol Tautomerism - Phenols

Phenols

In certain aromatic compounds such as phenol, the enol is important due to the aromatic character of the enol but not the keto form. Melting the naphthalene derivative 1,4-dihydroxynaphthalene 1 at 200 °C results in a 2:1 mixture with the keto form 2. Heating the keto form in benzene at 120°C for three days also affords a mixture (1:1 with first-order reaction kinetics). The keto product is kinetically stable and reverts back to the enol in presence of a base. The keto form can be obtained in a pure form by stirring the keto form in trifluoroacetic acid and toluene (1:9 ratio) followed recrystallisation from isopropyl ether.

When the enol form is complexed with chromium tricarbonyl, complete conversion to the keto form accelerates and occurs even at room temperature in benzene.


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