Chloranil Explained

Chloranil is a quinone with the molecular formula C₆Cl₄O₂. Also known as tetrachloro-1,4-benzoquinone, it is a yellow solid. Like the parent benzoquinone, chloranil is a planar molecule^[1] that functions as a mild oxidant.

Synthesis and use as reagent

Chloranil is produced by chlorination of phenol to give hexachlorocyclohexa-2,5-dien-1-one ("hexachlorophenol"). Hydrolysis of the dichloromethylene group in this dienone gives chloranil:^[2]

C₆H₅OH + 6 Cl₂ → C₆Cl₆O + 6 HCl

C₆Cl₆O + H₂O → C₆Cl₄O₂ + 2 HCl

Chloroanil serves as a hydrogen acceptor. It is more electrophilic than quinone itself. It is used for the aromatization reactions, such as the conversion of cyclohexadienes to the benzene derivatives.^[3]

Chloranil is used to test for free secondary amines. This test is useful for checking for the presence of proline derivatives. It is also a good test for the successful deprotection of a secondary amine. Secondary amines react with chloranil to give a brown/red/orange derivative, the colour depending on the amine. In these reactions, the amine displaces chloride from the ring of the quinone.

Commercial applications

It is a precursor to many dyes, such as pigment violet 23 and diaziquone (AZQ), a cancer chemotherapeutic agent.

See also

• Chloranilic acid
• 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)

Notes and References

1. J.-M. Lü, S. V. Rosokha, I. S. Neretin and J. K. Kochi, "Quinones as Electron Acceptors. X-Ray Structures, Spectral (EPR, UV-vis) Characteristics and Electron-Transfer Reactivities of Their Reduced Anion Radicals as Separated vs Contact Ion Pairs" Journal of the American Chemical Society 2006 128, 16708-16719.
2. François Muller and Liliane Caillard "Chlorophenols" in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim, 2011.
3. Derek R. Buckle "Chloranil" in Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley.