Menke nitration

What is Menke nitration?

The Menke nitration, named after Menke who reported it in 1925, is a reaction that involves the nitration of aromatic compounds using cupric nitrate Cu(NO3)2 and acetic anhydride Ac2O.

Menke nitration - general reaction scheme
Menke nitration

The Menke nitration is most suitable for aromatics with electron-donating substituents such as amino and hydroxyl groups, and gives a high ratio of ortho-product. For instance, the nitration of phenol with cupric nitrate in acetic acid results in only ortho-nitrophenol, while the nitration of aniline yields only ortho-nitroaniline. Under Menke conditions, several inorganic nitrates, including iron, manganese, cobalt, nickel, copper, mercury, lithium, aluminium, cerium, and uranium, have been found to be effective for the nitration of aromatics, with cupric nitrate being the most efficient reagent.

The Menke nitration is believed to generate diacetyl ortho-nitric acid and acetyl nitrate, which serve as effective nitrating agents, while excess humidity is scavenged by acetic anhydride Ac2O. Although chloroform is the most commonly used solvent, other solvents such as hexane, cyclohexane, nitromethane, acetonitrile, methylene chloride, and nitrobenzene can also be used. Notably, the nitration rate is faster in solvents with higher dielectric constants. The use of acidic montmorillonite clay supported cupric nitrate, known as “claycop,” has been proposed as an improved nitrating agent that can quantitatively convert toluene into nitrotoluene isomers.

The nitration of toluene can also be directed to produce >90% para-nitro isomer by using a clay (e.g., H-ZSM-5 zeolite) impregnated with cupric nitrate or ferric nitrate. By using excess amounts of claycop and small amounts of fuming nitric acid, polynitrated aromatics can be obtained, including those with deactivating groups such as chlorobenzene and benzaldehyde. Trifluoroacetic anhydride has also been used to enhance nitration yields.

References

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