Copper(II) nitrate

Copper(II) nitrate is the chemical compound with the formula Cu(NO₃)₂. Commonly referred to simply as copper nitrate, the anhydrous form is a blue, crystalline solid. Hydrated forms of copper nitrate, also blue, are commonly used in school laboratories to demonstrate chemical voltaic cell reactions. The hydrated and anhydrous species have remarkably different properties, illustrating the effect of water of crystallization.

The Roman numeral sign is to specify that the copper has an oxidation state of +2.

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Properties

Hydrated and anhydrous copper nitrates behave differently.

Anhydrous form

The bright blue anhydrous material, Cu(NO₃)₂, is a volatile solid, subliming in a vacuum. In the gas-phase, Cu(NO₃)₂ is square planar, each Cu center being surrounded by four oxygen atoms. Upon condensation, this monomer polymerizes.^[1]

Hydrated copper nitrate

Crystalline Cu(NO₃)₂(H₂O)_2.5 features octahedral Cu centers surrounded by water and the nitrate anions.^[1] This hydrate decomposes at ca. 170 °C into copper(II) oxide, nitrogen dioxide and oxygen:

2Cu(NO₃)_2(s) → 2CuO_(s) + 4NO_2(g) + O_2(g)

Copper nitrate can be used to generate nitric acid by heating it until decomposition and passing the fumes directly into water. This method is similar to the last step in the Ostwald process. The equations are as follows:

2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂

3NO₂ + H₂O → 2 HNO₃ + NO

Copper nitrate soaked splints of wood burn with an emerald green flame. Addition of Magnesium nitrate gives a lime green color.

Synthesis

Cu(NO₃)₂ forms when copper metal is treated with N₂O₄:^[2]

Cu + 2 N₂O₄ → Cu(NO₃)₂ + 2 NO

It can also be formed by reacting copper metal with an aqueous solution of silver nitrate, see this page for more info.

Use in organic synthesis

Copper nitrate, in combination with acetic anhydride, is an effective reagent for nitration of aromatic compounds, under what are known as "Menke conditions", in honor of the Dutch chemist who discovered that metal nitrates are effective reagents for nitration.^[3] Hydrated copper nitrate absorbed onto clay affords a reagent called "claycop". The resulting blue clay is used as a slurry, for example for the oxidation of thiols to disulfides. Claycop is also used to convert dithioacetals to carbonyls.^[4] A related reagent based on Montmorillonite has proven useful for the nitration of aromatic compounds.^[5]

• ICSC Copper and compounds fact sheet

References

1. ^ ^{a b} Wells, A.F. Structural Inorganic Chemistry, Oxford: Clarendon Press (1984). ISBN 0-19-855370-6.
2. ^ Jolly, W. L. "The Synthesis and Characterization of Inorganic Compounds" Prentice Hall, London, 1970
3. ^ Menke J.B. (1925). "Nitration with nitrates". Recueil des Travaux Chimiques des Payes-Bas 44: 141.
4. ^ Balogh, M. "Copper(II) Nitrate–K10 Bentonite Clay" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
5. ^ Collet, C.; Delville, A.; Laszlo, P. “Clays Direct Aromatic Nitration” Angewandte Chemie International Edition in English, 2003, Volume 29, Issue 5 , Pages 535 - 536. DOI 10.1002/anie.199005351.