Copper(II) acetate

Copper(II) acetate, also referred to as cupric acetate, is the chemical compound with the formula Cu₂(OAc)₄ where AcO^- is acetate (CH₃CO₂^-). The hydrated derivative, which contains one molecule of water for each Cu atom, is available commercially. Cu₂(OAc)₄ is a dark green crystalline solid, whereas Cu₂(OAc)₄(H₂O)₂ is more bluish-green. Since ancient times, copper acetates of some form have been used as fungicides and green pigments. Today, Cu₂(OAc)₄ is used as a source of copper(II) in inorganic synthesis and as a catalyst or an oxidizing agent in organic synthesis. Copper acetate, like all copper compounds, emits a blue-green glow in a flame.

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History

Copper(II) acetate is the primary component of verdigris,^{[citation needed]} the blue-green substance that forms on copper during long exposures to atmosphere. It was historically prepared in vineyards, since acetic acid is a byproduct of fermentation. Copper sheets were alternately layered with fermented grape skins and dregs left over from wine production and exposed to air. This would leave a blue substance on the outside of the sheet. This was then scraped off and dissolved in water. The resulting solid was used as a pigment, or combined with arsenic trioxide to form copper acetoarsenite, a powerful insecticide and fungicide called Paris or Schweinfurt green.

Uses in chemical synthesis

The uses for copper(II) acetate are more plentiful as a catalyst or oxidizing agent in organic syntheses. For example, Cu₂(OAc)₄ is used to couple two terminal alkynes to make a 1,3-diyne:^[1]

Cu₂(OAc)₄ + 2 RC≡CH → 2 CuOAc + RC≡C-C≡CR + 2 HOAc

The reaction proceeds via the intermediacy of copper(I) acetylides, which are then oxidized by the copper(II) acetate, releasing the acetylide radical. A related reaction involving copper acetylides is the synthesis of ynamines, terminal alkynes with amine groups using Cu₂(OAc)₄.^[2]

Structure

  Cu₂(OAc)₄(H₂O)₂ adopts the "Chinese lantern" structure seen also for related Rh(II) and Cr(II) tetraacetates.^[2][3] One oxygen atom on each acetate is bound to one copper at 1.97 Å (197 pm. Completing the coordination sphere are two water ligands, with Cu-O distances of 2.20 Å (220 pm). The two five-coordinate copper atoms are separated by only 2.65 Å (265 pm), which is close to the Cu--Cu separation in metallic copper.^[5]. The two copper centers interact resulting in a diminishing of the magnetic moment such that near 90 K, Cu₂(OAc)₄(H₂O)₂ is essentially diamagnetic due to cancellation of the two opposing spins. Cu₂(OAc)₄(H₂O)₂ was a critical step in the development of modern theories for antiferromagnetic coupling.^[4]

Synthesis

Copper(II) acetate has been synthesized for centuries by the method described in the history section. This method, however, leads to an impure copper(II) acetate. In a laboratory, a much purer form can be synthesized in a simple three-step procedure. The overall reaction is as follows:^[6]

2 CuSO₄^.5H₂O + 4 NH₃ + 4 CH₃COOH → Cu₂(OAc)₄(H₂O)₂ + 2 [NH₄]₂[SO₄] + 8 H₂O

The hydrate form can be dehydrated by heating at 100 °C in a vacuum:^[5]

Cu₂(OAc)₄(H₂O)₂ → Cu₂(OAc)₄ + 2 H₂O

Heating a mixture of anhydrous Cu₂(OAc)₄ and copper metal affords colorless, volatile cuprous acetate:^[6]

2 Cu + Cu₂(OAc)₄ → 4 CuOAc

References

1. ^ P. Vogel, J. Srogl "Copper(II) Acetate" in "EROS Encyclopedia of Reagents for Organic Synthesis" Copper(II) Acetate, 2005 John Wiley & Sons.
2. ^ van Niekerk, J. N.; Schoening, F. R. L. “X-Ray Evidence for Metal-to-Metal Bonds in Cupric and Chromous Acetate” Nature 1953, volume 171, pages 36-37. doi:10.1038/171036a0.
3. ^ Wells, A.F. (1984). Structural Inorganic Chemistry, Oxford: Clarendon Press.
4. ^ R. L. Carlin "Magnetochemistry" Springer: Berlin, 1986
5. ^ S. J. Kirchner, Q. Fernando "Copper(I) Acetate" Inorganic Syntheses, 1980, volume XX, pages 53-55.
6. ^ Parish, E. J.; Kizito, S. A. "Copper(I) Acetate" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley & Sons. DOI: 10.1002/047084289X.rc193.