Copper(II) hydroxide

Copper(II) hydroxide (chemical formula Cu(OH)₂) is the hydroxide of the metal copper. The typical color of copper hydroxide is blue. Some forms of copper(II) hydroxide are sold as "stabilized" copper hydroxide, quite likely a mixture of copper(II) carbonate and hydroxide. These are often greener in color.

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History

Copper(II) hydroxide has been known to man since copper smelting began around 5000 BCE although the alchemists were probably the first to manufacture it.^[1] This was easily done by mixing solutions of lye and blue vitriol, both chemicals which were known in antiquity.

It was produced on an industrial scale during the 17^th and 18^th centuries for use in pigments such as blue verditer and Bremen green.^[2] These pigments were used in ceramics and painting.^[3]

Chemical Properties

Synthesis

Copper(II) hydroxide can be produced by adding a small amount of sodium hydroxide to a dilute solution of copper(II) sulfate (CuSO₄ · 5H₂O). The precipitate produced in this manner, however, often contains an appreciable amount of sodium hydroxide impurity and a purer product can be attained if ammonium chloride is added to the solution beforehand. Alternatively, copper hydroxide is readily made by electrolysis of water (containing a little electrolyte such as sodium bicarbonate). A copper anode is used, often made from scrap copper.

"Copper in moist air slowly acquires a dull green coating. The green material is a 1:1 mole mixture of Cu(OH)₂ and CuCO₃."^[4]

2Cu(s) + H₂O(g) + CO₂(g) + O₂(g) ---> Cu(OH)₂(s) + CuCO₃(s)

This is the patina that forms on bronze and other copper alloy statues such as the Statue of Liberty.

Reactions

Moist samples of copper(II) hydroxide slowly turn black due to the formation of copper(II) oxide.^[5] When it is dry, however, copper(II) hydroxide does not decompose unless it is heated to 185°C.^[6]

Copper(II) hydroxide reacts with a solution of ammonia to form a deep blue solution consisting of the [Cu(NH₃)₄]²⁺ complex ion, but the hydroxide is reformed when the solution is diluted with water. Copper(II) hydroxide in ammonia solution, known as Schweizer's reagent, possesses the interesting ability to dissolve cellulose. This property led to it being used in the production of rayon, a cellulosic fiber.

Since copper(II) hydroxide is mildly amphoteric, it dissolves slightly in concentrated alkali, forming [Cu(OH)₄]^2-.^[7]

Use as an organic reagent

Copper(II) hydroxide has a rather specialized role in organic synthesis. Often, when it is utilized for this purpose, it is prepared in situ by mixing a soluble copper(II) salt and potassium hydroxide.

It is sometimes used in the synthesis of aryl amines. For example, copper(II) hydroxide catalyzes the reaction of ethylenediamine with 1-bromoanthraquinone or 1-amino-4-bromoanthraquinone to form 1-((2-aminoethyl)amino)anthraquinone or 1-amino-4-((2-aminoethyl)amino)anthraquinone, respectively.

Copper(II) hydroxide also converts acid hydrazides to carboxylic acids at room temperature. This is especially useful in synthesizing carboxylic acids with other fragile functional groups. The published yields are generally excellent as is the case with the production of benzoic acid and octanoic acid.

Natural occurrence

Copper(II) hydroxide is found in several different copper minerals, most notably azurite, malachite, antlerite, and brochantite. Azurite (2CuCO₃ • Cu(OH)₂ ) and malachite (CuCO₃ • Cu(OH)₂) are carbonates while antlerite (CuSO₄ • 2Cu(OH)₂) and brochantite (CuSO₄ • 3Cu(OH)₂) are sulfates. Copper(II) hydroxide is rarely found as an uncombined mineral because it slowly reacts with carbon dioxide from the atmosphere to form a basic copper(II) carbonate.

Uses

Copper(II) hydroxide has been used as an alternative to the Bordeaux mixture, a fungicide and nematacide.^[8] Nowadays, it is disfavored because of environmental contamination problems. Copper(II) hydroxide is also occasionally used as ceramic colorant.

Precautions

Copper(II) hydroxide is a skin, eye and respiratory irritant. Always wear safety glasses when handling copper hydroxide. In case of contact with eyes, rinse immediately with plenty of water and seek medical advice.

References

1. Roscoe, H. E., & Schorlemmer, C. (1879). A Treatise on Chemistry 2nd Ed, Vol 2, Part 2. MacMillan & Co. (p 498).
2. Paquette, Leo A. (1995). Encyclopedia of Reagents for Organic Synthesis, 8 Volume Set. Wiley. ISBN 0-4719-3623-5.