Copper(I) chloride

Copper(I) chloride (quite commonly called cuprous chloride), is the lower chloride of copper, with the formula CuCl. It occurs naturally as the mineral nantokite. This colorless solid is almost insoluble in water, and which tends to oxidize in air to green CuCl₂. It is a Lewis acid which reacts with suitable ligands such as ammonia or chloride ion to form complexes, many of which are water-soluble. It is even able to form a stable complex with carbon monoxide in the presence of aluminum chloride as in the COPure^SM process.

In aqueous solution, CuCl would be unstable with respect to disproportionation into Cu and CuCl₂, but is stable toward water due to its low solubility.^[1]

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Chemical properties

Copper(I) chloride is a Lewis acid, classified as soft according to the Hard-Soft Acid-Base concept. Thus, it tends to form stable complexes with soft Lewis bases such as triphenylphosphine:

CuCl + PPh₃ → [CuCl(PPh₃)]₄ (Ph = phenyl)

Although CuCl is insoluble in water, it dissolves in aqueous solutions containing suitable donor molecules. It readily forms complexes with halide ions, for example forming H₃O⁺ CuCl₂^- with concentrated hydrochloric acid. It also dissolves readily in solutions containing CN^-, S₂O₃^2- or NH₃.

Solutions of CuCl in HCl or NH₃ absorb carbon monoxide to form colourless complexes such as the crystalline halogen-bridged dimer [CuCl(CO)]₂. The same HCl solution can also react with acetylene gas to form [CuCl(C₂H₂)], while an NH₃ solution of CuCl forms an explosive copper(I) acetylide with acetylene. Complexes of CuCl with alkenes can be made by reduction of CuCl₂ by sulfur dioxide in the presence of the alkene in alcohol solution. Complexes with chelating alkenes such as 1,5-cyclooctadiene are particularly stable:^[2]

CuCl reacts with organometallic compounds such as methyllithium (CH₃Li) to form "Gilman reagents" such as (CH₃)₂CuLi, which find extensive use in organic synthesis. Grignard reagents react similarly.

Preparation

Copper(I) chloride may be prepared by the reduction of copper(II) salts such as CuSO₄ using sulfur dioxide or copper metal.^[3]

2 CuBr₂ + H₂O + SO₃^2- → 2 CuBr + SO₄^2- + 2 HBr

SO₂ may be prepared in situ from sodium bisulfite (NaHSO₃) or sodium metabisulfite (Na₂S₂O₅) and acid. The reduction is carried out in hydrochloric acid, and the resulting CuCl₂^- complex is diluted to precipitate white CuCl (by driving the equilibrium using Le Chatelier's principle).

(1) NaHSO₃(aq) + HCl (aq) → SO₂(aq) + NaCl + H₂O(l)

(2) 2 CuSO₄(aq) + SO₂(aq) + 2 H₂O(l) + 4 HCl(aq) → 2 HCuCl₂(aq) + 3 H₂SO₄(aq)

(3) HCuCl₂(aq) + H₂O(l) → CuCl(s) + H₃O⁺(aq) + Cl^-(aq)

Uses

A major chemical use for copper(I) chloride is as a catalyst for a variety of organic reactions. Compared to other "soft" Lewis acids, it is much more affordable than non-toxic silver(I) chloride and palladium(II) chloride, and much less toxic than lead(II) chloride and mercury(II) chloride. In addition, it can undergo redox chemistry via copper(II) or copper(III) intermediates. This combination of properties make copper(I) salts invaluable reagents.

One such application is in the Sandmeyer reaction.^[4] Treatment of an arenediazonium salt with CuCl leads to an aryl chloride, for example:

The reaction has wide scope, and usually gives good yields.

The observation that copper(I) halides catalyse 1,4-addition of Grignard reagents to alpha,beta-unsaturated ketones^[5] led to the development of organocuprate reagents that are widely used today in organic synthesis:^[1]

Although other copper(I) compounds such as copper(I) iodide are now more often used for this type of reaction, there are cases where copper(I) chloride is particularly effective:^[1]

Here, Bu indicates an n-butyl group. Without CuCl, the Grignard reagent alone gives a mixture of 1,2 and 1,4-addition products (i.e., the butyl adds at the closer to the C=O).

Copper(I) chloride is also an intermediate formed from copper(II) chloride in the Wacker process.

CuCl is used as a catalyst in Atom Transfer Radical Polymerization (ATRP).

Precautions

Copper salts do have some toxicity and should be handled with care; wear gloves and goggles. Avoid bringing CuCl into contact with alkynes.^[6]

References