Copper(I) chloride
Copper(I) chloride, commonly called cuprous chloride, is the lower chloride of copper, with the formula CuCl. The substance is a white solid sparingly soluble in water, but very soluble in concentrated hydrochloric acid. Impure samples appear green due to the presence of copper(II) chloride (CuCl). History. Copper(I) chloride was first prepared by Robert Boyle in the mid-seventeenth century from mercury(II) chloride ("Venetian sublimate") and copper metal: In 1799, J.L. Proust characterized the two different chlorides of copper. He prepared CuCl by heating CuCl at red heat in the absence of air, causing it to lose half of  its combined chlorine followed by removing residual CuCl by washing with water. An acidic solution of CuCl was formerly used for analysis of carbon monoxide content in gases, for example in Hempel's gas apparatus. This application was significant during the nineteenth and early twentieth centuries when coal gas was widely used for heating and lighting. Synthesis. Copper(I) chloride is produced industrially by the direct combination of copper metal and chlorine at 450–900 °C: Copper(I) chloride can also be prepared by reducing copper(II) chloride with sulfur dioxide, or with ascorbic acid (vitamin C) that acts as a reducing sugar : Many other reducing agents can be used. Properties. Copper(I) chloride has the cubic zincblende crystal structure at ambient conditions. Upon heating to 408 °C the structure changes to hexagonal. Several other crystalline forms of CuCl appear at high pressures (several GPa). Copper(I) chloride is a Lewis acid, which is classified as soft according to the Hard-Soft Acid-Base concept. Thus, it forms a series of complexes with soft Lewis bases such as triphenylphosphine: Although CuCl is insoluble in water, it dissolves in aqueous solutions containing suitable donor molecules. It forms complexes with halide ions, for example forming HO  CuCl in concentrated hydrochloric acid. Chloride is displaced by CN and SO. Solutions of CuCl in HCl or NH absorb carbon monoxide to form colourless complexes such as the chloride-bridged dimer [CuCl(CO)]. The same hydrochloric acid solutions also react with acetylene gas to form [CuCl(CH)]. Ammoniacal solutions of CuCl react with acetylenes to form the explosive copper(I) acetylide, CuC. Alkene complexes o can be prepared by reduction of CuCl by sulfur dioxide in the presence of the alkene in alcohol solution. Complexes with dienes such as 1,5-cyclooctadiene are particularly stable: In absence of other ligands, its aqueous solutions  are unstable with respect to disproportionation: In part for this reason samples in air assume a green coloration. Uses. The main use of copper(I) chloride is as a precursor to the fungicide copper oxychloride. For this purpose aqueous copper(I) chloride is generated by comproportionation and then air-oxidized: Copper(I) chloride catalyzes a variety of organic reactions, as discussed above. Its affinity for carbon monoxide in the presence of aluminium chloride is exploited in the COPure process. In organic synthesis. CuCl is used with carbon monoxide, aluminium chloride, and hydrogen chloride in the Gatterman-Koch reaction to form benzaldehydes. In the  Sandmeyer reaction. Treatment of an arenediazonium salt with CuCl leads to an aryl chloride, for example: The reaction has wide scope and usually gives good yields. Early investigators observed that copper(I) halides catalyse 1,4-addition of Grignard reagents to alpha,beta-unsaturated ketones led to the development of organocuprate reagents that are widely used today in organic synthesis: This finding led to the development of organocopper chemistry. For example, CuCl reacts with methyllithium (CHLi) to form "Gilman reagents" such as (CH)CuLi, which find extensive use in organic synthesis. Grignard reagents form similar organocopper compounds. Although other copper(I) compounds such as  copper(I) iodide are now more often used for these types of reactions, copper(I) chloride is still recommended in some cases: 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 C closer to the C=O). Copper(I) chloride is also an intermediate formed from copper(II) chloride in the Wacker process. In polymer chemistry. CuCl is used as a catalyst in Atom Transfer Radical Polymerization (ATRP). Niche uses. Copper(I) chloride is also used in pyrotechnics as a blue/green coloring agent. In  a flame test, copper chlorides, like all copper compounds, emit green-blue. Natural occurrence. Natural form of CuCl is the rare mineral nantokite.