Copper iodide is the inorganic compound with the formula CuI. It is also known as cuprous iodide. It is useful in a variety of applications ranging from organic synthesis to cloud seeding. Pure copper iodide is white, but samples are often tan or even, when found in nature as rare mineral marshite, reddish brown, but such color is due to the presence of impurities. It is common for samples of iodide-containing compounds to become discolored due to the facile aerobic oxidation of the iodide anion to molecular iodine.
Structure
Copper iodide, like most binary metal halides, is an inorganic polymer. It has a rich phase diagram, meaning that it exists in several crystalline forms. It adopts a zinc blende structure below 390 °C, a wurtzite structure between 390 and 440 °C, and a rock salt structure above 440 °C. The ions are tetrahedrally coordinated when in the zinc blende or the wurtzite structure, with a Cu-I distance of 2.338 Å. Copper bromide and copper chloride also transform from the zinc blende structure to the wurtzite structure at 405 and 435 °C, respectively. Therefore, the longer the copper – halide bond length, the lower the temperature needs to be to change the structure from the zinc blende structure to the wurtzite structure. The interatomic distances in copper bromide and copper chloride are 2.173 and 2.051 Å, respectively.
γ-CuI
β-CuI
α-CuI
Preparation
Copper iodide can be prepared by heating iodine and copper in concentrated hydriodic acid, HI. In the laboratory however, copper iodide is prepared by simply mixing an aqueous solution of sodium or potassium iodide and a soluble copper salt such copper sulfate. The CuI2 immediately decomposes to iodine and insoluble copper iodide, releasing I2. This reaction has been employed as a means of assaying copper samples, since the evolved I2 can be analyzed by redox titration. The reaction in itself may look rather odd, as using the rule of thumb for a proceeding redox reaction, Eooxidator − Eoreductor > 0, this reaction fails. The quantity is below zero, so the reaction should not proceed. But the equilibrium constant for the reaction is 1.38*10−13. By using fairly moderate concentrates of 0.1 mol/L for both iodide and Cu2+, the concentration of Cu+ is calculated as 3*10−7. As a consequence, the product of the concentrations is far in excess of the solubility product, so copperiodide precipitates. The process of precipitation lowers the copper concentration, providing an entropic driving force according to Le Chatelier's principle, and allowing the redox reaction to proceed.
Properties
CuI is poorly soluble in water, but it dissolves in the presence of NaI or KI to give the linear anion −. Dilution of such solutions with water reprecipitates CuI. This dissolution–precipitation process is employed to purify CuI, affording colorless samples. Copper iodide can be dissolved in acetonitrile, yielding a solution of different complex compounds. Upon crystallization, molecular or polymeric compounds can be isolated. Dissolution is also observed when a solution of the appropriate complexing agent in acetone or chloroform is used. For example, thiourea and its derivatives can be used. Solids that crystallize out of those solutions are composed ofhybrid inorganic chains.
Uses
CuI has several uses:
CuI is used as a reagent in organic synthesis. In combination with 1,2- or 1,3 diamine ligands, CuI catalyzes the conversion of aryl-, heteroaryl-, and vinyl-bromides into the corresponding iodides. NaI is the typical iodide source and dioxane is a typical solvent. Aryl halides are used to form carbon–carbon and carbon–heteroatom bonds in process such as the Heck, Stille, Suzuki, Sonogashira and Ullmann type coupling reactions. Aryl iodides, however, are more reactive than the corresponding aryl bromides or aryl chlorides. 2-Bromo-1-octen-3-ol and 1-nonyne are coupled when combined with dichlorobispalladium, CuI, and diethylamine to form 7-methylene-8-hexadecyn-6-ol.
CuI is used in cloud seeding, altering the amount or type of precipitation of a cloud, or their structure by dispersing substances into the atmosphere which increase water's ability to form droplets or crystals. CuI provides a sphere for moisture in the cloud to condense around, causing precipitation to increase and cloud density to decrease.
The structural properties of CuI allow CuI to stabilize heat in nylon in commercial and residential carpet industries, automotive engine accessories, and other markets where durability and weight are a factor.
CuI is used in the detection of mercury. Upon contact with mercury vapors, the originally white compound changes color to form copper tetraiodomercurate, which has a brown color.
CuI is used in designing and synthesizing Cu clusters, which is polymetal complex compounds.
