Chemical Structure and Application of Zinc iodide

Zinc iodide is an inorganic compound that appears as white or off-white powder at room temperature, exhibiting hygroscopicity and decomposing with discoloration upon exposure to light or air, while its aqueous solution is weakly acidic. Zinc iodide is soluble in water, ethanol, ether, and other solvents. It is primarily prepared via the direct reaction of zinc with iodine or through the metathesis of zinc sulfate and barium iodide. Commonly employed as a component in iodine‑starch reagent formulations, zinc iodide is used to detect oxidizing agents such as nitrites and free chlorine. For storage, it must be sealed, protected from light, and kept dry to slow decomposition, with quality control standards requiring ZnI₂ content between 99.0% and 102.0%.

Chemical Structure

In normally formed zinc iodide crystals, zinc is hexacoordinated by iodine atoms, adopting either a hexagonal close‑packed (cadmium iodide‑type) or cubic close‑packed (cadmium chloride‑type) layered structure. The tetragonal crystal form of zinc iodide shares the same structure as zinc chloride and zinc bromide, where zinc is tetra‑coordinated, and structural units composed of four zinc‑iodine tetrahedra share three vertices, resembling the structure of tetraphosphorus decaoxide. Electron diffraction and infrared spectroscopy studies indicate that zinc iodide exists as linear molecules in the vapor phase, with a Zn‑I bond length of 238 pm.

Property

Zinc iodide appears as white or off-white fine granular powder or colorless hexagonal crystals. It is highly hygroscopic and turns brown when exposed to air or light due to iodine release. Zinc iodide is soluble in ethanol, ether, ammonia water, sodium hydroxide, and ammonium carbonate solutions, slightly soluble in glycerol, and extremely soluble in water, with its aqueous solution having a pH around 5. It melts at 446°C and boils at 624°C, decomposing beyond this temperature. At room temperature, zinc iodide reacts with O₂ and H₂O to form ZnO and I₂; it also reacts with sulfuric acid to produce SO₂ and I₂, forms ammonia adducts with NH₃, and begins to decompose at 1150°C. At approximately 1150°C, zinc iodide vapor begins to decompose into zinc (or zinc monoiodide) and iodine. It is more reactive than other halides. When heated, it can be reduced by hydrogen. At room temperature, it reacts with oxygen and water to form zinc oxide and iodine. Exposure to air and light causes iodine to precipitate, turning it brown. Heating in the presence of oxygen or reacting with nitrogen dioxide at room temperature yields the same products. It can reduce sulfuric acid to sulfur dioxide while releasing iodine.

Preparation Methods

Zinc iodide can be prepared by directly heating metallic zinc with I₂, or through a metathesis reaction of barium iodide and zinc sulfate, followed by filtering off the barium sulfate precipitate and evaporating the filtrate to induce crystallization. Alternatively, it can be synthesized by refluxing an ether solution of I₂ in the presence of zinc, removing the ether under reduced pressure, and sublimating the product at 400°C to obtain zinc iodide.

Application

Zinc iodide is commonly employed as an X-ray opaque penetrant in industrial radiography to enhance the contrast between damaged and intact composite structures. It is also utilized in aqueous zinc-halogen rechargeable cells, where an electrolytic solution containing zinc iodide or its mixtures serves as a key component in both positive and negative electrode compartments. In electron microscopy, zinc iodide, combined with osmium tetroxide, functions as an effective staining agent. Furthermore, as a Lewis acid, zinc iodide acts as a catalyst in the conversion of methanol to triptane and hexamethylbenzene. Additionally, it can be applied as a topical antiseptic in medical contexts. [1]

Allenylation of terminal alkynes

Figure1: Zinc iodide-catalyzed allenylation of terminal alkynes

A straightforward, user-friendly, efficient protocol for the one pot, Zinc iodide-catalyzed allenylation of terminal alkynes with pyrrolidine and ketones, toward trisubstituted allenes, is described. Trisubstituted allenes can be obtained under either conventional heating or microwave irradiation conditions, which significantly reduces the reaction time. A sustainable, widely available, and low-cost metal salt catalyst is employed, and the reactions are carried out under solvent-free conditions. Among others, synthetically valuable allenes bearing functionalities such as amide, hydroxyl, or phthalimide can be efficiently prepared.[2]

Reference

[1] Bercaw J E, Diaconescu P L, Grubbs R H, et al. On the mechanism of the conversion of methanol to 2, 2, 3-trimethylbutane (triptane) over zinc iodide[J]. The Journal of Organic Chemistry, 2006, 71(23): 8907-8917.

[2] Zorba L P, Ega?a E, Gómez-Bengoa E, et al. Zinc Iodide catalyzed synthesis of trisubstituted allenes from terminal alkynes and ketones[J]. ACS omega, 2021, 6: 23329-23346.

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