Carbon tetraiodide

Carbon tetraiodide is CI4, a tetrahalomethane. Being bright red, it is a relatively rare example of a highly colored methane derivative. It is only 2% by weight carbon, although other methane derivatives are known with still less carbon. The tetrahedral molecule features C-I distances of 2.12 ± 0.02 Å. The molecule is slightly crowded with short I---I contacts of 3.459 ± 0.03 Å, and possibly for this reason, it is thermally and photochemically unstable. Hexaiodoethane is unknown, probably for the same reason.

Properties, synthesis, uses
CI4 is slightly reactive towards water, giving iodoform and I2. Otherwise it is soluble in nonpolar organic solvents. It decomposes thermally and photochemically to tetraiodoethylene, I2C=CI2. Its synthesis entails AlCl3-catalyzed halide exchange, which is conducted at room temperature:

CCl4 + 4 EtI ? CI4 + 4 EtCl

The product crystallizes from the reaction solution.
CI4 is used as an iodination reagent, often upon reaction with base. Ketones are converted to 1,1-diiodoethenes upon treatment with PPh3 and CI4. Alcohols are converted in and to iodide, by a mechanism similar to the Appel reaction. In an Appel reaction carbon tetrachloride is used to generate the chloride from alcohols.

Safety considerations
Manufacturers recommend that CI4 be stored near 0 °C. As a ready source of iodine, it is an irritant. LD50: 178 mg kg–1. In general perhalogenated organic compounds should be considered toxic.