Carbon tetraiodide explained

Carbon tetraiodide is a tetrahalomethane with the molecular formula CI4. Being bright red, it is a relatively rare example of a highly colored methane derivative. It is only 2.3% by weight carbon, although other methane derivatives are known with still less carbon.

Structure

The tetrahedral molecule features C-I distances of 2.12 ± 0.02 Å.^[1] The molecule is slightly crowded with short contacts between iodine atoms of 3.459 ± 0.03 Å, and possibly for this reason, it is thermally and photochemically unstable.

Carbon tetraiodide crystallizes in tetragonal crystal structure (a 6.409, c 9.558 (.10⁻¹ nm)).^[2]

It has zero dipole moment due to its symmetrically substituted tetrahedral geometry.

Properties, synthesis, uses

Carbon tetraiodide is slightly reactive towards water, giving iodoform and I₂. It is soluble in nonpolar organic solvents. It decomposes thermally and photochemically to, C₂I₄. Its synthesis entails AlCl₃-catalyzed halide exchange, which is conducted at room temperature:^[3]

CCl4 + 4 EtI -> CI4 + 4 EtClThe product crystallizes from the reaction solution.

Carbon tetraiodide is used as an iodination reagent, often upon reaction with bases.^[4] Ketones are converted to 1,1-diiodoalkenes upon treatment with triphenylphosphine (PPh₃) and carbon tetraiodide. 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 alkyl chlorides from alcohols.

Safety considerations

Manufacturers recommend that carbon tetraiodide be stored near . As a ready source of iodine, it is an irritant. Its LD₅₀ on rats is 18 mg/kg. In general, organic compounds should be considered toxic, with the narrow exception of small perfluoroalkanes (essentially inert due to the strength of the C-F bond).

Further reading

• Sorros H., Hinkam J. B. . 1945 . The Redistribution Reaction. XI. Application to the Preparation of Carbon Tetraiodide and Related Halides . . 67 . 10 . 1643 . 10.1021/ja01226a004 .

Notes and References

1. Finbak, Chr. . Hassel, O. . Kristallstruktur und Molekülbau von CI₄ und CBr₄ . Zeitschrift für Physikalische Chemie . 1937 . B36 . 301–308. 10.1515/zpch-1937-3621. 99718985 .
2. Pohl, S. . Die Kristallstruktur von CI₄ . Zeitschrift für Kristallographie . 1982 . 159 . 1–4 . 211–216 . 10.1524/zkri.1982.159.14.211. 102246815 .
3. Book: McArthur, R. E. . Simons, J. H. . Inorganic Syntheses . Carbon Tetraiodide . Inorganic Syntheses . 1950 . III . 37–39 . 10.1002/9780470132340.ch8. 9780470132340 .
4. Book: P. R. Schreiner, A. A. Fokin . Carbon Tetraiodide . Encyclopedia of Reagents for Organic Synthesis . L. Paquette . 2005 . John Wiley & Sons, Ltd.