This column is about a halogenated solvent based on iodine. The solvent has been tested in critical applications and found useful. But you can’t buy it today or maybe ever. This column is about inertness, perfluoro-n-butyliodide (PFBI), and commercial risk.

PARTIAL INERTNESS
In the 1990s, some chemists recognized the inherent value in the use of — and problems with the use of —halogenated solvents. They sought to retain as much as possible of the former while reducing the latter.
The core problem was the absolute reactivity of halogen atoms with others.

• Fluorine, by far the most electronegative (reactive), produced exceptionally stable (inert) compounds.
  These compounds are excellent refrigerants but become essentially permanent residents of the stratosphere when emitted.
• Compounds with chlorine and bromine atoms are considerably less stable because these atoms aren’t as reactive as fluorine is.
• It was compounds with fluorine and either chlorine or bromine that provided the greatest value as cleaning solvents, but the greatest threat to the planet. The fluorine atoms provided inertness and the hydrogen-bonding and polar intermolecular forces manifested by the chlorine and bromine atoms matched well with those of many soils. Chlo-rofluorocarbon (CFC)-113 was both the most useful and notorious example. Emitted CFC-113 was inert enough to migrate to the stratosphere and reactive enough to liberate chlorine atoms that catalyzed the destruction of ozone molecules.
• Iodine is much less electronegative (Table 1) for the ratings on the Pauling scale of atomic electroneg-ativity. Iodine-carbon bonds are very weak.Iodat-ed compounds are not very stable.

SACRIFICIAL INERTNESS
The approach taken by these chemists was to replace the chlorine and bromine atoms in heavily fluorinated molecules with iodine. This work was supportedby the U.S. Department of Energy, and led by Jon Nimitz.

The basic idea was to retain the refrigerant and sol-vation properties of a heavily fluorinated solvent, and make the molecule when emitted very unstable in the atmosphere. It couldn’t be an ozone-depleting or a global-warmingcompound if it didn’t reach the stratosphere.

After evaluating various structures, the chemists chose PFBI as their development candidate.