Chlorine trifluoride

Chlorine trifluoride is the chemical compound with the formula ClF3. This colourless, poisonous, corrosive and very reactive gas condenses to a pale-greenish yellow liquid, the form in which it is most often sold (pressurized at room temperature). The compound is primarily of interest as a component in rocket fuels, in industrial cleaning and etching operations primarily in the semiconductor industry , nuclear reactor fuel processing and other industrial operations.

Preparation, structure, and properties
It was first reported by Ruff and Krug who prepared it by fluorination of chlorine; this also produced ClF and the mixture was separated by distillation.
 * 3 F2 +  Cl2  →  2 ClF3

ClF3 is approximately T-shaped. This structure is explicable in the context of VSEPR theory, which considers also lone pairs of electrons as occupying two equatorial positions of a hypothetic trigonal bipyramid. The elongated Cl-Faxial bonds are consistent with hypervalent bonding.

Pure ClF3 is stable to 180° in glass vessels, but above this temperature it decomposes by a free radical mechanism to the elements.

The main use of ClF3 is to produce uranium hexafluoride, UF6 as part of nuclear fuel processing and reprocessing, by the reaction:
 * U + 3ClF3 &rarr; UF6 + 3ClF

Hazards
ClF3 is a very strong oxidizing and fluorination agent. ClF3 is extremely reactive with most inorganic and organic materials and will initiate the combustion of many materials without an ignition source and these reactions are often violent or in some cases explosive. Several metals give chlorides and fluorides, phosphorus yields PCl3 plus PF5, sulfur SCl2 plus SF4. ClF3 is also violently water reactive in which it hydrolyses to a variety of hazardous chemicals such as hydrofluoric acid. H2S explodes on being mixed with ClF3 at room temperature.

The ability to surpass the oxidizing ability of oxygen leads to corrosivity against oxide-containing materials often thought as incombustible. In an industrial accident, a spill of 900 kg of chlorine trifluoride burned itself through 30 cm of concrete and 90 cm of gravel beneath. Any equipment that comes into contact with chlorine trifluoride must be carefully selected and cleaned, because any contamination can ignite on contact.

Exposure of larger amounts of chlorine trifluoride, as a liquid or as a gas, ignites tissue. The hydrolysis reaction with water is violent and exposure results in a thermal burn. The product of hydrolysis is hydrofluoric acid, which is corrosive to human tissue, absorbs through skin, selectively attacks bone and stimulates pain nerves, and causes a potentially lethal poisoning.

Military applications
Under the code name N-stoff ("substance N"), chlorine trifluoride was investigated for military applications by the Kaiser Wilhelm Institute in Nazi Germany from slightly before the start of World War II. Tests were made against mock-ups of the Maginot Line fortifications, and it was found to be an effective combined incendiary weapon and poison gas. From 1938 construction commenced on a partly bunkered, partly subterranean 31.76 km² munitions factory at Falkenhagen which was intended to produce 50 tonnes of N-stoff per month, plus Sarin. However by the time it was captured by the advancing Red Army in 1944, the factory had produced only about 30 to 50 tonnes, at a cost of over 100 German Reichsmark per kilogram. N-stoff was never used in war.

Rocket propellant
Chlorine trifluoride has been investigated as a high-performance storable oxidizer in rocket propellant systems. Handling concerns, however, prevented its use. Clark summarized the difficulties, "It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured.  It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water—with which it reacts explosively."