Chlorine trifluoride

Chlorine trifluoride is an interhalogen compound with the formula ClF3. This colorless, poisonous, corrosive, and extremely 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 plasmaless cleaning and etching operations in the semiconductor industry,[9][10] in nuclear reactor fuel processing,[11] and other industrial operations.[12]

Chlorine trifluoride
Systematic IUPAC name
Trifluoro-λ3-chlorane[1] (substitutive)
Other names
3D model (JSmol)
ECHA InfoCard 100.029.301
EC Number
  • 232-230-4
MeSH chlorine+trifluoride
RTECS number
  • FO2800000
UN number 1749
Molar mass 92.45 g·mol−1
Appearance Colorless gas or greenish-yellow liquid
Odor sweet, pungent, irritating, suffocating[2][3]
Density 3.779 g/L[4]
Melting point −76.34 °C (−105.41 °F; 196.81 K)[4]
Boiling point 11.75 °C (53.15 °F; 284.90 K)[4] (decomposes @ 180 °C (356 °F; 453 K))
Exothermic hydrolysis[5]
Solubility Reacts with benzene, toluene, ether, alcohol, acetic acid, selenium tetrafluoride, nitric acid, sulfuric acid, alkali, hexane.[5] Soluble in CCl4 but can be explosive in high concentrations.
Vapor pressure 175 kPa
-26.5·10−6 cm3/mol[6]
Viscosity 91.82 μPa s
63.9 J K−1mol−1
281.6 J K−1mol−1
−163.2 kJ mol−1
−123.0 kJ mol−1
Main hazards explosive when exposed to organics, reacts violently with water[3]
Safety data sheet
GHS pictograms
GHS Signal word Danger
NFPA 704 (fire diamond)
Flash point noncombustible [3]
Lethal dose or concentration (LD, LC):
95 ppm (rat, 4 hr)
178 ppm (mouse, 1 hr)
230 ppm (monkey, 1 hr)
299 ppm (rat, 1 hr)
NIOSH (US health exposure limits):
PEL (Permissible)
C 0.1 ppm (0.4 mg/m3)[3]
REL (Recommended)
C 0.1 ppm (0.4 mg/m3)[3]
IDLH (Immediate danger)
20 ppm[3]
Related compounds
Related compounds
Chlorine pentafluoride

Chlorine monofluoride
Bromine trifluoride
Iodine trifluoride

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation, structure, and properties

It was first reported in 1930 by Ruff and Krug who prepared it by fluorination of chlorine; this also produced ClF and the mixture was separated by distillation.[13]

3 F2 + Cl2 → 2 ClF3

ClF3 is approximately T-shaped, with one short bond (1.598 Å) and two long bonds (1.698 Å).[14] This structure agrees with the prediction of VSEPR theory, which predicts lone pairs of electrons as occupying two equatorial positions of a hypothetic trigonal bipyramid. The elongated Cl-F axial bonds are consistent with hypervalent bonding.

Pure ClF3 is stable to 180 °C in quartz vessels; above this temperature it decomposes by a free radical mechanism to its constituent elements.


Reactions with many metals give chlorides and fluorides; phosphorus yields phosphorus trichloride (PCl3) and phosphorus pentafluoride (PF5); and sulfur yields sulfur dichloride (SCl2) and sulfur tetrafluoride (SF4). ClF3 also violently reacts with water, oxidizing it to give oxygen or, in controlled quantities, oxygen difluoride (OF2), as well as hydrogen fluoride and hydrogen chloride:

ClF3 + 2H2O 3HF + HCl + O2
ClF3 + H2O HF + HCl + OF2

It will also convert many metal oxides to metal halides and oxygen or oxygen difluoride.

One of the main uses of ClF3 is to produce uranium hexafluoride, UF6, as part of nuclear fuel processing and reprocessing, by the fluorination of uranium metal:

U + 3 ClF3 → UF6 + 3 ClF

The compound can also dissociate under the scheme:

ClF3 → ClF + F2


Semiconductor industry

In the semiconductor industry, chlorine trifluoride is used to clean chemical vapour deposition chambers.[15] It has the advantage that it can be used to remove semiconductor material from the chamber walls without the need to dismantle the chamber.[15] Unlike most of the alternative chemicals used in this role, it does not need to be activated by the use of plasma since the heat of the chamber is enough to make it decompose and react with the semiconductor material.[15]

Rocket propellant

Chlorine trifluoride has been investigated as a high-performance storable oxidizer in rocket propellant systems. Handling concerns, however, severely limit its use. John Drury 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. It can be kept in some of the ordinary structural metals—steel, copper, aluminum, etc.—because of the formation of a thin film of insoluble metal fluoride that protects the bulk of the metal, just as the invisible coat of oxide on aluminum keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.[2][16][17]

The discovery of chlorine pentafluoride rendered ClF3 obsolete as an oxidizer.

Proposed 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 not long 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 14,000 m2 munitions factory, the Falkenhagen industrial complex, which was intended to produce 90 tonnes of N-Stoff per month, plus sarin. However, by the time it was captured by the advancing Red Army in 1945, the factory had produced only about 30 to 50 tonnes, at a cost of over 100 German Reichsmark per kilograma. N-Stoff was never used in war.[18][19]


ClF3 is a very strong oxidizing and fluorinating agent. It is extremely reactive with most inorganic and organic materials, such as glass, and will initiate the combustion of many otherwise non-flammable materials without any ignition source. These reactions are often violent, and in some cases explosive. Vessels made from steel, copper, or nickel are not consumed by ClF3 because a thin layer of insoluble metal fluoride will form, but molybdenum, tungsten, and titanium form volatile fluorides and are consequently unsuitable. Any equipment that comes into contact with chlorine trifluoride must be meticulously cleaned and then passivated, because any contamination left may burn through the passivation layer faster than it can re-form. Chlorine trifluoride has also been known to corrode materials otherwise known to be non-corrodible such as iridium, platinum, and gold.

The fact that its oxidizing ability surpasses oxygen's leads to corrosivity against oxide-containing materials often thought as incombustible. Chlorine trifluoride and gases like it have been reported to ignite sand, asbestos, and other highly fire-retardant materials. It will also ignite the ashes of materials that have already been burned in oxygen. In an industrial accident, a spill of 900 kg of chlorine trifluoride burned through 30 cm of concrete and 90 cm of gravel beneath.[20][17] There is exactly one known fire control/suppression method capable of dealing with chlorine trifluoride - the use of nitrogen and noble gases: the surrounding area must be flooded with nitrogen or helium. Barring that, the area must simply be kept cool until the reaction ceases.[21] The compound reacts with water-based suppressors, and oxidizes even in the absence of atmospheric oxygen, rendering traditional atmosphere-displacement suppressors such as CO2 and halon ineffective. It ignites glass on contact.[22]

Exposure to larger amounts of chlorine trifluoride, as a liquid or as a gas, ignites living tissue. The hydrolysis reaction with water is violent and exposure results in a thermal burn. The products of hydrolysis are mainly hydrofluoric acid and hydrochloric acid, usually released as acidic steam or vapor due to the highly exothermic nature of the reaction.

See also


^a Using data from Economic History Services and The Inflation Calculator, we can calculate that 100 Reichsmark in 1941 is approximately equivalent to US$540 in 2006. Reichsmark exchange rate values from 1942 to 1944 are fragmentary.


  1. "Chlorine trifluoride – Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification and Related Records. Retrieved 9 October 2011.
  2. ClF3/Hydrazine Archived 2007-02-02 at the Wayback Machine at the Encyclopedia Astronautica.
  3. NIOSH Pocket Guide to Chemical Hazards. "#0117". National Institute for Occupational Safety and Health (NIOSH).
  4. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. p. 4.58. ISBN 978-1-4398-5511-9.
  5. Chlorine fluoride (ClF3) Archived 2013-10-29 at the Wayback Machine at Guidechem Chemical Network
  6. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. p. 4.132. ISBN 978-1-4398-5511-9.
  7. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. p. 5.8. ISBN 978-1-4398-5511-9.
  8. "Chlorine trifluoride". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  9. Habuka, Hitoshi; Sukenobu, Takahiro; Koda, Hideyuki; Takeuchi, Takashi; Aihara, Masahiko (2004). "Silicon Etch Rate Using Chlorine Trifluoride". Journal of the Electrochemical Society. 151 (11): G783–G787. doi:10.1149/1.1806391.
  10. Xi, Ming et al. (1997) U.S. Patent 5,849,092 "Process for chlorine trifluoride chamber cleaning"
  11. Board on Environmental Studies and Toxicology, (BEST) (2006). Acute Exposure Guideline Levels for Selected Airborne Chemicals: Volume 5. Washington D.C.: National Academies Press. p. 40. ISBN 978-0-309-10358-9. (available from National Academies Press)
  12. Boyce, C. Bradford and Belter, Randolph K. (1998) U.S. Patent 6,034,016 "Method for regenerating halogenated Lewis acid catalysts"
  13. Otto Ruff, H. Krug (1930). "Über ein neues Chlorfluorid-CIF3". Zeitschrift für anorganische und allgemeine Chemie. 190 (1): 270–276. doi:10.1002/zaac.19301900127.
  14. Smith, D. F. (1953). "The Microwave Spectrum and Structure of Chlorine Trifluoride". The Journal of Chemical Physics. 21 (4): 609–614. Bibcode:1953JChPh..21..609S. doi:10.1063/1.1698976.
  15. "In Situ Cleaning of CVD Chambers". Semiconductor International. June 1, 1999.
  16. Clark, John D. (2001). Ignition!. UMI Books on Demand. ISBN 978-0-8135-0725-5.
  17. Clark, John D. (1972). Ignition! An Informal History of Liquid Rocket Propellants. Rutgers University Press. p. 214. ISBN 978-0-8135-0725-5.
  18. Müller, Benno (24 November 2005). "A poisonous present". Nature. Review of: Kampfstoff-Forschung im Nationalsozialismus: Zur Kooperation von Kaiser-Wilhelm-Instituten, Militär und Industrie [Weapons Research in National Socialism] by Florian Schmaltz (Wallstein, 2005, 676 pages). 438 (7067): 427. Bibcode:2005Natur.438..427M. doi:10.1038/438427a.
  19. "Germany 2004".
  20. Safetygram. Air Products
  21. "Chlorine Trifluoride Handling Manual". Canoga Park, CA: Rocketdyne. September 1961. p. 24. Retrieved 2012-09-19.
  22. Patnaik, Pradyot (2007). A comprehensive guide to the hazardous properties of chemical substances (3rd ed.). Wiley-Interscience. p. 478. ISBN 978-0-471-71458-3.

Further reading

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