Chlorine trifluoride
Encyclopedia
Chlorine trifluoride is an interhalogen 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 in the semiconductor industry, in nuclear reactor fuel processing, and other industrial operations.
; this also produced ClF and the mixture was separated by distillation.
ClF3 is approximately T-shaped, with one short bond (1.598 Å
) and two long bonds (1.698 Å). 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-Faxial bonds are consistent with hypervalent bonding.
Pure ClF3 is stable to 180° in glass vessels; 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:
and fluorination agent. It is extremely reactive with most inorganic and organic materials, even plastics, and will initiate the combustion of many materials without an ignition source. These reactions are often violent, and in some cases explosive
. Reaction with several metals give chloride
s and fluoride
s; phosphorus
yields PCl3 and PF5
; and sulfur
yields SCl2
and SF4
. ClF3 is also violently reactive with water, 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 checked and cleaned, because any contamination can ignite on contact. In addition, most general-purpose fire control/suppression hardware (Class A/B/C/K) is either incapable of suppressing this oxidation or can aggravate it; chlorine trifluoride has been reported to burn sand, asbestos
, and other highly fire-retardant materials, reacts violently with water-based suppressors, and oxidizes in the absence of atmospheric oxygen, rendering atmosphere-displacement suppressors such as Halon and CO2 ineffective. It ignites glass on prolonged 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 mainly hydrofluoric acid
and hydrochloric acid
, usually released as steam or vapor due to the highly exothermic nature of the reaction. Hydrofluoric acid is corrosive to human tissue, absorbs through skin, selectively attacks bone and stimulates pain nerves, and causes a potentially lethal poisoning. Hydrochloric acid
is secondary in its danger to living organisms, but is more corrosive to most inorganic materials than hydrofluoric acid.
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 bunker
ed, partly subterranean 31.76 km² munitions factory at Falkenhagen which was intended to produce 50 tonne
s 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.
systems. Handling concerns, however, prevented its use. John D. Clark
summarized the difficulties:
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
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 in the semiconductor industry, in nuclear reactor fuel processing, and other industrial operations.
Preparation, structure, and properties
It was first reported in 1930 by Ruff and Krug who prepared it by fluorination of chlorineChlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
; this also produced ClF and the mixture was separated by distillation.
- 3 F2 + Cl2 → 2 ClF3
ClF3 is approximately T-shaped, with one short bond (1.598 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
) and two long bonds (1.698 Å). This structure agrees with the prediction of VSEPR theory
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
, which predicts 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; above this temperature it decomposes by a free radical mechanism to the elements.
The main use of ClF3 is to produce uranium hexafluoride
Uranium hexafluoride
Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...
, UF6, as part of nuclear fuel processing and reprocessing, by the reaction:
- U + 3 ClF3
→ UF6 + 3 ClF
Hazards
ClF3 is a very strong oxidizingOxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
and fluorination agent. It is extremely reactive with most inorganic and organic materials, even plastics, and will initiate the combustion of many materials without an ignition source. These reactions are often violent, and in some cases explosive
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
. Reaction with several metals give chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
s and fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
s; phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
yields PCl3 and PF5
Phosphorus pentafluoride
Phosphorus pentafluoride, PF5, is a phosphorus halide. It's a colourless gas at room temperature and pressure.-Structure:Single-crystal X-ray studies indicate PF5 molecule has two distinct P−F bonds : P−Fax = 158.0 pm and P−Feq = 152.2 pm...
; and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
yields SCl2
Sulfur dichloride
Sulfur dichloride is the chemical compound with the formula SCl2. This cherry-red liquid is the simplest sulfur chloride and one of the most common. It is used as a precursor to organosulfur compounds.-Chlorination of sulfur:...
and SF4
Sulfur tetrafluoride
Sulfur tetrafluoride is the chemical compound with the formula SF4. This species exists as a gas at standard conditions. It is a corrosive species that releases dangerous HF upon exposure to water or moisture...
. ClF3 is also violently reactive with water, in which it hydrolyses
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
to a variety of hazardous chemicals such as hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
. H2S
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
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 checked and cleaned, because any contamination can ignite on contact. In addition, most general-purpose fire control/suppression hardware (Class A/B/C/K) is either incapable of suppressing this oxidation or can aggravate it; chlorine trifluoride has been reported to burn sand, asbestos
Asbestos
Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...
, and other highly fire-retardant materials, reacts violently with water-based suppressors, and oxidizes in the absence of atmospheric oxygen, rendering atmosphere-displacement suppressors such as Halon and CO2 ineffective. It ignites glass on prolonged 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 mainly hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
and hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
, usually released as steam or vapor due to the highly exothermic nature of the reaction. Hydrofluoric acid is corrosive to human tissue, absorbs through skin, selectively attacks bone and stimulates pain nerves, and causes a potentially lethal poisoning. Hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
is secondary in its danger to living organisms, but is more corrosive to most inorganic materials than hydrofluoric acid.
Military applications
Under the code nameCode name
A code name or cryptonym is a word or name used clandestinely to refer to another name or word. Code names are often used for military purposes, or in espionage...
N-stoff ("substance N"), chlorine trifluoride was investigated for military applications by the Kaiser Wilhelm Institute
Kaiser Wilhelm Institute
The Kaiser Wilhelm Society for the Advancement of Science was a German scientific institution established in 1911. It was implicated in Nazi science, and after the Second World War was wound up and its functions replaced by the Max Planck Society...
in Nazi Germany
Nazi Germany
Nazi Germany , also known as the Third Reich , but officially called German Reich from 1933 to 1943 and Greater German Reich from 26 June 1943 onward, is the name commonly used to refer to the state of Germany from 1933 to 1945, when it was a totalitarian dictatorship ruled by...
from slightly before the start of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
. Tests were made against mock-ups of the Maginot Line
Maginot Line
The Maginot Line , named after the French Minister of War André Maginot, was a line of concrete fortifications, tank obstacles, artillery casemates, machine gun posts, and other defences, which France constructed along its borders with Germany and Italy, in light of its experience in World War I,...
fortifications, and it was found to be an effective combined incendiary weapon and poison gas
Chemical warfare
Chemical warfare involves using the toxic properties of chemical substances as weapons. This type of warfare is distinct from Nuclear warfare and Biological warfare, which together make up NBC, the military acronym for Nuclear, Biological, and Chemical...
. From 1938 construction commenced on a partly bunker
Bunker
A military bunker is a hardened shelter, often buried partly or fully underground, designed to protect the inhabitants from falling bombs or other attacks...
ed, partly subterranean 31.76 km² munitions factory at Falkenhagen which was intended to produce 50 tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...
s of N-stoff per month, plus Sarin
Sarin
Sarin, or GB, is an organophosphorus compound with the formula [2CHO]CH3PF. It is a colorless, odorless liquid, which is used as a chemical weapon. It has been classified as a weapon of mass destruction in UN Resolution 687...
. However, by the time it was captured by the advancing Red Army
Red Army
The Workers' and Peasants' Red Army started out as the Soviet Union's revolutionary communist combat groups during the Russian Civil War of 1918-1922. It grew into the national army of the Soviet Union. By the 1930s the Red Army was among the largest armies in history.The "Red Army" name refers to...
in 1944, the factory had produced only about 30 to 50 tonnes, at a cost of over 100 German Reichsmark
German reichsmark
The Reichsmark was the currency in Germany from 1924 until June 20, 1948. The Reichsmark was subdivided into 100 Reichspfennig.-History:...
per kilogram. N-stoff was never used in war.
Rocket propellant
Chlorine trifluoride has been investigated as a high-performance storable oxidizer in rocket propellantRocket propellant
Rocket propellant is mass that is stored in some form of propellant tank, prior to being used as the propulsive mass that is ejected from a rocket engine in the form of a fluid jet to produce thrust. A fuel propellant is often burned with an oxidizer propellant to produce large volumes of very hot...
systems. Handling concerns, however, prevented its use. John D. Clark
John D. Clark
John Drury Clark, Ph.D. was a noted American rocket fuel developer, chemist, and science fiction writer and fan. He was instrumental in the revival of interest in Robert E. Howard's Conan stories and influenced the writing careers of L. Sprague de Camp, Fletcher Pratt, and other authors.- Life and...
summarized the difficulties:
It is, of course, extremely toxic, but that's the least of the problem. It is hypergolicHypergolic fuelA rocket propellant combination used in a rocket engine is called hypergolic when the propellants spontaneously ignite when they come into contact. Strictly speaking it is the combination that is hypergolic, but in less precise usage the individual propellants are also referred to as 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, aluminium, etc. — because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium 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."
Semiconductor industry
In the semiconductor industry, chlorine trifluoride is used to clean chemical vapour deposition chambers. It has the advantage that it can be used to remove semiconductor material from the chamber walls without having to dismantle the chamber. 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.External links
- National Pollutant Inventory - Fluoride and compounds fact sheet
- NIST Standard Reference Database
- WebElements
- Sand Won't Save You This Time, blog post by Derek LoweDerek Lowe (chemist)Derek Lowe is a chemistry blogger and medicinal chemist working on preclinical drug discovery. Lowe got his BA from Hendrix College and his PhD in organic chemistry from Duke University, doing his doctoral research studying the organic synthesis of natural products, before spending time in Germany...
on the hazards of handling ClF3