Chlorine trifluoride is an
interhalogen
In chemistry, an interhalogen compound is a molecule which contains two or more different halogen atoms ( fluorine, chlorine, bromine, iodine, or astatine) and no atoms of elements from any other group.
Most interhalogen compounds known are b ...
compound with the formula ClF
3. This colorless, poisonous, corrosive, and extremely reactive
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
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 in
plasmaless cleaning and
etching
Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other types ...
operations in the
semiconductor industry, in
nuclear reactor fuel processing, as a component in
rocket fuel
Rocket propellant is the reaction mass of a rocket. This reaction mass is ejected at the highest achievable velocity from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical ...
s, and other industrial operations.
Preparation, structure, and properties
It was first reported in 1930 by Ruff and Krug who prepared it by
fluorination
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, ...
of
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
; this also produced
ClF (chlorine monofluoride) and the mixture was separated by
distillation
Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heat ...
.
:3 F
2 + Cl
2 → 2 ClF
3
The
molecular geometry
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that dete ...
of ClF
3 is approximately
T-shaped
Many shapes have metaphorical names, i.e., their names are metaphors: these shapes are named after a most common object that has it. For example, "U-shape" is a shape that resembles the letter U, a bell-shaped curve has the shape of the vertical ...
, with one short bond (1.598
Å) and two long bonds (1.698 Å). This structure agrees with the prediction of
VSEPR theory
Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm the ...
, 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
In chemistry, a hypervalent molecule (the phenomenon is sometimes colloquially known as expanded octet) is a molecule that contains one or more main group elements apparently bearing more than eight electrons in their valence shells. Phosphorus ...
.
Pure ClF
3 is stable to in
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
vessels; above this temperature, it
decomposes by a
free radical
A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing.
Ageing
Ailments of unknown cause
Biogerontology
Biological processes
Causes of death
Cellular processes
Gerontology
Life extension
Metabo ...
mechanism to its constituent elements.
Reactions
Reactions with many metals give
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
s and
fluorides. With
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, it yields
phosphorus trichloride
Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxi ...
(PCl
3) and
phosphorus pentafluoride
Phosphorus pentafluoride, P F5, is a phosphorus halide. It is a colourless, toxic gas that fumes in air.
Preparation
Phosphorus pentafluoride was first prepared in 1876 by the fluorination of phosphorus pentachloride using arsenic trifluoride, w ...
(PF
5), while
sulfur yields
sulfur dichloride (SCl
2) and
sulfur tetrafluoride (SF
4). ClF
3 also reacts with water to give
hydrogen fluoride and
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
, along with oxygen and
oxygen difluoride
Oxygen difluoride is a chemical compound with the formula . As predicted by VSEPR theory, the molecule adopts a "bent" molecular geometry. It is strong oxidizer and has attracted attention in rocketry for this reason. With a boiling point of -144.7 ...
(OF
2):
:ClF
3 + H
2O
→ HF + HCl + OF
2
:ClF
3 + 2H
2O
→ 3HF + HCl + O
2
It will also convert many
metal oxides to
metal halides
Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, su ...
and oxygen or oxygen difluoride.
It occurs as a
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
in the complex .
One of the main uses of ClF
3 is to produce
uranium hexafluoride
Uranium hexafluoride (), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with alumin ...
, UF
6, as part of nuclear fuel processing and reprocessing, by the fluorination of
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
metal:
:U + 3 ClF
3 → UF
6 + 3 ClF
The
compound
Compound may refer to:
Architecture and built environments
* Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall
** Compound (fortification), a version of the above fortified with defensive struc ...
can also
dissociate
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid ...
under the scheme:
:ClF
3 → ClF + F
2
Uses
Semiconductor industry
In the
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
industry, chlorine trifluoride is used to clean
chemical vapour deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (substra ...
chambers.
It has the advantage that it can be used to remove semiconductor material from the chamber walls without the need 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 sufficient to make it decompose and react with the semiconductor material.
Rocket propellant
Chlorine trifluoride has been investigated as a high-performance storable
oxidizer
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...
in
rocket propellant systems. Handling concerns, however, severely limit its use.
John Drury Clark
John Drury Clark, Ph.D. (August 15, 1907 – July 6, 1988) was an American rocket fuel developer, chemist, and science fiction writer. He was instrumental in the revival of interest in Robert E. Howard's '' Conan'' stories and influenced the wr ...
summarized the difficulties:
It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic
A hypergolic propellant is a rocket propellant combination used in a rocket engine, whose components spontaneously ignite when they come into contact with each other.
The two propellant components usually consist of a fuel and an oxidizer. T ...
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.
Chlorine pentafluoride
Chlorine pentafluoride is an interhalogen compound with formula ClF5. This colourless gas is a strong oxidant that was once a candidate oxidizer for rockets. The molecule adopts a square pyramidal structure with C4v symmetry, as confirmed by ...
(ClF
5) has also been investigated as a potential rocket oxidizer. It offered improved
specific impulse
Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...
over chlorine trifluoride, but with all of the same difficulties in handling. Neither compound has been used in any operational rocket propulsion system.
Proposed military applications
Under the
code name N-Stoff ("substance N"), chlorine trifluoride was investigated for military applications by the
Kaiser Wilhelm Institute
The Kaiser Wilhelm Society for the Advancement of Science (German: ''Kaiser-Wilhelm-Gesellschaft zur Förderung der Wissenschaften'') was a German scientific institution established in the German Empire in 1911. Its functions were taken over by ...
in
Nazi Germany
Nazi Germany (lit. "National Socialist State"), ' (lit. "Nazi State") for short; also ' (lit. "National Socialist Germany") (officially known as the German Reich from 1933 until 1943, and the Greater German Reich from 1943 to 1945) was ...
not long before the start of
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
. Tests were made against mock-ups of the
Maginot Line fortifications, and it was found to be an extremely effective
incendiary weapon
Incendiary weapons, incendiary devices, incendiary munitions, or incendiary bombs are weapons designed to start fires or destroy sensitive equipment using fire (and sometimes used as anti-personnel weaponry), that use materials such as napalm, th ...
and
poison gas
Many gases have toxic properties, which are often assessed using the LC50 (median lethal dose) measure. In the United States, many of these gases have been assigned an NFPA 704 health rating of 4 (may be fatal) or 3 (may cause serious or perman ...
. From 1938, construction commenced on a partly
bunker
A bunker is a defensive military fortification designed to protect people and valued materials from falling bombs, artillery, or other attacks. Bunkers are almost always underground, in contrast to blockhouses which are mostly above ground. T ...
ed, partly subterranean munitions factory, the
Falkenhagen industrial complex, which was intended to produce 90
tonne
The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
s of N-Stoff per month, plus
sarin (a deadly
nerve agent
Nerve agents, sometimes also called nerve gases, are a class of organic chemicals that disrupt the mechanisms by which nerves transfer messages to organs. The disruption is caused by the blocking of acetylcholinesterase (AChE), an enzyme that ...
). However, by the time it was captured by the advancing
Red Army
The Workers' and Peasants' Red Army ( Russian: Рабо́че-крестья́нская Кра́сная армия),) often shortened to the Red Army, was the army and air force of the Russian Soviet Federative Socialist Republic and, afte ...
in 1945, 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.
Hazards
ClF
3 is a very strong
oxidizer
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...
and
fluorinating agent. It is extremely reactive with most inorganic and organic materials, and will combust many otherwise non-flammable materials
without any ignition source. These reactions are often violent, and in some cases
explosive, especially with flammable materials.
Steel,
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
, and
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
are not consumed because a
passivation layer of insoluble metal fluoride will form which prevents further corrosion, but
molybdenum,
tungsten
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
, and
titanium
Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...
are unsuitable as the fluorides that they form are volatile. Any equipment that comes into contact with ClF
3 must be meticulously cleaned and then passivated, because any contamination left may burn through the unfluorinated material faster than it can re-form. ClF
3 will quickly corrode even
noble metals like iridium, platinum, or gold, oxidizing them to chlorides and fluorides.
This oxidizing power, surpassing that of oxygen, causes ClF
3 to react vigorously with many other materials often thought of as incombustible and refractory. It is known to ignite sand,
asbestos, glass, and even ashes of substances that have already burned in oxygen. In one particular industrial accident, a spill of 900 kg of ClF
3 burned through 30 cm of concrete and 90 cm of gravel beneath.
There is exactly one known fire control/suppression method capable of dealing with ClF
3 – the use of nitrogen and noble gases: the surrounding area must be flooded with nitrogen or argon. Barring that, the area must simply be kept cool until the reaction ceases. The compound reacts with water-based suppressors and CO
2, rendering them ineffective.
Exposure to larger amounts of ClF
3, as a liquid or as a gas, ignites living tissue, resulting in severe chemical and thermal burns. ClF
3 reacts violently with water and exposure to the reaction also results in burns. The products of hydrolysis are mainly
hydrofluoric acid and
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
, which are usually released as steam or vapor due to the highly exothermic nature of the reaction.
See also
*
Dioxygen difluoride
Dioxygen difluoride is a compound of fluorine and oxygen with the molecular formula O2F2. It can exist as an orange-colored solid which melts into a red liquid at . It is an extremely strong oxidant and decomposes into oxygen and fluorine even ...
*
Chlorine fluoride
*
List of stoffs
During World War II, Germany fielded many aircraft and rockets whose fuels, and oxidizers, were designated (letter)-''Stoff'' (). The following list of stoffs refers to the World War II aerospace meanings if not noted otherwise.
Meaning of ''st ...
Notes
Using data fro
Economic History Servicesan
The Inflation Calculator we can calculate that 100 Reichsmark in 1941 is approximately equivalent to $4,652.50 USD in 2021. Reichsmark exchange rate values from 1942 to 1944 are fragmentary.
References
Further reading
*
*
*
*
*
*
*
External links
*
ttp://webbook.nist.gov/chemistry/ NIST Standard Reference DatabaseCDC – NIOSH Pocket Guide to Chemical Hazards – Chlorine TrifluorideWebElementsSand Won't Save You This Time blog post by
Derek Lowe on the hazards of handling ClF
3
{{Fluorine compounds
Fluorides
Interhalogen compounds
Inorganic chlorine compounds
Incendiary weapons
Rocket oxidizers
Fluorinating agents
Oxidizing agents
Chlorine(III) compounds
Substances discovered in the 1930s