Halomethane
Encyclopedia
Halomethane compounds are derivatives of methane
(CH4) with one or more of the hydrogen
atoms replaced with halogen
atoms (F
, Cl, Br
, or I
). Halomethanes are both naturally occurring, especially in marine environments, and man-made, most notably as refrigerants, solvents, propellants, and fumigants. Many, including the chlorofluorocarbon
s, have attracted wide attention because they become active when exposed to ultraviolet light found at high altitudes and destroy the Earth's protective ozone layer
.
The physical properties of the halomethanes are tunable by changes in the number and identity of the halogen atoms. In general, they are volatile but less so than methane because of their polarizability of the halides. The polarizability of the halides and the polarity of the molecules makes them useful as solvents. The halomethanes are far less flammable than methane. Broadly speaking, reactivity of the compounds is greatest for the iodides and lowest for the fluorides.
or methanol
and from halogen
s or the halogen halides. They are usually prepared by one of three methods.
This method is useful for the production of CH4-xClx (x = 1, 2, 3, or 4). The main problems with the method are that it cogenerates HCl and it affords mixtures.
halomethanes, especially the bromine derivatives. Traces of halomethanes arise through the introduction of other non-natural industrial materials. Small amounts of chloromethanes arise from the interaction of chlorine sources with various carbon compounds. The biosyntheses of these halomethanes are catalyzed by the chloroperoxidase and bromoperoxidase
enzymes, respectively. An idealized equation is:
For example, consider Halon 1211:
C F Cl Br
1 2 1 1
Halon 1211 has one carbon atom, two fluorine atoms, one chlorine atom and one bromine atom. A single carbon only has four bonds, all of which are taken by the halogen atoms, so there is no hydrogen. Thus its formula is CF2BrCl , and its IUPAC name is therefore bromochlorodifluoromethane.
The decoding system for CFC-01234a is:
Other coding systems are in use as well.
. The main chemical used under this trademark is dichlorodifluoromethane. The word Freon is a registered trademark
belonging to DuPont
.
Hydrofluorocarbons (HFCs) contain no chlorine. They are composed entirely of carbon, hydrogen, and fluorine. They have no known effects at all on the ozone layer. Only compounds containing chlorine and bromine are thought to harm the ozone layer. Fluorine itself is not ozone-toxic.http://www.epa.gov/ozone/ods.html However, HFCs and perfluorocarbons do have activity in the entirely different realm of greenhouse gas
es, which do not destroy ozone, but cause global warming
. Two groups of haloalkanes, hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs), are targets of the Kyoto Protocol
. Allan Thornton President of Environmental Investigation Agency
, an environmental watchdog, says that HFCs are up to 12,500 times as potent as carbon dioxide in global warming. Wealthy countries are clamping down on these gases. Thornton says that many countries are needlessly producing these chemicals just to get the carbon credits. Thus, as a result of carbon trading rules under the Kyoto Protocol, nearly half the credits from developing countries are from HFCs, with China scoring billions of dollars from catching and destroying HFCs that would be in the atmosphere as industrial byproducts.
and a degreaser. In the food industry
, it is used to decaffeinate
coffee
and tea
as well as to prepare extracts of hops
and other flavorings. Its volatility has led to its use as an aerosol spray propellant and as a blowing agent
for polyurethane
foam
s.
inhalers for drugs used to treat asthma
. The conversion of these devices and treatments from CFC to halocarbons that do not have the same effect on the ozone layer is well under way. The hydrofluoroalkane propellant's ability to solubilize medications and excipient
s is markedly different from CFCs and as a result requires a considerable amount of effort to reformulate (a significant amount of development effort has also been required to develop non-CFC alternatives to CFC-based refrigerants, particularly for applications where the refrigeration mechanism cannot be modified or replaced). They have now been outlawed universally in the U.S.
atoms that combine readily with active hydrogen atoms, quenching flame propagation reaction even when adequate fuel, oxygen, and heat remains. The chemical reaction
in a flame
proceeds as a free radical chain reaction
; by sequestering the radicals which propagate the reaction, halons are able to "poison" the fire at much lower concentration
s than are required by fire suppressants using the more traditional methods of cooling, oxygen deprivation, or fuel dilution.
For example, Halon 1301 total flooding systems are typically used at concentrations no higher than 7% v/v in air, and can suppress many fires at 2.9% v/v. By contrast, carbon dioxide
fire suppression flood systems are operated from 34% concentration by volume (surface-only combustion of liquid fuels) up to 75% (dust traps). Carbon dioxide can cause severe distress at concentrations of 3–6%, and has caused death by respiratory paralysis in a few minutes at 10% concentration. Halon 1301 causes only slight giddiness at its effective concentration of 5%, and even at 15% persons remain conscious but impaired and suffer no long term effects. (Experimental animals have also been exposed to 2% concentrations of Halon 1301 for 30 hours per week for 4 months, with no discernible health effects at all.) Halon 1211 also has low toxicity, although it is more toxic than Halon 1301, and thus considered unsuitable for flooding systems.
However, Halon 1301 fire suppression is not completely non-toxic; very high temperature flame, or contact with red-hot metal, can cause decomposition of Halon 1301 to toxic byproducts. The presence of such byproducts is readily detected because they include hydrobromic acid
and hydrofluoric acid
, which are intensely irritating. Halons are very effective on Class A (organic solids), B (flammable liquids and gases) and C (electrical) fires, but they are totally unsuitable for Class D (metal) fires, as they will not only produce toxic gas and fail to halt the fire, but in some cases pose a risk of explosion. Halons can be used on Class K (kitchen oils and greases) fires, but offer no advantages over specialised foams.
Halon 1211 is typically used in hand-held extinguishers, in which a stream of liquid halon is directed at a smaller fire by a user. The stream evaporates under reduced pressure, producing strong local cooling, as well as a high concentration of halon in the immediate vicinity of the fire. In this mode, extinguishment is achieved by cooling and oxygen deprivation at the core of the fire, as well as radical quenching over a larger area. After fire suppression, the halon moves away with the surrounding air, leaving no residue.
Halon 1301 is more usually employed in total flooding systems. In these systems, banks of halon cylinders are kept pressurised to about 4 MPa
(600 psi
) with compressed nitrogen
, and a fixed piping network leads to the protected enclosure. On triggering, the entire measured contents of one or more cylinders are discharged into the enclosure in a few seconds, through nozzle
s designed to ensure uniform mixing throughout the room. The quantity dumped is pre-calculated to achieve the desired concentration, typically 3–7% v/v. This level is maintained for some time, typically with a minimum of ten minutes and sometimes up to a twenty minute "soak" time, to ensure all items have cooled so reignition is unlikely to occur, then the air in the enclosure is purged, generally via a fixed purge system that is activated by the proper authorities. During this time the enclosure may be entered by persons wearing SCBA. (There exists a common myth that this is because halon is highly toxic; in fact, it is because it can cause giddiness and mildly impaired perception, and also due to the risk of combustion byproducts.)
Flooding systems may be manually operated or automatically triggered by a VESDA or other automatic detection system. In the latter case, a warning siren and strobe lamp will first be activated for a few seconds to warn personnel to evacuate the area. The rapid discharge of halon and consequent rapid cooling fills the air with fog
, and is accompanied by a loud, disorienting noise.
Due to environmental concerns, alternatives are being deployed.
Halon 1301 is also used in the F-16 fighter to prevent the fuel vapors in the fuel tank
s from becoming explosive; when the aircraft enters an area with the possibility of unfriendly fire, Halon 1301 is injected into the fuel tanks for one-time use. Due to environmental concerns, trifluoroiodomethane (CF3I) is being considered as an alternative.
and the production of fine chemicals. Chlorodifluoromethane is the main intermediate en route to tetrafluoroethylene
, the monomeric precursor to Teflon.
, which is highly toxic.
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
(CH4) with one or more of the hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atoms replaced with halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
atoms (F
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
, Cl, Br
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
, or I
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
). Halomethanes are both naturally occurring, especially in marine environments, and man-made, most notably as refrigerants, solvents, propellants, and fumigants. Many, including the chlorofluorocarbon
Chlorofluorocarbon
A chlorofluorocarbon is an organic compound that contains carbon, chlorine, and fluorine, produced as a volatile derivative of methane and ethane. A common subclass are the hydrochlorofluorocarbons , which contain hydrogen, as well. They are also commonly known by the DuPont trade name Freon...
s, have attracted wide attention because they become active when exposed to ultraviolet light found at high altitudes and destroy the Earth's protective ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...
.
Structure and properties
Like methane itself, halomethanes are tetrahedral molecules. The halogen atoms differ greatly in size from hydrogen and from each other. Consequently, the various halomethanes deviate from the perfect tetrahedral symmetry of methane.The physical properties of the halomethanes are tunable by changes in the number and identity of the halogen atoms. In general, they are volatile but less so than methane because of their polarizability of the halides. The polarizability of the halides and the polarity of the molecules makes them useful as solvents. The halomethanes are far less flammable than methane. Broadly speaking, reactivity of the compounds is greatest for the iodides and lowest for the fluorides.
Industrial routes
The halomethanes are produced on a massive scale easily from abundant precursors, i.e. natural gasNatural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
or methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
and from halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
s or the halogen halides. They are usually prepared by one of three methods.
- Free radical chlorination of methane:
- CH4 + Cl2 → CH3Cl + HCl
This method is useful for the production of CH4-xClx (x = 1, 2, 3, or 4). The main problems with the method are that it cogenerates HCl and it affords mixtures.
- HalogenationHalogenationHalogenation is a chemical reaction that incorporates a halogen atom into a molecule in substitution of hydrogen atom. Halogenation takes place in the gas phase. There are four types of halogenation: fluorination, chlorination, bromination, and iodination...
of methanol. This method is used for the production of the monochloride, bromide, and iodide.
- CH3OH + HCl → CH3Cl + H2O
- 4 CH3OH + 3 Br2 + S → 4 CH3Br + H2SO4 + 2 HBr
- 3 CH3OH + 3 I2 + P → 3 CH3I + HPO(OH)2 + 3 HI
- Halogen exchange. The method is mainly used to produce fluorinated derivatives from the chlorides.
- HCCl3 + 2 HF → HCF2Cl + 2 HCl
In nature
Many marine organisms biosynthesizeBiosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
halomethanes, especially the bromine derivatives. Traces of halomethanes arise through the introduction of other non-natural industrial materials. Small amounts of chloromethanes arise from the interaction of chlorine sources with various carbon compounds. The biosyntheses of these halomethanes are catalyzed by the chloroperoxidase and bromoperoxidase
Bromoperoxidase
Bromoperoxidases are enzymes that catalyse the bromination of hydrocarbons. The enzymes accomplish this reaction via the following reaction:Related chloroperoxidase enzymes effect chlorination....
enzymes, respectively. An idealized equation is:
- CH4 + Cl- + 1/2 O2 → CH3Cl + OH-
Classes of compounds
Halons are usually defined as hydrocarbons where the hydrogen atoms have been replaced by bromine, along with other halogens. They are referred to by a system of code numbers similar to (but simpler than) the system used for freons. The first digit specifies the number of carbon atoms in the molecule, the second is the number of fluorine atoms, the third is the chlorine atoms, and the fourth is the number of bromine atoms. If the number includes a fifth digit, the fifth number indicates the number of iodine atoms (though iodine in halon is rare). Any bonds not taken up by halogen atoms are then allocated to hydrogen atoms.For example, consider Halon 1211:
C F Cl Br
1 2 1 1
Halon 1211 has one carbon atom, two fluorine atoms, one chlorine atom and one bromine atom. A single carbon only has four bonds, all of which are taken by the halogen atoms, so there is no hydrogen. Thus its formula is CF2
ANSI/ASHRAE Standard 34-1992
The refrigerant naming system is mainly used for fluorinated and chlorinated short alkanes for refrigerant use. In the US, the standard is specified in ANSI/ASHRAE Standard 34-1992, with additional annual supplements. The specified ANSI/ASHRAE prefixes were FC (fluorocarbon) or R (refrigerant), but today most are prefixed by a more specific classification:- CFC—list of chlorofluorocarbons
- HCFC—list of hydrochlorofluorocarbons
- HFC—list of hydrofluorocarbons
- FC—list of fluorocarbonFluorocarbonFluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes that contain only single bonds are more chemically and thermally stable than alkanes...
s - PFC—list of perfluorocarbons (completely fluorinated)
The decoding system for CFC-01234a is:
- 0 = Number of double bonds (omitted if zero)
- 1 = Carbon atoms -1 (omitted if zero)
- 2 = Hydrogen atoms +1
- 3 = Fluorine atoms
- 4 = Replaced by Bromine ("B" prefix added)
- a = Letter added to identify isomers, the "normal" isomer in any number has the smallest mass difference on each carbon, and a, b, or c are added as the masses diverge from normal.
Other coding systems are in use as well.
Hydrofluoro compounds (HFC)
Freon is a trade name for a group of chlorofluorocarbons used primarily as a refrigerantRefrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
. The main chemical used under this trademark is dichlorodifluoromethane. The word Freon is a registered trademark
Trademark
A trademark, trade mark, or trade-mark is a distinctive sign or indicator used by an individual, business organization, or other legal entity to identify that the products or services to consumers with which the trademark appears originate from a unique source, and to distinguish its products or...
belonging to DuPont
DuPont
E. I. du Pont de Nemours and Company , commonly referred to as DuPont, is an American chemical company that was founded in July 1802 as a gunpowder mill by Eleuthère Irénée du Pont. DuPont was the world's third largest chemical company based on market capitalization and ninth based on revenue in 2009...
.
Hydrofluorocarbons (HFCs) contain no chlorine. They are composed entirely of carbon, hydrogen, and fluorine. They have no known effects at all on the ozone layer. Only compounds containing chlorine and bromine are thought to harm the ozone layer. Fluorine itself is not ozone-toxic.http://www.epa.gov/ozone/ods.html However, HFCs and perfluorocarbons do have activity in the entirely different realm of greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
es, which do not destroy ozone, but cause global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
. Two groups of haloalkanes, hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs), are targets of the Kyoto Protocol
Kyoto Protocol
The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change , aimed at fighting global warming...
. Allan Thornton President of Environmental Investigation Agency
Environmental Investigation Agency
The Environmental Investigation Agency is an NGO founded in 1984 by Dave Currey, Jennifer Lonsdale and Allan Thornton, three environmental activists in the United Kingdom. Its stated goal is to investigate and expose crimes against wildlife and the environment...
, an environmental watchdog, says that HFCs are up to 12,500 times as potent as carbon dioxide in global warming. Wealthy countries are clamping down on these gases. Thornton says that many countries are needlessly producing these chemicals just to get the carbon credits. Thus, as a result of carbon trading rules under the Kyoto Protocol, nearly half the credits from developing countries are from HFCs, with China scoring billions of dollars from catching and destroying HFCs that would be in the atmosphere as industrial byproducts.
Overview of principal halomethanes
Virtually every permutation of hydrogen, fluorine, chlorine, bromine, and iodine on one carbon atom have been evaluated experimentally.| Overview of Halomethanes | ||||
|---|---|---|---|---|
| Systematic name | Common/Trivial name(s) |
Code | Use | Chem. formula |
| Tetrachloromethane | Carbon tetrachloride Carbon tetrachloride Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent... , Freon 10 |
CFC-10 | Formerly in fire extinguishers | CCl4 |
| Tetrabromomethane Tetrabromomethane Tetrabromomethane, CBr4, also known as carbon tetrabromide, is a carbon bromide. Both names are acceptable under IUPAC nomenclature, depending on whether it is considered as an organic or an inorganic compound.- Physical properties :... |
Carbon tetrabromide | CBr4 | ||
| Tetraiodomethane | Carbon tetraiodide Carbon tetraiodide Carbon tetraiodide is CI4, a tetrahalomethane. Being bright red, it is a relatively rare example of a highly colored methane derivative. It is only 2% by weight carbon, although other methane derivatives are known with still less carbon.... |
CI4 | ||
| Tetrafluoromethane Tetrafluoromethane Tetrafluoromethane, also known as carbon tetrafluoride, is the simplest fluorocarbon . It has a very high bond strength due to the nature of the carbon–fluorine bond. It can also be classified as a haloalkane or halomethane... |
Carbon tetrafluoride, Freon 14 | PFC-14 (CFC-14 and HF-14 also used, although formally incorrect) |
CF4 | |
| Chloromethane Chloromethane Chloromethane, also called methyl chloride, R-40 or HCC 40, is a chemical compound of the group of organic compounds called haloalkanes. It was once widely used as a refrigerant. It is a colorless extremely flammable gas with a minorly sweet odor, which is, however, detected at possibly toxic levels... |
Methyl chloride | methylation agent, e.g. for methyl trichlorosilane | CH3Cl | |
| Dichloromethane Dichloromethane Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents... |
Methylene chloride | solvent | CH2Cl2 | |
| Trichloromethane | Chloroform Chloroform Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous... |
solvent | CHCl3 | |
| Trichlorofluoromethane Trichlorofluoromethane Trichlorofluoromethane, also called freon-11, CFC-11, or R-11, is a chlorofluorocarbon. It is a colorless, nearly odorless liquid that boils at about room temperature.- Uses :It was the first widely used refrigerant... |
Freon-11, R-11 | CFC-11 | CCl3F | |
| Dichlorodifluoromethane Dichlorodifluoromethane Dichlorodifluoromethane , is a colorless gas, and usually sold under the brand name Freon-12, is a chlorofluorocarbon halomethane , used as a refrigerant and aerosol spray propellant. Complying with the Montreal Protocol, its manufacture was banned in the United States along with many other... |
Freon-12, R-12 | CFC-12 | CCl2F2 | |
| Chlorotrifluoromethane Chlorotrifluoromethane Chlorotrifluoromethane, R-13, CFC-13, or Freon 13, is a non-flammable, non-corrosive chlorofluorocarbon and also a mixed halomethane. It is used as a refrigerant, however, due to concerns about its ozone-depleting potential, its use has been phased out due to the Montreal Protocol.-Physical... |
CFC-13 | CClF3 | ||
| Chlorodifluoromethane Chlorodifluoromethane Chlorodifluoromethane or difluoromonochloromethane is a hydrochlorofluorocarbon . This colorless gas is better known as HCFC-22, or R-22. It was once commonly used as a propellant and in air conditioning applications... |
R-22 | HCFC-22 | CHClF2 | |
| Trifluoromethane | Fluoroform Fluoroform Fluoroform is the chemical compound with the formula CHF3. It is one of the "haloforms", a class of compounds with the formula CHX3 . Fluoroform is used in diverse niche applications and is produced as a by-product of the manufacture of Teflon... |
HFC-23 | in semiconductor industry, refrigerant | CHF3 |
| Chlorofluoromethane Chlorofluoromethane Chlorofluoromethane or Freon 31 is a gaseous mixed halomethane .Its crystal structure is monoclinic with space group P21 and lattice constants a = 6.7676, b = 4.1477, c = 5.0206 , β = 108.205°.... |
Freon 31 | refrigerant (phased out) | CH2ClF | |
| Difluoromethane Difluoromethane Difluoromethane, also called HFC-32 or R-32, is an organic compound of the dihalogenoalkane variety. It is based on methane, except that two of the four hydrogen atoms have been replaced by fluorine atoms... |
HFC-32 | refrigerant with zero ozone depletion potential Ozone depletion potential The ozone depletion potential of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane being fixed at an ODP of 1.0. Chlorodifluoromethane , for example, has an ODP of 0.055... |
CH2F2 | |
| Fluoromethane Fluoromethane Fluoromethane, also known as methyl fluoride, Freon 41, Halocarbon-41 and HFC-41, is a non-toxic, liquefiable, and flammable gas at standard temperature and pressure. It is made of carbon, hydrogen, and fluorine. The name stems from the fact that it is methane plus fluorine, minus a hydrogen... |
Methyl fluoride | HFC-41 | semiconductor manufacture | CH3F |
| Bromomethane | Methyl bromide | soil sterilant and fumigant, currently being phased out. It strongly depletes ozone layer and is highly toxic. | CH3Br | |
| Dibromomethane Dibromomethane Dibromomethane or methylene bromide, or methylene dibromide is a halomethane. It is slightly soluble in water, but very soluble in carbon tetrachloride, diethyl ether and methanol.Its refractive index is 1.5419 .-Preparation:... |
Methylene bromide | solvent and chemical intermediate. | CH2Br2 | |
| Tribromomethane | Bromoform Bromoform Bromoform is a pale yellowish liquid with a sweet odor similar to chloroform, a halomethane or haloform. Its refractive index is 1.595 . Bromoform is produced naturally by phytoplankton and seaweeds in the ocean and this is thought to be the predominant source to the environment... |
for separation of heavy minerals | CHBr3 | |
| Bromochloromethane Bromochloromethane Bromochloromethane or methylene bromochloride and Halon 1011 is a mixed halomethane. It is a heavy low-viscosity liquid with refractive index 1.4808.... |
Halon 1011 | formerly in fire extinguishers | CH2BrCl | |
| Bromochlorodifluoromethane Bromochlorodifluoromethane Bromochlorodifluoromethane, also known by the trade name Halon 1211, or BCF, or Halon 1211 BCF, or Freon 12B1, is a haloalkane with the chemical formula CF2ClBr.... |
BCF, Halon 1211 BCF, or Freon 12B1 | Halon 1211 | CBrClF2 | |
| Bromotrifluoromethane Bromotrifluoromethane Bromotrifluoromethane is an organic halide with the chemical formula CBrF3. Alternative names are Halon 1301, R13B1, Halon 13B1 and BTM.- Table of physical properties :- Uses :... |
BTM, Halon 1301 BTM, or Freon 13BI | Halon 1301 | CBrF3 | |
| Trifluoroiodomethane | Trifluoromethyl iodide | Freon 13T1 | organic synthesis | CF3I |
| Methyl iodide | Iodomethane | organic synthesis | CH3I | |
Applications
Because they have many applications and easily prepared, halomethanes have been of intense commercial interest.Solvents
Dichloromethane is the most important halomethane-based solvent. Its volatility, low flammability, and ability to dissolve a wide range of organic compounds makes this colorless liquid a useful solvent. It is widely used as a paint stripperPaint stripper
Paint stripper, or paint remover, is the generic name given to solvent mixtures designed to remove paint and other finishes and also to clean the underlying surface. The principal active ingredient is usually dichloromethane...
and a degreaser. In the food industry
Food industry
The food production is a complex, global collective of diverse businesses that together supply much of the food energy consumed by the world population...
, it is used to decaffeinate
Decaffeination
Decaffeination is the act of removing caffeine from coffee beans, cocoa, tea leaves and other caffeine-containing materials. Despite removal of caffeine, many decaffeinated drinks still have around 1-2% of the...
coffee
Coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...
and tea
Tea
Tea is an aromatic beverage prepared by adding cured leaves of the Camellia sinensis plant to hot water. The term also refers to the plant itself. After water, tea is the most widely consumed beverage in the world...
as well as to prepare extracts of hops
Hops
Hops are the female flower clusters , of a hop species, Humulus lupulus. They are used primarily as a flavoring and stability agent in beer, to which they impart a bitter, tangy flavor, though hops are also used for various purposes in other beverages and herbal medicine...
and other flavorings. Its volatility has led to its use as an aerosol spray propellant and as a blowing agent
Blowing agent
A blowing agent is a substance which is capable of producing a cellular structure via a foaming process in a variety of materials that undergo hardening or phase transition, such as polymers, plastics, and metals. They are typically applied when the blown material is in a liquid stage...
for polyurethane
Polyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...
foam
Foam
-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...
s.
Propellants
One major use of CFCs has been as propellants in aerosolAerosol spray
Aerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles. This is used with a can or bottle that contains a liquid under pressure. When the container's valve is opened, the liquid is forced out of a small hole and emerges as an aerosol or mist...
inhalers for drugs used to treat asthma
Asthma
Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...
. The conversion of these devices and treatments from CFC to halocarbons that do not have the same effect on the ozone layer is well under way. The hydrofluoroalkane propellant's ability to solubilize medications and excipient
Excipient
An excipient is generally a pharmacologically inactive substance used as a carrier for the active ingredients of a medication. In many cases, an "active" substance may not be easily administered and absorbed by the human body; in such cases the substance in question may be dissolved into or...
s is markedly different from CFCs and as a result requires a considerable amount of effort to reformulate (a significant amount of development effort has also been required to develop non-CFC alternatives to CFC-based refrigerants, particularly for applications where the refrigeration mechanism cannot be modified or replaced). They have now been outlawed universally in the U.S.
Fire extinguishing
At high temperatures, halons decompose to release halogenHalogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
atoms that combine readily with active hydrogen atoms, quenching flame propagation reaction even when adequate fuel, oxygen, and heat remains. The chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
in a flame
Flame
A flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...
proceeds as a free radical chain reaction
Chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
; by sequestering the radicals which propagate the reaction, halons are able to "poison" the fire at much lower concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
s than are required by fire suppressants using the more traditional methods of cooling, oxygen deprivation, or fuel dilution.
For example, Halon 1301 total flooding systems are typically used at concentrations no higher than 7% v/v in air, and can suppress many fires at 2.9% v/v. By contrast, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
fire suppression flood systems are operated from 34% concentration by volume (surface-only combustion of liquid fuels) up to 75% (dust traps). Carbon dioxide can cause severe distress at concentrations of 3–6%, and has caused death by respiratory paralysis in a few minutes at 10% concentration. Halon 1301 causes only slight giddiness at its effective concentration of 5%, and even at 15% persons remain conscious but impaired and suffer no long term effects. (Experimental animals have also been exposed to 2% concentrations of Halon 1301 for 30 hours per week for 4 months, with no discernible health effects at all.) Halon 1211 also has low toxicity, although it is more toxic than Halon 1301, and thus considered unsuitable for flooding systems.
However, Halon 1301 fire suppression is not completely non-toxic; very high temperature flame, or contact with red-hot metal, can cause decomposition of Halon 1301 to toxic byproducts. The presence of such byproducts is readily detected because they include hydrobromic acid
Hydrobromic acid
Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at 124.3 °C and contains 47.6% HBr by weight, which is 8.89 mol/L. Hydrobromic acid has a pKa of −9, making it a...
and 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 ....
, which are intensely irritating. Halons are very effective on Class A (organic solids), B (flammable liquids and gases) and C (electrical) fires, but they are totally unsuitable for Class D (metal) fires, as they will not only produce toxic gas and fail to halt the fire, but in some cases pose a risk of explosion. Halons can be used on Class K (kitchen oils and greases) fires, but offer no advantages over specialised foams.
Halon 1211 is typically used in hand-held extinguishers, in which a stream of liquid halon is directed at a smaller fire by a user. The stream evaporates under reduced pressure, producing strong local cooling, as well as a high concentration of halon in the immediate vicinity of the fire. In this mode, extinguishment is achieved by cooling and oxygen deprivation at the core of the fire, as well as radical quenching over a larger area. After fire suppression, the halon moves away with the surrounding air, leaving no residue.
Halon 1301 is more usually employed in total flooding systems. In these systems, banks of halon cylinders are kept pressurised to about 4 MPa
MPA
-Academic degrees:* Master of Professional Accountancy* Master of Public Administration* Master of Public Affairs* Master of Physician's Assistant-Chemicals:* Medroxyprogesterone acetate, also known by the brand name Depo-Provera* Morpholide of pelargonic acid...
(600 psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
) with compressed nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, and a fixed piping network leads to the protected enclosure. On triggering, the entire measured contents of one or more cylinders are discharged into the enclosure in a few seconds, through nozzle
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
s designed to ensure uniform mixing throughout the room. The quantity dumped is pre-calculated to achieve the desired concentration, typically 3–7% v/v. This level is maintained for some time, typically with a minimum of ten minutes and sometimes up to a twenty minute "soak" time, to ensure all items have cooled so reignition is unlikely to occur, then the air in the enclosure is purged, generally via a fixed purge system that is activated by the proper authorities. During this time the enclosure may be entered by persons wearing SCBA. (There exists a common myth that this is because halon is highly toxic; in fact, it is because it can cause giddiness and mildly impaired perception, and also due to the risk of combustion byproducts.)
Flooding systems may be manually operated or automatically triggered by a VESDA or other automatic detection system. In the latter case, a warning siren and strobe lamp will first be activated for a few seconds to warn personnel to evacuate the area. The rapid discharge of halon and consequent rapid cooling fills the air with fog
Fog
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...
, and is accompanied by a loud, disorienting noise.
Due to environmental concerns, alternatives are being deployed.
Halon 1301 is also used in the F-16 fighter to prevent the fuel vapors in the fuel tank
Fuel tank
A fuel tank is safe container for flammable fluids. Though any storage tank for fuel may be so called, the term is typically applied to part of an engine system in which the fuel is stored and propelled or released into an engine...
s from becoming explosive; when the aircraft enters an area with the possibility of unfriendly fire, Halon 1301 is injected into the fuel tanks for one-time use. Due to environmental concerns, trifluoroiodomethane (CF3I) is being considered as an alternative.
Chemical building blocks
Chloromethane and bromomethane are to introduce methyl groups in organic synthesisOrganic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
and the production of fine chemicals. Chlorodifluoromethane is the main intermediate en route to tetrafluoroethylene
Tetrafluoroethylene
Tetrafluoroethylene is a chemical compound with the formula C2F4. It is the simplest alkene fluorocarbon. This gaseous species is used primarily in the industrial preparation of polymers.-Properties:...
, the monomeric precursor to Teflon.
Safety
Haloalkanes are diverse in their behavior, making generalizations impossible. Few are acutely dangerous but many pose risks for prolonged exposure. Some problematic aspects include carcinogenicity (e.g., methyl iodide) and liver damage (e.g., carbon tetrachloride). Under certain combustion conditions, chloromethanes convert to phosgenePhosgene
Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. In low concentrations, its odor resembles...
, which is highly toxic.
External links
- Gas conversion table
- Nomenclature FAQ
- History of Halon use by the US Navy
- Ozone Loss: The Chemical Culprits
- http://www.eia-international.org Environmental Investigation AgencyEnvironmental Investigation AgencyThe Environmental Investigation Agency is an NGO founded in 1984 by Dave Currey, Jennifer Lonsdale and Allan Thornton, three environmental activists in the United Kingdom. Its stated goal is to investigate and expose crimes against wildlife and the environment...
- http://www.eia-global.org Environmental Investigation AgencyEnvironmental Investigation AgencyThe Environmental Investigation Agency is an NGO founded in 1984 by Dave Currey, Jennifer Lonsdale and Allan Thornton, three environmental activists in the United Kingdom. Its stated goal is to investigate and expose crimes against wildlife and the environment...
in the USA