Halomethane

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A methane molecule in 3D space filling model. Methane-3D-space-filling.svg
A methane molecule in 3D space filling model.

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 human-made, most notably as refrigerants, solvents, propellants, and fumigants. Many, including the chlorofluorocarbons, 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.

Contents

Structure and properties

Like methane itself, halomethanes are tetrahedral molecules. The halogen atoms differ greatly in size and charge from hydrogen and from each other. Consequently, most halomethanes deviate from the perfect tetrahedral symmetry of methane. [1]

The physical properties of halomethanes depend on the number and identity of the halogen atoms in the compound. In general, halomethanes are volatile but less so than methane because of the 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.

Production

Industrial routes

The halomethanes are produced on an industrial scale from abundant precursors such as natural gas or methanol, and from halogens or halides. They are usually prepared by one of three methods. [2]

CH4 + Cl2 → CH3Cl + HCl

This method is useful for the production of CH4−nCln (n = 1, 2, 3, or 4). The main problems with this method are that it cogenerates HCl and it produces mixtures of different products. Using CH4 in large excess generates primarily CH3Cl and using Cl2 in large excess generates primarily CCl4, but mixtures of other products will still be present.

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
CH3Cl + HF → CH3F + HCl
CH2Cl2 + HF → CH2FCl + HCl
CH2Cl2 + 2 HF → CH2F2 + 2 HCl
CH2Cl2 + F2 → CH2F2 + Cl2
CHCl3 + HF → CHFCl2 + HCl
CHCl3 + 2 HF → CHF2Cl + 2 HCl
CHCl3 + F2 → CHF2Cl + Cl2
CHCl3 + 3 HF → CHF3 + 3 HCl
CHCl3 + F2 + HF → CHF3 + Cl2 + HCl
CCl4 + HF → CFCl3 + HCl
CCl4 + 2 HF → CF2Cl2 + 2 HCl
CCl4 + F2 → CF2Cl2 + Cl2
CCl4 + 3 HF → CF3Cl + 3 HCl
CCl4 + F2 + HF → CF3Cl + Cl2 + HCl
CCl4 + 4 HF → CF4 + 4 HCl
CCl4 + F2 + 2 HF → CF4 + Cl2 + 2 HCl
CCl4 + 2 F2 → CF4 + 2 Cl2
CH4 + HOCl → CH3Cl + H2O
CH3OH + HOCl → CH3Cl + H2O2

Traces of halomethanes in the atmosphere arise through the introduction of other non-natural, industrial materials.

In nature

Many marine organisms biosynthesize halomethanes, especially bromine-containing compounds. [3] 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:

2 CH4 + 2 Cl− + O2 → 2 CH3Cl + 2 OH−

Classes of compounds

Halons are usually defined as hydrocarbons where the hydrogen atoms have been replaced by bromine, along with other halogens. [4] 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. This halon has number 1211 in its name, which tells it has 1 carbon atom, 2 fluorine atoms, 1 chlorine atom, and 1 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 CF2ClBr, hence its IUPAC name is bromochlorodifluoromethane.

ANSI/ASHRAE Standard 34-1992

The refrigerant naming system is mainly used for fluorinated and chlorinated short alkanes used as refrigerants. In the United States, the standard is specified in ANSI/ASHRAE Standard 34–1992, with additional annual supplements. [5] The specified ANSI/ASHRAE prefixes were FC (fluorocarbon) or R (refrigerant), but today most are prefixed by a more specific classification:

The decoding system for CFC-01234a is:

Other coding systems are in use as well.

Hydrofluoro compounds (HFC)

Hydrofluorocarbons (HFCs) contain no chlorine. They are composed entirely of carbon, hydrogen, and fluorine. They have no known effects on the ozone layer; fluorine itself is not ozone-toxic. [6] [7] However, HFCs and perfluorocarbons (PFCs) are greenhouse gases, which cause global warming. Two groups of haloalkanes, hydrofluorocarbons (HFCs) and perfluorocarbons, are targets of the Kyoto Protocol. [8] Allan Thornton, President of the Environmental Investigation Agency, a non-governmental, environmental watchdog, says that HFCs are up to 12,500 times as potent as carbon dioxide in global warming. [9] The higher global warming potential has two causes: HFCs remain in the atmosphere for long periods of time, and they have more chemical bonds than CO2, which means that they are able to absorb more solar energy per molecule than carbon dioxide. 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. [10]

Overview of principal halomethanes

Most permutations of hydrogen, fluorine, chlorine, bromine, and iodine on one carbon atom have been evaluated experimentally.

Overview of Halomethanes
Systematic nameCommon/trivial
name(s)		CodeUseChemical formula
Tetrachloromethane Carbon tetrachloride, Freon 10

(Freon is a trade name for a group of chlorofluorocarbons used primarily as a refrigerant. The main chemical used under this trademark is dichlorodifluoromethane. The word Freon is a registered trademark belonging to DuPont.)

CFC-10Formerly in fire extinguishersCCl4
Tetrabromomethane Carbon tetrabromideCBr4
Tetraiodomethane Carbon tetraiodide CI4
Tetrafluoromethane Carbon tetrafluoride, Freon 14PFC-14
(CFC-14 and HF-14 also used, although formally incorrect)		CF4
Chloromethane Methyl chlorideMethylation agent; e.g., for methyl trichlorosilaneCH3Cl
Dichloromethane Methylene chlorideSolventCH2Cl2
Trichloromethane Chloroform SolventCHCl3
Trichlorofluoromethane Freon-11, R-11CFC-11CCl3F
Dichlorodifluoromethane Freon-12, R-12CFC-12CCl2F2
Chlorotrifluoromethane CFC-13CClF3
Chlorodifluoromethane R-22HCFC-22CHClF2
Trifluoromethane Fluoroform HFC-23In semiconductor industry, refrigerantCHF3
Chlorofluoromethane Freon 31Refrigerant (phased out)CH2ClF
Difluoromethane HFC-32Refrigerant with zero ozone depletion potential CH2F2
Fluoromethane Methyl fluorideHFC-41Semiconductor manufactureCH3F
Bromomethane Methyl bromideSoil sterilant and fumigant, currently being phased out. It strongly depletes the ozone layer and is highly toxic.CH3Br
Dibromomethane Methylene bromideSolvent and chemical intermediate.CH2Br2
Tribromomethane Bromoform For separation of heavy mineralsCHBr3
Bromochloromethane Halon 1011Formerly in fire extinguishersCH2BrCl
Bromochlorodifluoromethane BCF, Halon 1211 BCF, or Freon 12B1Halon 1211CBrClF2
Bromotrifluoromethane BTM, Halon 1301 BTM, or Freon 13BIHalon 1301CBrF3
Trifluoroiodomethane Trifluoromethyl iodideFreon 13T1Organic synthesisCF3I
Iodomethane Methyl iodideOrganic synthesisCH3I

Applications

Because they have many applications and are 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. [2] It is widely used as a paint stripper and a degreaser. In the food industry, it was previously used to decaffeinate coffee and tea as well as to prepare extracts of hops and other flavorings. [11] Its volatility has led to its use as an aerosol spray propellant and as a blowing agent for polyurethane foams.

Propellants

One major use of CFCs has been as propellants of aerosols, including metered-dose inhalers for drugs used to treat asthma. The conversion of these devices and treatments from CFC to propellants that do not deplete the ozone layer is almost complete. Production and import is now banned in the United States.

Fire extinguishing

Halon-based fire extinguishing system inside Diefenbunker, a nuclear fallout bunker in Canada. Halon-based fire extinguishing system.JPG
Halon-based fire extinguishing system inside Diefenbunker, a nuclear fallout bunker in Canada.

At high temperatures, halons decompose to release halogen atoms that combine readily with active hydrogen atoms, quenching flame propagation reactions even when adequate fuel, oxygen, and heat remain. 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 halt the fire at much lower concentrations than are required by fire suppressants using the more traditional methods of cooling, oxygen deprivation, or fuel dilution. As of 2023, due to ozone depletion problems, halon fire extinguishers are largely banned in some countries and alternatives are being deployed by the US military. [12]

Halon 1301 total flooding systems are typically used at concentrations no higher than 7% by volume in air, and can suppress many fires at 2.9% v/v. By contrast, carbon dioxide fire suppression flood systems operate 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% those exposed 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.[ citation needed ]) 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 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 1301 is common 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 nozzles 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 due to the risk of combustion byproducts.)

Halon-based hand-held fire extinguishers Halon extinguishers.jpg
Halon-based hand-held fire extinguishers

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.

Halon 1301 is also used in the F-16 fighter to prevent the fuel vapors in the fuel tanks from becoming explosive; when the aircraft enters an area with the possibility of attack, Halon 1301 is injected into the fuel tanks for one-time use. Due to ozone depletion, trifluoroiodomethane (CF3I) is being considered as an alternative. [13]

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, fire is extinguished 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 diffuses, leaving no residue.

Chemical building blocks

Chloromethane and bromomethane are used to introduce methyl groups in organic synthesis. Chlorodifluoromethane is the main precursor of tetrafluoroethylene, which is the monomeric precursor to Teflon. [1]

Safety

Haloalkanes are diverse in their properties, making generalizations difficult. Few are acutely toxic, but many pose risks from prolonged exposure. Some problematic aspects include carcinogenicity and liver damage (e.g., carbon tetrachloride). Under certain combustion conditions, chloromethanes convert to phosgene, which is highly toxic.

See also

Related Research Articles

<span class="mw-page-title-main">Hydrocarbon</span> Organic compound consisting entirely of hydrogen and carbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may be similar to that of gasoline or lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases, liquids, low melting solids or polymers.

<span class="mw-page-title-main">Halogen</span> Group of chemical elements

The halogens are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would exclude tennessine as its chemistry is unknown and is theoretically expected to be more like that of gallium. In the modern IUPAC nomenclature, this group is known as group 17.

<span class="mw-page-title-main">Chlorofluorocarbon</span> Class of organic compounds

Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and propane.

<span class="mw-page-title-main">Haloalkane</span> Group of chemical compounds derived from alkanes containing one or more halogens

The haloalkanes are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes that contain chlorine, bromine, and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases. Methyl iodide, a naturally occurring substance, however, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, see Halomethane. Haloalkane or alkyl halides are the compounds which have the general formula "RX" where R is an alkyl or substituted alkyl group and X is a halogen.

Chloroform, or trichloromethane, is an organochloride with the formula CHCl3 and a common solvent. It is a volatile, colorless, sweet-smelling, dense liquid produced on a large scale as a precursor to refrigerants and PTFE. Chloroform was once used as an inhalational anesthetic between the 19th century and the first half of the 20th century. It is miscible with many solvents but it is only very slightly soluble in water.

<span class="mw-page-title-main">Carbon tetrachloride</span> Carbon compound

Carbon tetrachloride, also known by many other names (such as carbon tet for short and tetrachloromethane, also recognised by the IUPAC), is a chemical compound with the chemical formula CCl4. It is a non-flammable, dense, colourless liquid with a "sweet" chloroform-like odour that can be detected at low levels. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, an anthelmintic and a cleaning agent, but has since been phased out because of environmental and safety concerns. Exposure to high concentrations of carbon tetrachloride can affect the central nervous system and degenerate the liver and kidneys. Prolonged exposure can be fatal.

Bromotrifluoromethane, commonly referred to by the code numbers Halon 1301, R13B1, Halon 13B1 or BTM, is an organic halide with the chemical formula CBrF3. It is used for gaseous fire suppression as a far less toxic alternative to bromochloromethane.

Chloromethane, also called methyl chloride, Refrigerant-40, R-40 or HCC 40, is an organic compound with the chemical formula CH3Cl. One of the haloalkanes, it is a colorless, sweet-smelling, flammable gas. Methyl chloride is a crucial reagent in industrial chemistry, although it is rarely present in consumer products, and was formerly utilized as a refrigerant. Most chloromethane is biogenic.

In chemistry, halogenation is a chemical reaction which introduces one or more halogens into a chemical compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, drugs. This kind of conversion is in fact so common that a comprehensive overview is challenging. This article mainly deals with halogenation using elemental halogens. Halides are also commonly introduced using salts of the halides and halogen acids. Many specialized reagents exist for and introducing halogens into diverse substrates, e.g. thionyl chloride.

In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chloroform (CHCl3), dichloromethane (CH2Cl2), and hexachlorobutadiene. It proceeds by a free-radical chain mechanism.

<span class="mw-page-title-main">Fluoromethane</span> Chemical compound

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 (CH4) with a fluorine atom substituted for one of the hydrogen atoms. It is used in semiconductor manufacturing processes as an etching gas in plasma etch reactors.

In chemistry, trihalomethanes (THMs) are chemical compounds in which three of the four hydrogen atoms of methane are replaced by halogen atoms. Trihalomethanes with all the same halogen atoms are called haloforms. Many trihalomethanes find uses in industry as solvents or refrigerants. Some THMs are also environmental pollutants, and a few are considered carcinogenic.

<span class="mw-page-title-main">Oxygen fluoride</span> Any binary compound of oxygen and fluorine

Oxygen fluorides are compounds of elements oxygen and fluorine with the general formula OnF2, where n = 1 to 6. Many different oxygen fluorides are known:

Fluoroform, or trifluoromethane, is the chemical compound with the formula CHF3. It is a hydrofluorocarbon as well as being a part of the haloforms, a class of compounds with the formula CHX3 with C3v symmetry. Fluoroform is used in diverse applications in organic synthesis. It is not an ozone depleter but is a greenhouse gas.

<span class="mw-page-title-main">1,1,1,2,3,3,3-Heptafluoropropane</span> Chemical compound

1,1,1,2,3,3,3-Heptafluoropropane, also called heptafluoropropane, HFC-227ea, HFC-227 or FM-200, as well as apaflurane (INN), is a colourless, odourless gaseous halocarbon commonly used as a gaseous fire suppression agent.

Trifluoroiodomethane, also referred to as trifluoromethyl iodide is a halomethane with the formula CF3I. It is an experimental alternative to Halon 1301 (CBrF3) in unoccupied areas. It would be used as a gaseous fire suppression flooding agent for in-flight aircraft and electronic equipment fires.

In chemistry, a reaction intermediate, or intermediate, is a molecular entity arising within the sequence of a stepwise chemical reaction. It is formed as the reaction product of an elementary step, from the reactants and/or preceding intermediates, but is consumed in a later step. It does not appear in the chemical equation for the overall reaction.

<span class="mw-page-title-main">Bromochloromethane</span> Chemical compound

Bromochloromethane or methylene bromochloride and Halon 1011 is a mixed halomethane. It is a heavy low-viscosity liquid with refractive index 1.4808.

<span class="mw-page-title-main">Dichlorofluoromethane</span> Chemical compound

Dichlorofluoromethane or Freon 21 or R 21 is a halomethane or hydrochlorofluorocarbon with the formula CHCl2F. It is a colorless and odorless gas. It is produced by fluorination of chloroform using a catalyst such as antimony trifluoride:

Tetrahalomethanes are fully halogenated methane derivatives of general formula CFkCllBrmInAtp, where:Tetrahalomethanes are on the border of inorganic and organic chemistry, thus they can be assigned both inorganic and organic names by IUPAC: tetrafluoromethane - carbon tetrafluoride, tetraiodomethane - carbon tetraiodide, dichlorodifluoromethane - carbon dichloride difluoride.

References

  1. 1 2 Günter Siegemund, Werner Schwertfeger, Andrew Feiring, Bruce Smart, Fred Behr, Herward Vogel, Blaine McKusick “Fluorine Compounds, Organic” Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. doi : 10.1002/14356007.a11_349
  2. 1 2 Manfred Rossberg, Wilhelm Lendle, Gerhard Pfleiderer, Adolf Tögel, Eberhard-Ludwig Dreher, Ernst Langer, Heinz Rassaerts, Peter Kleinschmidt, Heinz Strack, Richard Cook, Uwe Beck, Karl-August Lipper, Theodore R. Torkelson, Eckhard Löser, Klaus K. Beutel, Trevor Mann “Chlorinated Hydrocarbons” in Ullmann's Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. doi : 10.1002/14356007.a06_233.pub2.
  3. ↑ Gordon W. Gribble (1998). "Naturally Occurring Organohalogen Compounds". Acc. Chem. Res. 31 (3): 141–152. doi:10.1021/ar9701777.
  4. ↑ John Daintith (2008). Oxford Dictionary of Chemistry. Oxford University Press. ISBN   978-0-19-920463-2.
  5. ↑ "ASHRAE Bookstore". Archived from the original on 2006-06-15. Retrieved 2009-10-07.
  6. ↑ "Ozone Layer Protection". US EPA. October 14, 2020.
  7. ↑ Tressaud, Alain (2006). "Fluorine in the Atmosphere" (PDF). Advances in Fluorine Science. 1: 1–32. doi:10.1016/S1872-0358(06)01001-3.
  8. ↑ Lerner & K. Lee Lerner, Brenda Wilmoth (2006). "Environmental issues : essential primary sources". Thomson Gale. Retrieved 2006-09-11.
  9. ↑ US EPA, OAR (February 15, 2013). "Ozone Layer Protection". US EPA.
  10. ↑ All Things Considered, NPR News, 5:24 p.m., December 11, 2007.
  11. ↑ Office of Environmental Health Hazard Assessment (September 2000). "Dichloromethane" (PDF). Public Health Goals for Chemicals in Drinking Water. California Environmental Protection Agency. Archived from the original (PDF) on 2009-10-09.
  12. ↑ 3-III-2 HALON 1301 REPLACEMENTS Archived 2008-04-19 at the Wayback Machine
  13. ↑ "Credit Card Freedom | Red Smoothie Detox & Dessert Photographs". www.afrlhorizons.com. Archived from the original on July 11, 2007.
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