What are halocarbons?
Halocarbons comprise a wide range of gases. They are compounds that only contain carbon and one or more halogens such as fluorine, chlorine and bromine. Halocarbons are mostly man-made chemicals and have been used in a range of applications over the past century including as solvents, fire-fighting agents, and refrigerants.
What is the most abundant halocarbon in the atmosphere?
However, CFC-12 is still the most abundant halocarbon in the atmosphere. What impact do halocarbons have on the climate? The most widely known concern related to the emissions of halocarbons is their ability to destroy stratospheric ozone.
Why are halocarbons bad for the environment?
However, another major concern about emissions of halocarbons is that many of them are powerful greenhouse gases.
Is Freon a halocarbon?
Several, but not all, of the Freon s ( q.v.) are halocarbons, as is the resin polychlorotrifluoroethylene (Kel-F). The nonflammability, low chemical reactivity, and low toxicity of many of the halocarbons are their most valuable properties ( see organohalogen compound ).
Are halocarbons found in nature?
Halocarbons are man-made synthetic halogenated compounds not found in nature. They include the following ozone-depleting substances: Chlorofluorocarbons (CFCs)
What products contain halocarbons?
They are used in the manufacture of aerosol sprays, blowing agents for foams and packing materials, as solvents, and as refrigerants. CFCs are classified as halocarbons, a class of compounds that contain atoms of carbon and halogen atoms.
What are examples of halocarbons?
Halocarbons are the basis of remarkably strong and inert plastics; Teflon® and polyvinyl chloride are two familiar examples. Another type of halocarbon, the chlorofluorocarbons (CFCs), were essentially an outgrowth of our early nuclear industry.
How do you identify halocarbons?
halocarbon, any chemical compound of the element carbon and one or more of the halogens (bromine, chlorine, fluorine, iodine); two important subclasses of halocarbons are the chlorocarbons, containing only carbon and chlorine, and the fluorocarbons, containing only carbon and fluorine.
Where do CFCs come from?
The most common source of CFCs are refrigerants, but fire suppression systems for aircraft and aerosols also emit CFCs into the atmosphere.
Where are chlorofluorocarbons found?
Chlorofluorocarbons (CFCs) are anthropogenic compounds that have been released into the atmosphere since the 1930s in various applications such as in air-conditioning, refrigeration, blowing agents in foams, insulations and packing materials, propellants in aerosol cans, and as solvents.
Is halocarbons a greenhouse gas?
Another major concern about emissions of halocarbons is that many of them are powerful greenhouse gases. However, another major concern about emissions of halocarbons is that many of them are powerful greenhouse gases.
Are HFCs halocarbons?
Emissions Scenarios. Emissions of halocarbons (CFCs, HCFCs, halons, PFCs, and HFCs) and other halogenated compounds (SF6) on a substance-by- substance basis are described in detail in Fenhann (2000).
Are CFCs man made?
Man-made compounds such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HCFCs) and halons destroy ozone in the upper atmosphere (stratosphere). The stratospheric ozone layer makes life possible by shielding the earth from harmful ultraviolet (UV-B) rays generated from the sun.
How are halocarbons released?
When the CFCs move upward into the stratosphere, the atmospheric layer just above the troposphere, they are broken down by the ultraviolet radiation from the sun to release free chlorine atoms. The chlorine then reacts with ozone to deplete the ozone layer.
How are halocarbons prepared?
How may halocarbons be prepared? A halogen can replace a hydrogen atom on an alkane to produce a halocarbon. The substituents of organic molecules often contain oxygen, nitrogen, sulfur, and/or phosphorus. They are called functional groups because they are the chemically functional parts of the molecules.
How do you reduce halocarbons?
Figure 1: Pathways to Reduce Halocarbon Emissions. Policies can focus on two pathways to reduce halocarbon emissions: (1) managing and reducing leaks in existing equipment, and (2) reducing halocarbon use. Regulations can mandate or incentivize mitigating environmental damage.
What are halocarbons?
They are compounds that only contain carbon and one or more halogens such as fluorine, chlorine and bromine. Halocarbons are mostly man-made chemicals and have been used in a range of applications over the past century including as solvents, fire-fighting agents, and refrigerants. Prior to the 1930s, refrigerators and air conditioning systems were ...
What is the most abundant halocarbon in the atmosphere?
However, CFC-12 is still the most abundant halocarbon in the atmosphere.
Why are halocarbons important?
Halocarbons: What Are They and Why Are They Important? CFCs and other halocarbons have long been known for causing an ozone hole over the Antarctic, but many of them are also powerful greenhouse gases. CFC-12 was used as a refrigerant in domestic refrigerators. Despite many advantages including being cheap, non-toxic, non-flammable, ...
What are the major concerns about halocarbons?
Another major concern about emissions of halocarbons is that many of them are powerful greenhouse gases. However, another major concern about emissions of halocarbons is that many of them are powerful greenhouse gases. Several of them stay in the atmosphere for decades and are thousands of times more efficient (per kg emitted) ...
CHLOROFLUOROCARBONS AND OTHER HALOCARBONS
Halocarbon measurements have been made from a variety of platforms including ground-based sites, ships, aircraft, and balloons. There are several well-established global networks making regular, long-term measurements of certain species using in situ, canister, and remote sensing techniques.
Atmospheric Gases
Halocarbons are greenhouse gases that can contribute to climate change, and they are also largely responsible for stratospheric ozone loss over recent decades. Because of their dual impact, they are of particular concern to environmental health.
Stratospheric Ozone
Despite falling stratospheric halocarbon levels, there remains considerable geographic and seasonal variation in stratospheric ozone levels, reflected in marked annual variation in both Antarctic and Arctic ozone ‘holes.’ Although ozone depletion is most marked at the poles (with Antarctic depletion greater than Arctic) due to the enhancement of free radical production under extremely cold temperatures, polar ozone depletion contributes to depletion of ozone at midlatitudes through circulation of ozone-poor air.
Volume 2
Robyn M. Lucas, ... Rachel E. Neale, in Encyclopedia of Environmental Health (Second Edition), 2019
Geochemistry of Estuaries and Coasts
Several marine-derived halocarbons with short atmospheric lifetimes, most notably iodomethane (methyl iodide: CH3 I), bromomethane (methyl bromide: CH 3 Br), dibromomethane (CH 2 Br 2 ), tribromomethane (bromoform: CHBr 3 ), and iodoethane (ethyl iodide: C 2 H 5 I), influence the oxidation capacity of the troposphere via the production of BrO and IO radicals ( Barrie et al., 1988; Carpenter and Liss, 2000; Allan et al., 2001; Bottenheim et al., 2002) and can contribute to stratospheric ozone depletion ( Montzka et al., 2003; Yang et al., 2005; Salawitch, 2006 ).
Future climates of the world: a modelling perspective
Gases such as H 2 O, CO 2, O 3, CH 4, N 2O and the halocarbons have atmospheric abundances affected by the presence of organisms, and are present at concentrations sufficient to influence the planetary heat balance through their absorption of outgoing infrared radiation.
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What are the two subclasses of halocarbons?
Halocarbon, any chemical compound of the element carbon and one or more of the halogens (bromine, chlorine, fluorine, iodine); two important subclasses of halocarbons are the chlorocarbons, containing only carbon and chlorine, and the fluorocarbons, containing only carbon and fluorine.
What is an organohalogen compound?
organohalogen compound, any of a class of organic compounds that contain at least one halogen (fluorine [F], chlorine [Cl], bromine [Br], or iodine [I]) bonded to carbon. They are subdivided into alkyl, vinylic, aryl, and acyl halides. In alkyl halides all four bonds to the carbon that bears the halogen…
Is Freon a halocarbon?
Several, but not all, of the Freon s ( q.v.) are halocarbons, as is the resin polychlorotrifluoroethylene (Kel-F). The nonflammability, low chemical reactivity, and low toxicity of many of the halocarbons are their most valuable properties ( see organohalogen compound ).
What are the CFCs?
Chlorofluorocarbons (CFCs) are nontoxic, nonflammable chemicals containing atoms of carbon, chlorine, and fluorine. They are used in aerosol sprays, blowing agents for foams and packing materials, as solvents, and as refrigerants. CFCs are classified as halocarbons, a class of compounds that contain atoms of carbon and halogen atoms.
Why were CFCs used?
Refrigerators in the late 1800s and early 1900s used the toxic gases, ammonia (NH 3 ), methyl chloride (CH 3 Cl), or sulfur dioxide (SO 2 ), as refrigerants.
CFCs can destroy stratospheric ozone
Whereas CFCs are safe to use in most applications and are inert in the lower atmosphere, they decompose in the upper atmosphere or stratosphere. In 1974, two University of California chemists, Professor F. Sherwood Rowland and Dr.
The Montreal Protocol
On September 16, 1987, 27 nations signed a global environmental treaty, the Montreal Protocol to Reduce Substances that Deplete the Ozone Layer that had a provision to reduce production levels of these compounds by 50% relative to 1986 before the year 2000.
The Substitutes: HCFCs and HFCs
Besides recycling, the demand for CFCs has been further reduced by the use of substitutes. Some applications, for example degreasing of metals and cleaning solvents for circuit boards, which once used CFCs now use halocarbon-free fluids, water (sometimes as steam), and diluted citric acids.
The Future: Recovery of the Ozone Hole and Nitrous Oxide (N 2 O)
Scientists hope that stratosphere ozone will continue on its track to recovery, but natural and manmade changes can influence the year-to-year variability of stratosphere ozone concentrations. Atmospheric N 2 O is the third strongest greenhouse gas.
Overview
History and context
A few halocarbons are produced in massive amounts by microorganisms. For example, several million tons of methyl bromide are estimated to be produced by marine organisms annually. Most of the halocarbons encountered in everyday life – solvents, medicines, plastics – are man-made. The first synthesis of halocarbons was achieved in the early 1800s. Production began accelerating when their useful properties as solvents and anesthetics were discovered. Develop…
Chemical families
Halocarbons are typically classified in the same ways as the similarly structured organic compounds that have hydrogen atoms occupying the molecular sites of the halogen atoms in halocarbons. Among the chemical families are:
• haloalkanes—compounds with carbon atoms linked by single bonds
• haloalkenes—compounds with one or more double bonds between carbon atoms
Organoiodine compounds, including biological derivatives
Organoiodine compounds, called organic iodides, are similar in structure to organochlorine and organobromine compounds, but the C-I bond is weaker. Many organic iodides are known, but few are of major industrial importance. Iodide compounds are mainly produced as nutritional supplements.
The thyroxin hormones are essential for human health, hence the usefulness of iodized salt.
Uses
The first halocarbon commercially used was Tyrian purple, a natural organobromide of the Murex brandaris marine snail.
Common uses for halocarbons have been as solvents, pesticides, refrigerants, fire-resistant oils, ingredients of elastomers, adhesives and sealants, electrically insulating coatings, plasticizers, and plastics. Many halocarbons have specialized uses in industry. One halocarbon, sucralose, is a s…
Hazards
The stability of halocarbons tended to encourage beliefs that they were mostly harmless, although in the mid-1920s physicians reported workers in polychlorinated naphthalene (PCN) manufacturing suffering from chloracne (Teleky 1927), and by the late 1930s it was known that workers exposed to PCNs could die from liver disease (Flinn & Jarvik 1936) and that DDT would kill
See also
• Halogenation
• Carbon–fluorine bond
• Fluorinated gases
• List of refrigerants
External links
• Media related to Organohalides at Wikimedia Commons