Encyclopedia
The
haloalkanes are a group of chemical compounds, consisting of
alkanes, such as
methane or
ethane, with one or more
halogens linked, such as chlorine or
fluorine, making them a type of organic halide. They are known under many chemical and trivial names. As
fire extinguishants, propellants and solvents they have or had wide use. Some haloalkanes have negative effects on the
environment such as
ozone depletion. The most widely known family within this group are the chlorofluorocarbons .
General
A
haloalkane, also known as
alkyl halogenide,
halogenalkane or
halogenoalkane, and
alkyl halide is a chemical compound derived from an
alkane by substituting one or more
hydrogen atoms with
halogen atoms. Substitution with
fluorine, chlorine, bromine and iodine results in
fluoroalkanes,
chloroalkanes,
bromoalkanes and
iodoalkanes, respectively. Mixed compounds are also possible, examples are the chlorofluorocarbons which are mainly responsible for
ozone depletion. Haloalkanes are used in semiconductor device fabrication, as refrigerants,
foam blowing agents, solvents,
aerosol spray propellants,
fire extinguishing agents, and chemical reagents.
Freon is a trade name for a group of chlorofluorocarbons used primarily as a
refrigerant. The word
Freon is a registered
trademark belonging to
DuPont.
Chlorofluoro compounds
Chlorofluorocarbons are haloalkanes with both chlorine and
fluorine. They were formerly used widely in industry, for example as refrigerants, propellants, and cleaning solvents. Their use has been generally prohibited by the
Montreal Protocol, because of effects on the
ozone layer .
Hydrochlorofluorocarbons are of a class of haloalkanes where not all hydrogen has been replaced by chlorine or fluorine. They are used primarily as chlorofluorocarbon substitutes, as the environmental effects are only about 10% of the CFCs. When the chlorine is reduced to zero, these compounds are known as
hydrofluorocarbons , with even fewer environmental effects.
Polymer haloalkanes
Chlorinated or fluorinated alkenes can be used for polymerization, resulting in polymer haloalkanes with notable chemical resistance properties. Important examples include polychloroethene , and polytetrafluoroethene , but many more halogenated polymers exist.
History
Original development
Carbon tetrachloride was used in fire extinguishers and glass "anti-fire grenades" from the late
nineteenth century until around the end of
World War II. Experimentation with chloroalkanes for fire suppression on military
aircraft began at least as early as the
1920s.
American engineer Thomas Midgley developed Chlorofluorocarbons in 1928 as a replacement for
ammonia ,
chloromethane , and
sulfur dioxide that are toxic but in common use at the time as refrigerants. The new compound developed had to have a low boiling point, be non-toxic and be generally non-reactive. In a demonstration for the American Chemical Society, Midgley flamboyantly demonstrated all these properties by inhaling a breath of the gas and using it to blow out a candle.
Midgley specifically developed CCl
2F
2. However, one of the attractive features is that there exists a whole family of the compounds, each having a unique boiling point which can suit different applications. In addition to their original application as refrigerants, chlorofluoroalkanes have been used as propellants in aerosol cans, cleaning solvents for circuit boards and as blowing agents for making expanded plastics .
Development of alternatives
During World War II various early chloroalkanes were in standard use in military aircraft by some combatants, but these early halons suffered from excessive toxicity. Nevertheless after the war they slowly became more common in civil aviation as well.
In the
1960s, fluoroalkanes and bromofluoroalkanes became available and were quickly recognized as one of the most effective fire-fighting materials discovered. Much early research with Halon 1301 was conducted under the auspices of the US Armed Forces, while Halon 1211 was, initially, mainly developed in the UK. By the late 1960s they were standard in many applications where water and dry-powder extinguishers posed a threat of damage to the protected property, including computer rooms, telecommunications switches, laboratories, museums and art collections. Beginning with
warships, in the
1970s, bromofluoroalkanes also progressively came to be associated with rapid knockdown of severe fires in confined spaces with minimal risk to personnel.
Work on alternatives for chlorofluorocarbons in refrigerants began in the late
1970s after the first warnings of damage to
stratospheric ozone were published in the journal
Nature in 1974 by
Molina and Rowland . Adding
hydrogen and thus creating hydrochlorofluorocarbons , chemists made the compound less stable in the lower atmosphere enabling it to break down before reaching the ozone layer. Later alternatives dispense with the chlorine, creating hydrofluorocarbons with even shorter lifetimes in the lower atmosphere.
By the early
1980s, bromofluoroalkanes were in common use on aircraft, ships and large vehicles as well as in computer facilities and galleries. However, concern was beginning to be felt about the impact of chloroalkanes and bromoalkanes on the
ozone layer. The Vienna Convention on Ozone Layer Protection did not cover bromofluoroalkanes as it was thought, at the time, that emergency discharge of extinguishing systems was too small in volume to produce a significant impact, and too important to human safety for restriction.
However, by the time of the
Montreal Protocol it was realised that deliberate and accidental discharges during system tests and maintenance accounted for substantially larger volumes than emergency discharges, consequently halons were brought into the treaty but with many exceptions.
Phase out
Use of certain chloroalkanes as solvents for large scale application, such as dry cleaning, have been phased out, for example, by the IPPC directive on
greenhouse gases in 1994 and by the Volatile Organic Compounds directive of the
EU in 1997. Permitted chlorofluoroalkane uses are medicinal only.
Finally, bromofluoroalkanes have been largely phased out and the possession of such equipment is prohibited in some countries like the Netherlands and Belgium, from 1 January 2004, based on the
Montreal Protocol and guidelines of the European Union.
Production of new stocks ceased in most countries as of 1994. However many countries still require aircraft to be fitted with halon fire suppression systems because no safe and completely satisfactory alternative has been discovered for this application. There are also a few other, highly specialised, uses. These programs recycle halon through "halon banks" coordinated by the Halon Recycling Corporation to ensure that discharge to the atmosphere occurs only in a genuine emergency and to conserve remaining stocks.
Nomenclature
IUPAC nomenclature
The formal naming of haloalkanes should follow
IUPAC nomenclature, which put the halogen as a prefix to the alkane. For example,
ethane with bromine becomes
bromoethane,
methane with four chlorine groups becomes
tetrachloromethane. However, many of these compounds have already an established trivial name, which is endorsed by the IUPAC nomenclature, for example
chloroform and methylene chloride . For unambiguity, this article follows the systematic naming scheme throughout.
Alternative nomenclature for refrigerants
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 or R , but today most are prefixed by a more specific classification:
Fluorocarbons are chemical compounds that contain carbon [i]-fluorine [i] bond [i]s. ...
s
- PFC—list of perfluorocarbons
The decoding system for CFC-01234a is:
- 0 = Number of double bonds
- 1 = Carbon atoms -1
- 2 = Hydrogen atoms +1
- 3 = Fluorine atoms
- 4 = Replaced by Bromine
- 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 too.
Overview of named compounds
>| Overview of haloalkanes |
|---|
| This table gives an overview of most haloalkanes in general use or commonly known. Listing includes bulk commodity products as well as laboratory chemicals. |
| Systematic name | Common/Trivial
name | Code | Chem. formula |
|---|
| Halomethanes |
| Chloromethane | Methyl chloride | | CH3Cl |
| Dichloromethane | Methylene chloride | | CH2Cl2 |
| Trichloromethane | Chloroform | | CHCl3 |
| Tetrachloromethane | Carbon tetrachloride, Freon 10 | CFC-10 | CCl4 |
| Tetrafluoromethane | Carbon tetrafluoride, Freon 14 | CFC-14 | CF4 |
| Trichlorofluoromethane | Freon-11, R-11 | CFC-11 | CCl3F |
| Dichlorodifluoromethane | Freon-12, R-12 | CFC-12 | CCl2F2 |
| Chlorotrifluoromethane | | CFC-13 | CClF3 |
| Chlorodifluoromethane | R-22 | HCFC-22 | CHClF2 |
| Trifluoromethane | Fluoroform | HFC-23 | CHF3 |
| Difluoromethane | | HFC-32 | CH2F2 |
| Fluoromethane | Methyl fluoride | HFC-41 | CH3F |
| Dibromomethane | Methylene bromide | | CH2Br2 |
| Tribromomethane | Bromoform | | CHBr3 |
| Bromochlorodifluoromethane | BCF, Halon 1211 BCF, or Freon 12B1 | Halon 1211 | CBrClF2 |
| Bromotrifluoromethane | BTM, Halon 1301 BTM, or Freon 13BI | Halon 1301 | CBrF3 |
| Trifluoroiodomethane | Trifluoromethyl iodide | Freon 13T1 | CF3I |
| Haloethanes |
| 1,1,1-Trichloroethane | Methyl chloroform, tri | | Cl3C-CH3 |
| Hexachloroethane | | CFC-110 | C2Cl6 |
| 1,1,2-Trichloro-1,2,2-trifluoroethane | Trichlorotrifluoroethane | CFC-113 | Cl2FC-CClF2 |
| 1,1,1-trichloro-2,2,2-trifluoroethane | | CFC-113a | Cl3C-CF3 |
| 1,2-Dichloro-1,1,2,2-tetrafluoroethane | Dichlorotetrafluoroethane | CFC-114 | ClF2C-CClF2 |
| 1-Chloro-1,1,2,2,2-pentafluoroethane | Chloropentafluoroethane | CFC-115 | ClF2C-CF3 |
| 2-Chloro-1,1,1,2-tetrafluoroethane | | HFC-124 | CHF2CF3 |
| 1,1,2,2,2-pentafluoroethane | Pentafluoroethane | HFC-125 | CHF2CF3 |
| 1,1,2,2-Tetrafluoroethane | | HFC-134 | F2HC-CHF2 |
| 1,1,1,2-Tetrafluoroethane | R-134a | HFC-134a, Suva-134a | F3C-CH2F |
| 1,1-Dichloro-1-fluoroethane | | HCFC-141b | Cl2FC-CH3 |
| 1-Chloro-1,1-difluoroethane | | HCFC-142b | ClF2C-CH3 |
| 1,2-Dichloroethane | Ethylene dichloride | Freon 150 | ClH2C-CH2Cl |
| 1,1-Dichloroethane | Ethylidene dichloride | Freon 150a | Cl2HC-CH3 |
| 1,1-Difluoroethane | | HFC-152a | F2HC-CH3 |
| Longer haloalkanes, polymers |
| 1,1,1,2,3,3,3-Heptafluoropropane | | HFC-227ea, FE-227, FM-200 | F3C-CHF-CF3 |
| Decafluorobutane | perfluorobutane | R610, PFB, CEA-410 | F3C-CF2-CF2-CF3 |
| Polychloroethene | polyvinyl chloride, PVC | | -[CHCl-CH2]x- |
| Polytetrafluoroethene | Polytetrafluoroethylene,
PTFE, Teflon | | -[CF2-CF2]x- |
| | |
Synthesis
Alkyl halides can be synthesized from alkanes, alkenes, or alcohols.
From alkanes
Alkanes react with halogens by free radical
halogenation. In this reaction a hydrogen atom is removed from the alkane, then replaced by a halogen atom by reaction with a diatomic halogen molecule. Thus:
- Step 1: X2 ? 2 X·
- Step 2: X· + R-H ? R· + HX
- Step 3: R· + X2 ? R-X + X·
Steps 2 and 3 keep repeating, each providing the reactive intermediate needed for the other step. This is called a
radical chain reaction. This reaction continues until the radicals are used up by one of three termination steps.
- Step 4: R· + X· ? R-X
- Step 5: 2 X· ? X2
- Step 6: 2 R· ? R-R
Note that Step 4 results in the same product as Step 3, the desired haloalkane, but through the destruction of two radicals. Step 5 is just the reverse of Step 1 and Step 6 accounts for the small contamination of this reaction by larger alkanes and their subsequent haloalkanes.
From alkenes
An
alkene reacts with a hydrogen halides like
hydrogen chloride or
hydrogen bromide to form a haloalkane. The double bond of the alkene is replaced by two new bonds, one to the halogen and one to the hydrogen atom of the hydrohalic acid.
Markovnikov's rule states that in this reaction, the halogen becomes attached to the more substituted carbon more likely. Example:
- H3C-CH=CH2 + HBr ? H3C-CHBr-CH3 + H3C-CH2-CH2Br .
Alkenes also react with halogens to form haloalkanes with two neighboring halogen atoms. This is sometimes known as "decolorizing" the halogen since the reagent X
2 is colored and the product is usually colorless. Example:
- 2H3C-CH=CH2 + Br2 ? 2H3C-CHBr-CH2Br
From alcohols
Tertiary alkanol reacts with
hydrochloric acid directly to produce tertiary chloroalkane, but if primary or secondary alkanol is used, an activator such as
zinc chloride is needed. Alternatively the conversion may be performed directly using
thionyl chloride. Alkanol may likewise be converted to bromoalkane using
hydrobromic acid or
phosphorus tribromide or iodoalkane using red
phosphorus and iodine . Two examples:
- 3C-OH + HCl.H2O ? 3C-Cl + 2 H2O
- CH3-6-OH + SOCl2 ? CH3-6-Cl + SO2 + HCl
Reactions of haloalkanes
Haloalkanes are reactive towards
nucleophiles. They are
polar molecules: the carbon to which the halogen is attached is slightly electropositive where the halogen is slightly electronegative. This results in an electron deficient carbon which, inevitably, attracts
nucleophiles.
Substitution reactions
Substitution reactions involve the replacement of the halogen with another molecule - thus leaving
saturated hydrocarbons, as well as the halogen product.
Hydrolysis--a reaction in which
water breaks a bond--is a good example of the nucleophilic nature of halogenoalkanes. The polar bond attracts a hydroxide ion, OH
-. . This OH
- is a nucleophile with a clearly negative charge, as it has excess electrons it donates them to the carbon, which results in a
covalent bond between the two. Thus C-X is broken by heterolytic fission resulting in a halide ion, X
-. As can be seen, the OH is now attached to the alkyl group, creating an
alcohol. .
One should note that within the halogen series, the C-X bond weakens as one goes to heavier halogens, and this affects the rate of reaction. Thus, the C-I of an iodoalkane generally reacts faster than the C-F of a fluoroalkane.
Apart from hydrolysis, there are a few other isolated examples of nucleophilic substitution:
Elimination reactions
Rather than creating a molecule with the halogen substituted with something else, one can completely
eliminate both the halogen and a nearby hydrogen, thus forming an
alkene. For example, with bromoethane and NaOH in ethanol, the hydroxide ion OH
- attracts a hydrogen atom - thus removing a hydrogen and bromine from bromoethane. This results in C
2H
4 , H
2O and Br
-.
Applications
Propellant
One major use of CFCs has been as propellants in
aerosol 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. There are some differences between asthma inhalers using CFCs and the newer propellants, but the conversion has not proven difficult. They have now been outlawed in 50 states universally.
Fire extinguishing
At high temperatures, halons decompose to release halogen atoms that combine readily with active hydrogen atoms, quenching the 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
concentrations 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 up to 75% . Carbon dioxide can cause severe distress at concentrations of 3 to 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. 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 , B and C fires, but they are totally unsuitable for Class D 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 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 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.
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 fighters to prevent the fuel vapors in the
fuel tanks from becoming explosive; when the aircraft enters area with the possibility of unfriendly fire, Halon 1301 is injected into the fuel tanks for one-time use. Due to environmental concerns,
trifluoromethyl iodide is being considered as an alternative.
Environmental issues
Since the late 1970s the use of CFCs has been heavily regulated because of its destructive effects on the ozone layer. This damage was discovered by Sherry Rowland and
Mario Molina, who first published a paper suggesting the connection in 1974. It turns out that one of CFCs' most attractive features—their unreactivity—has been instrumental in making them one of the most significant pollutants. CFCs' lack of reactivity gives them a lifespan which can exceed 100 years in some cases. This gives them time to diffuse into the upper
stratosphere. Here, the sun's
ultraviolet radiation is strong enough to break off the chlorine atom, which on its own is a highly reactive
free radical. This
catalyses the break up of ozone into oxygen by means of a variety of mechanisms, of which the simplest is:
- Cl· + O3 ? ClO· + O2
- ClO· + O3 ? Cl· + 2 O2
Since the chlorine is regenerated at the end of these reactions, a single Cl atom can destroy many thousands of ozone molecules. Reaction schemes similar to this one are believed to be the cause of the
ozone hole observed over the poles and upper latitudes of the Earth. Decreases in stratospheric ozone may lead to increases in
skin cancer.
In 1975, the
US state of
Oregon enacted the world's first ban of CFCs . The United States and several European countries banned the use of CFC's in aerosol spray cans in 1978, but continued to use them in refrigeration, foam blowing, and as solvents for cleaning electronic equipment. By 1985, scientists observed a dramatic seasonal depletion of the ozone layer over
Antarctica. International attention to CFCs resulted in a meeting of world diplomats in
Montreal in 1987. They forged a treaty, the
Montreal Protocol, which called for drastic reductions in the production of CFCs. On March 2, 1989, 12
European Community nations agreed to ban the production of all CFCs by the end of the century. In 1990, diplomats met in
London and voted to significantly strengthen the Montreal Protocol by calling for a complete elimination of CFCs by the year 2000. By the year 2010 CFCs should be
completely eliminated from developing countries as well.
Because the only available CFC gases in countries adhering to the treaty is from recycling, their prices have gone up considerably. A worldwide end to production should also terminate the smuggling of this material, such as from Mexico to the United States.
A number of substitutes for CFC's have been introduced. Hydrochlorofluorocarbons are much more reactive than CFC's, so a large fraction of the HCFCs emitted break down in the troposphere, and hence are removed before they have a chance to affect the ozone layer.
Nevertheless, a significant fraction of the HCFCs do break down in the
stratosphere and they have contributed to more chlorine buildup there than originally predicted. Development of non-chlorine based chemical compounds as a substitute for CFCs and HCFCs continues. One such class are the hydrofluorocarbons , which contain only hydrogen and fluorine. One of these compounds,
HFC-134a, is now used in place of CFC-12 in automobile air conditioners.
There is concern that halons are being broken down in the
atmosphere to bromine, which reacts with
ozone, leading to
depletion of the
ozone layer . These issues are complicated: the kinds of fires that require halon extinguishers to be put out will typically cause more damage to the ozone layer than the halon itself, not to mention human and property damage. However, fire extinguisher systems must be tested regularly, and these tests may lead to damage. As a result, some regulatory measures have been taken, and halons are being phased out in most of the world.
In the United States purchase and use of freon gases is regulated by the
Environmental Protection Agency, and substantial fines have been levied for their careless venting. Also, licenses, good for life, are required to buy or use these chemicals. The EPA website discusses these rules in great detail, and also lists numerous private companies that are approved to give examinations for these certificates.
There are two kinds of licenses. Obtaining a "Section 609" license to use CFCs to recharge old car air conditioners is fairly easy and requires only an online multiple choice test offered by several companies. Companies that use unlicensed technicians for CFC recharge operations are subject to a US$15,000 fine per technician by the EPA.
The "Section 608" license, needed to recharge CFC-using stationary and non-automobile mobile units, is also multiple choice but more difficult. A general knowledge test is required, plus separate exams for small size units, and for high and low pressure systems. These are respectively called Parts I, II, and III. A person who takes and passes all tests receives a "Universal" license; otherwise, one that is endorsed only for the respectively passed Parts. While the general knowledge and Part I exams can be taken online, taking them before a proctor lets the applicant pass these tests with lower scores.
Safety
Freon in
copper tubing open to the environment can turn into
phosgene gas after coming in contact with extreme heat, such as while
brazing or in a
fire situation. Other ways that phosgene can be created is by passing the Freon through an internal combustion engine, or by inhaling it through a lit cigarette, cigar or pipe. Phosgene is a substance that was used as a
chemical weapon in
World War I. Low exposure can cause irritation, but high levels cause fluid to collect in the
lungs, possibly resulting in death.
See also
References
- B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
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