Haloalkane

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. They are known under many chemical and trivial names. As fire extinguisher, propellant, and solvents, they have or had wide use. Some haloalkanes have negative effects on the environment, such as ozone depletion.

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 (CFCs) 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 (CFC, HCFC, HFC)

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CFC molecules

Chlorofluorocarbons (CFC) 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 (see ozone depletion).

Hydrochlorofluorocarbons (HCFCs) is one of a class of haloalkanes where not all hydrogen has been replaced by chlorine or fluorine. They are used primarily as chlorofluorocarbon (CFC) substitutes, as the environmental effects are less than for CFCs. When the chlorine is reduced to zero, these compounds are known as hydrofluorocarbons (HFCs), with even less environmental effects.

Bromofluoro compounds (halons)

Halon is the group of haloalkanes with bromine as well as chlorine or fluorine groups. The two most common ones are bromochlorodifluoromethane (Halon 1211, CF₂BrCl) and bromotrifluoromethane (Halon 1301, CF₃Br). Halons are very stable and were widely used in fire extinguishers where water and other alternatives would be ineffective and dangerous (e.g., when dealing with fires involving live electrical circuits) or cause unacceptable collateral damage (e.g., with electronic equipment.)

Polymer haloalkanes

Chlorinated or fluorinated alkenes can be used for polymerization, resulting in polymer haloalkanes with notable chemical resistance properties. Important examples include polychloroethene (polyvinyl chloride, PVC), and polytetrafluoroethene (PTFE, Teflon), but many more halogenated polymers exist.

History

Original development

Tetrachloromethane 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.

Chlorofluorocarbons (CFC) were developed by the American engineer Thomas Midgley in 1928 as a replacement for ammonia (NH₃), chloromethane (CH₃Cl), and sulfur dioxide (SO₂), then a toxic but common 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 Association, Midgley flamboyantly demonstrated all these properties by inhaling a breath of the gas and using it to blow out a candle.

Midgley specifically developed CCl₂F₂. 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 (such as the expanded polystyrene used in packaging materials and disposable coffee cups).

Development on alternatives

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During World War II, various early chloroalkanes were in standard use in 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 recognised 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. Adding hydrogen and thus creating hydrochlorofluorocarbons (HCFC), chemists made the compound less stable in the lower atmosphere enabling them to break down before reaching the ozone layer. Later alternatives even fully excluded the chlorine, creating hydrofluorocarbons (HFC) with even shorted 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 possible impact of chloroalkanes and bromoalkanes on the ozone layer. The Vienna Convention on Ozone Layer Protection did not cover bromofluoroalkanes as it was felt that emergency discharge of 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 discharges during system tests and maintenance accounted for substantially larger volumes than emergency discharges, and so halons were brought into the treaty, but with many exceptions.

Phase out

Use of chloroalkanes as solvents for large scale application, such as dry cleaning, have been phased out, a.o. by the IPPC directive on greenhouse cases in 1994 and the the Volatile Organic Compounds (VOC) directive of the EU in 1997. Also chlorofluoroalkanes are minimized to medicinal use only.

At last, bromofluoroalkanes have been generally phased out and the possession of such equipment is prohibited in some countries like the Netherlands and Belgium from January 1, 2004, based on the Montreal Protocol and guidelines of the European Union. Production of new stocks has ceased in most (probably all) countries as of 1994. However many countries still require aircraft to be fitted with halon fire suppression systems, as no safe and completely satisfactory alternative has been discovered for this application. There are also a few other highly specialised users. These programs recycle halon through "halon banks", coordinated by the Halon Recycling Corporation (http://www.halon.org/), 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 (trichloromethane) and methylene chloride (dichloromethane). 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. The standard is specified in the ANSI/ASHRAE Standard 34-1992 with additional annual supplements [1] (http://resourcecenter.ashrae.org/store/ashrae/newstore.cgi?itemid=23260&view=item&categoryid=311&categoryparent=311&page=1&loginid=2022241). The specified ANSI/ASHRAE prefixes were FC (fluorocarbon), or R (refrigerant), but today most are prefixed by a more specific classification:

CFC chlorofluorocarbons
HCFC hydrogenchlorofluorocarbons
HFC hydrogenfluorocarbons
FC fluorocarbons
PFC 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 = Chlorine atoms 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.

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(s)	Code	Chem. formula
Halomethanes
chloromethane	methyl chloride		CH₃Cl
dichloromethane	methylene chloride		CHCl₂
trichloromethane	chloroform		CHCl₃
tetrachloromethane	carbon tetrachloride, tet		CCl₄
trichlorofluoromethane	Freon-11, R-11	CFC-11	CCl₃F
dichlorodifluoromethane	Freon-12, R-12	CFC-12	CCl₂F₂
chlorotrifluoromethane		CFC-13	CCl₃F
chlorodifluoromethane		HCFC-22	CHClF₂
trifluoromethane	fluoroform	HFC-23	CHF₃
fluoromethane	methyl fluoride	HFC-41	CH₃F
dibromomethane	methylene bromide		CH₂Br₂
tribromomethane	bromoform		CHBr₃
bromochlorodifluoromethane		Halon 1211	CBrClF₂
bromotrifluoromethane		Halon 1301	CBrF₃
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		CFC-114	ClF₂C-CClF₂
1-chloro-1,1,2,2,2-pentafluoroethane	chloropentafluoroethane	CFC-115	ClF₂C-CF₃
1,1,2,2-tetrafluoroethane		HFC-134	F₂HC-CHF₂
1,1,1,2-tetrafluoroethane		HFC-134a Suva-134a	F₃C-CH₂F
1,1-dichloro-1-fluoroethane		HCFC-141b	Cl₂FC-CH₃
1-chloro-1,1-difluoroethane		HCFC-142b	ClF₂C-CH₃
1,1,difluoroethane		HFC-152a	F₂HC-CH₃
Longer haloalkanes, polymers
1,1,1,2,3,3,3-heptafluoropropane		HFC-227ea	F₃C-CHF-CF₃
polychloroethene	polyvinyl chloride, PVC		-[CHCl-CH₂]_x-
polytetrafluoroethene	polytetrafluoroethylene, PTFE, Teflon		-[CF₂-CF₂]_x-
many other halogenated polymers

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· (Initiation Step)

Step 2: X· + R-H → R· + HX (1st Propagation Step)

Step 3: R· + X₂ → R-X + X· (2nd Propagation Step)

Steps 2 and 3 keep repeating, each providing the reactive intermediate needed for the other step. This is called a radical chain reaction.

From alkenes

An alkene reacts with a hydrogen halogenides (HX) like hydrogen chloride (HCl) or hydrogen bromide (HBr) 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 (primary product) + H₃C-CH₂-CH₂Br (secondary product).

Alkenes also react with halogens (X₂) to form haloalkanes with two neighboring halogen atoms. This is sometimes known as "decolorizing" the halogen since the reagent X₂ is colored and the product is usually colorless. Example:

H₃C-CH=CH₂ + Br₂ → H₃C-CHBr-CH₂Br

From alkanol (alcohol)

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 (equivalent to phosphorus triiodide). Two examples:

(H₃C)₃C-OH + HCl.H₂O → (H₃C)₃C-Cl + 2 H₂O

H₃-(CH₂)₆-OH + SOCl₂ → H₃-(CH₂)₆-Cl + SO₂ + HCl

The reactivity towards nucleophiles

There is a polarity about halogenoalkanes - the carbon to which the halogen is attached is slightly electropositive where the halogen is slightly electronegative. This results in an electron deficient (electrophilic) 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^-. (NaOH_(aq) being a common source of this ion). 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 bromide ion, Br^-. As can be seen, the OH is now attached to the alkyl group, creating an alcohol. (Hydrolysis of bromoethane, for example, yields ethanol).

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:

If one adds ammonia (NH₃) to bromoethane, an amine (CH₃CH₂NH₂) will form along with HBr.
If one adds cyanide (CN^-) to bromoethane, a nitrile (CH₃CH₂CN) will form along with Br^-.

(One should note that a nitrile can be further hydrolyzed into a carboxylic acid.)

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 C₂H₄ (ethylene), H₂O and Br^-.

Applications

Solvent

text to be added

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. (By contrast, a significant amount of development effort has been required to develop non-CFC alternatives to CFC-based refrigerants, particularly for applications where the refrigeration mechanism cannot be modified or replaced.)

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 (surface-only combustion of liquid fuels) up to 75% (dust traps). 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. (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.

Halon canisters used in a fire-suppression system

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 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 minumum of ten (1) minutes and sometimes up to a twenty (20) 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 detetion 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.

Environmental issues

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Ozone-depleting gas trends

There has been a movement since the late 1970s to ban CFCs because of their destructive effect 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 + O → Cl + 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 (but more complicated) 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, Oregon enacted the world's first ban of CFCs (legislation introduced by Walter F. Brown). 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.

A number of substitutes for CFC's have been introduced. Hydrochlorofluorocarbons (HCFCs) 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 (HFCs), 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 (this is similar to the case of chlorofluorocarbons such as freon). 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.

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. 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.

References

B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
Nomenclature FAQ (http://www.faqs.org/faqs/sci/chem-faq/part3/section-1.html)
Numbering Scheme for Ozone-Depleting Substances and their Substitutes (http://www.epa.gov/ozone/geninfo/numbers.html)
Class I Ozone-Depleting Substances (http://www.epa.gov/ozone/ods.html)
CFC Smuggling (http://www.alternet.org/story.html?StoryID=10109)
CFC Illegal Trade (http://www.american.edu/projects/mandala/TED/cfctrade.htm)
cfk or cfc (http://www.cfk.be.tt/) |dutch|
Numbering Scheme for Ozone-Depleting Substances and their Substitutes (http://www.epa.gov/ozone/geninfo/numbers.html)
Class I Ozone-Depleting Substances (http://www.epa.gov/ozone/ods.html)
Class II Ozone-Depleting Substances (HCFC's) (http://www.epa.gov/ozone/ods2.html)
History of Halon use by the US Navy (http://books.nap.edu/books/0309057825/html/63.html)
MSDS for Halon 1211 (PDF file, 39 kB) (http://www.ansul.com/AnsulGetDoc.asp?FileID=13402)
MSDS for Halon 1301 (PDF file, 39 kB) (http://www.ansul.com/AnsulGetDoc.asp?FileID=13403)de:Halogenalkan

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