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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. This reaction continues until the radicals are used up by one of three termination steps.

Step 4: R· + X· → R-X (1st termination step)Step 5: 2 X· → X2 (2nd termination step)Step 6: 2 R· → R-R (3rd termination step)

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

Preparation of haloalkane:

  • An alkene reacts with a dry hydrogen halide (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 with the halogen and one with the hydrogen atom of the hydrohalic acid. Markovnikov's rule states that in this reaction, the halogen is more likely to become attached to the more substituted carbon. This is a electrophilic addition reaction. It gives Markwonikoff addition product. For example:
H3C-CH=CH2 + HBr → H3C-CHBr-CH3 (primary product) + H3C-CH2-CH2Br (secondary product).

Water must be absent otherwise there will be a side product( water). The reaction is necessarily to be carried out in a dry inert solvent such as CCl4 or directly in the gaseous phase.

  • Alkenes also react with halogens (X2) to form haloalkanes with two neighboring halogen atoms( Dihaloalkane). This is sometimes known as "decolorizing" the halogen, since the reagent X2 is colored and the product is usually colorless. For 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 which is called the Darzen's process. The Darzen's process is one of the most convenient methods known because the bi-products are gaseous and thus escape, leaving behind pure alkyl 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 H2OCH3-(CH2)6-OH + SOCl2 → CH3-(CH2)6-Cl + SO2 + HCl

By substitution of alkanol in the absence of water

Halogenating agents are:

  1. Phosphorus pentachloride
  2. Thionyl chloride
  3. hydrogen chloride
  4. Phosphorus with Bromine
  5. Phosphorus with Iodine
  6. Hydrogen chloride with zinc chloride

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 (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 halide ion, X-. 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:

  • Ammonia (NH3) and bromoethane yields a mixture of ethylamine, diethylamine, and triethylamine (as their bromide salts), and tetraethylammonium bromide.
  • Cyanide (CN-) added to bromoethane will form propionitrile (CH3CH2CN), a nitrile, and Br-. Nitriles can be further hydrolyzed into carboxylic acids.

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

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. The hydrofluoroalkane propellants ability to solubilize medications and excipients is markedly different from CFCs and as a result require 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 in all 50 U.S. 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 percent v/v in air, and can suppress many fires at 2.9 percent v/v. By contrast, carbon dioxide fire suppression flood systems are operated from 34 percent concentration by volume (surface-only combustion of liquid fuels) up to 75 percent (dust traps). Carbon dioxide can cause severe distress at concentrations of 3 to 6 percent, and has caused death by respiratory paralysis in a few minutes at 10 percent concentration. Halon 1301 causes only slight giddiness at its effective concentration of 5 percent, and even at 15 percent persons remain conscious but impaired and suffer no long term effects. (Experimental animals have also been exposed to 2 percent 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 nozzles designed to ensure uniform mixing throughout the room. The quantity dumped is pre-calculated to achieve the desired concentration, typically 3-7 percent 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.4

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, trifluoroiodomethane (CF3I) is being considered as an alternative.5

Environmental issues

Pollution
Air pollution
Acid rain • Air Pollution Index • Air Quality Index • Atmospheric dispersion modeling • Chlorofluorocarbon • Global dimming • Global warming • Haze • Indoor air quality • Ozone depletion • Particulate • Smog • Roadway air dispersion
Water pollution
Eutrophication • Hypoxia • Marine pollution • Ocean acidification • Oil spill • Ship pollution • Surface runoff • Thermal pollution • Wastewater • Waterborne diseases • Water quality • Water stagnation
Soil contamination
Bioremediation • Herbicide • Pesticide •Soil Guideline Values (SGVs)
Radioactive contamination
Actinides in the environment • Environmental radioactivity • Fission product • Nuclear fallout • Plutonium in the environment • Radiation poisoning • radium in the environment • Uranium in the environment
Other types of pollution
Invasive species • Light pollution • Noise pollution • Radio spectrum pollution • Visual pollution
Government acts
Clean Air Act • Clean Water Act • Kyoto Protocol • Water Pollution Control Act • Environmental Protection Act 1990
Major organizations
DEFRA • Environmental Protection Agency • Global Atmosphere Watch • Greenpeace • National Ambient Air Quality Standards
Related topics
Natural environment

Since the late 1970s the use of CFCs has been heavily regulated because of its destructive effects on the ozone layer. After the development of his atmospheric CFC detector, James Lovelock was the first to detect the presence of CFC's in the air, finding a concentration of 60 parts per trillion of CFC-11 over Ireland. In a self-funded research expedition ending in 1973, Lovelock went on to measure the concentration of CFC-11 in both the arctic and Antarctic, finding the presence of the gas in each of 50 air samples collected, but incorrectly concluding that CFC's are not hazardous to the environment. The experiment did however provide the first useful data on the presence of CFC's in the atmosphere. The damage caused by CFC's discovered by Sherry Rowland and Mario Molina who, after hearing a lecture on the subject of Lovelocks work, embarked on research resulting in the first published 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 catalyzes the break up of ozone into oxygen by means of a variety of mechanisms, of which the simplest is:

Cl· + O3 → ClO· + O2ClO· + 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 (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, the US state of 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 CFCs 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.

Ozone-depleting gas trends

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 CFCs have been introduced. Hydrochlorofluorocarbons (HCFCs) are much more reactive than CFCs, 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.

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 (pre-1993 model year) 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 (such as home refrigerator) 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 (which has to be done for Parts II and III) lets the applicant pass these tests with lower scores.

Safety

Haloalkanes 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 Haloalkane 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

  • Alkane
  • Halogen
  • Halide

Notes

  1. ↑ K. Lee Lerner and Brenda Wilmoth. Environmental issues : essential primary sources. (Detroit: Thomson Gale, 2006)
  2. ↑ 1.www.halom.org.Retrieved March 13, 2008.
  3. ↑ 2.Retrieved March 13, 2008.
  4. ↑ 3.Retrieved March 13, 2008.
  5. ↑ Defense Tech Briefs 4.Retrieved March 13, 2008.

References

  • B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
  • Lerner, K. Lee Brenda Wilmoth Lerner, eds. Environmental issues: essential primary sources. Detroit: Thomson Gale, 2006.
  • McMurry, John. 2004. Organic Chemistry, 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052.
  • Moeller, Dade W. 2005. Environmental Health, 3rd ed. Cambridge, MA: Harvard University Press. ISBN 0674014944.
  • Morrison, Robert T., and Robert N. Boyd. 1992. Organic Chemistry,'. 6th ed. Englewood Cliffs, NJ: Prentice Hall. ISBN 0136436692.
  • Solomons, T.W. Graham, and Fryhle, Craig B. 2004. Organic Chemistry, 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998.
Functional groups
Chemical class: Alcohol • Aldehyde • Alkane • Alkene • Alkyne • Amide • Amine • Azo compound • Benzene derivative • Carboxylic acid • Cyanate • Ester • Ether • Haloalkane • Imine • Isocyanide • Isocyanate • Ketone • Nitrile • Nitro compound • Nitroso compound • Peroxide • Phosphoric acid • Pyridine derivative • Sulfone • Sulfonic acid • Sulfoxide • Thioether • Thiol • Toluene derivative

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