GREENHOUSE CHEMICALS CARBON DIOXIDE – 83% Carbon dioxide, CO2, is one of the gases in our atmosphere, being uniformly distributed over the earth's surface at a concentration of about 0.033% or 330 ppm. Commercially, CO2 finds uses as a refrigerant (dry ice is solid CO2), in beverage carbonation, and in fire extinguishers. In the United States, 10.89 billion pounds of carbon dioxide were produced by the chemical industry in 1995, ranking it 22nd on the list of top chemicals produced. Because the concentration of carbon dioxide in the atmosphere is low, it is not practical to obtain the gas by extracting it from air. Most commercial carbon dioxide is recovered as a by-product of other processes, such as the production of ethanol by fermentation and the manufacture of ammonia. Some CO2 is obtained from the combustion of coke or other carbon-containing fuels. C(coke) + O2(g) CO2(g) Carbon dioxide is released into our atmosphere when carbon-containing fossil fuels such as oil, natural gas, and coal are burned in air. As a result of the tremendous world-wide consumption of such fossil fuels, the amount of CO2 in the atmosphere has increased over the past century, now rising at a rate of about 1 ppm per year. Major changes in global climate could result from a continued increase in CO2 concentration. In addition to being a component of the atmosphere, carbon dioxide also dissolves in the water of the oceans. At room temperature, the solubility of carbon dioxide is 3 about 90 cm of CO2 per 100 mL of water. In aqueous solution, carbon dioxide exists in many forms. First, it simply dissolves. CO2(g) CO2(aq) Then, an equilibrium is established between the dissolved CO2 and H2CO3, carbonic acid. CO2(aq) + H2O(l) H2CO3(aq) Only about 1% of the dissolved CO2 exists as H2CO3. Carbonic acid is a weak acid which dissociates in two steps. + 7 H2CO3 H + HCO3¯ Ka1 = 4.2 × 10¯ + 2 11 HCO3¯ H + CO3 ¯ Ka2 = 4.8 × 10¯ As carbon dioxide dissolves in sea water, an equilibrium is established 2 involving the carbonate ion, CO3 ¯. The carbonate anion interacts with cations in seawater. According to the solubility rules, "all carbonates are insoluble except those of ammonium and Group IA elements." Therefore, the carbonate ions cause the 2+ 2+ precipitation of certain ions. For example, Ca and Mg ions precipitate from large 9 bodies of water as carbonates. For CaCO3, the value of Ksp is 5 × 10¯ , and for 3 MgCO3, Ksp is 2 × 10¯ . Extensive deposits of limestone (CaCO3) and dolomite (mixed CaCO3 and MgCO3) have been formed in this way. Calcium carbonate is also the main constituent of marble, chalk, pearls, coral reefs, and clam shells. Although "insoluble" in water, calcium carbonate dissolves in acidic solutions. The carbonate ion behaves as a Brønsted base. + 2+ CaCO3(s) + 2 H (aq) Ca (aq) + H2CO3(aq) The aqueous carbonic acid dissociates, producing carbon dioxide gas. H2CO3(aq) H2O(l) + CO2(g) In nature, surface water often becomes acidic because atmospheric CO 2 dissolves in it. This acidic water can dissolve limestone. 2+ CO2(aq) + H2O(l) + CaCO3(s) Ca (aq) + 2 HCO3¯(aq) This reaction occurs in three steps. 2+ 2 CaCO3(s) Ca (aq) + CO3 ¯(aq) CO2(aq) + H2O(l) H2CO3(aq) 2 H2CO3(aq) + CO3 ¯(aq) 2 HCO3¯(aq) In the third step, carbonate ions accept hydrogen ions from carbonic acid. This reaction often occurs underground, when rainwater saturated with CO2 seeps through a layer of limestone. As the water dissolves calcium carbonate, it forms openings in the limestone. Caves from which the limestone has been dissolved are often prevalent in areas where there are large deposits of CaCO3 (e.g., Mammoth Cave, Carlsbad Caverns, and Cave of the Mounds). If the water containing dissolved Ca(HCO3)2 reaches the ceiling of a cavern, the water will evaporate. As it evaporates, carbon dioxide escapes, and calcium carbonate deposits on the ceiling. 2+ Ca (aq) + 2 HCO3(aq) H2O(g) + CO2(g) + CaCO3(s) A new use for liquid carbon dioxide currently under development is as a dry-cleaning solvent. Currently, most laundries use chlorinated hydrocarbons as dry-cleaning solvents. These chlorinated hydrocarbons are probable human carcinogens, so the search is on for replacements. Carbon dioxide does not exist in liquid form at atmospheric pressure at any temperature. The pressure-temperature phase diagram of CO2 shows that liquid carbon dioxide at 20°C requires a pressure of 30 atmospheres. The lowest pressure at which liquid CO2 exists is at the triple point, namely 5.11 atm at -56.6°C. The high pressures needed for liquid CO2 require specialized washing machines. Like chlorinated hydrocarbons, liquid carbon dioxide is an effective solvent for grease and oils. Liquid CO2 has some advantages over chlorinated hydrocarbons-- items that cannot be dry cleaned with chlorinated hydrocarbons, such as leather, fur, and some synthetics, can be safely cleaned with liquid carbon dioxide. More information about alternative dry-cleaning solvents can be found in the Innovations section of Environmental Health Perspectives, Volume 104, Number 5. METHANE – 9% Methane is a colorless, odorless gas with a wide distribution in nature. It is the principal component of natural gas, a mixture containing about 75% CH4, 15% ethane (C2H6), and 5% other hydrocarbons, such as propane (C3H8) and butane (C4H10). The "firedamp" of coal mines is chiefly methane. Anaerobic bacterial decomposition of plant and animal matter, such as occurs under water, produces marsh gas, which is also methane. At room temperature, methane is a gas less dense than air. It melts at –183°C and boils at –164°C. It is not very soluble in water. Methane is combustible, and mixtures of about 5 to 15 percent in air are explosive. Methane is not toxic when inhaled, but it can produce suffocation by reducing the concentration of oxygen inhaled. A trace amount of smelly organic sulfur compounds (tertiary-butyl mercaptan, (CH3)3CSH and dimethyl sulfide, CH3–S–CH3) is added to give commercial natural gas a detectable odor. This is done to make gas leaks readily detectible. An undetected gas leak could result in an explosion or asphyxiation. (The attached scratch-and-sniff sheet from Madison Gas & Electric Company is for your use outside of class.) Methane is synthesized commercially by the distillation of bituminous coal and by heating a mixture of carbon and hydrogen. It can be produced in the laboratory by heating sodium acetate with sodium hydroxide and by the reaction of aluminum carbide (Al4C3) with water. In the chemical industry, methane is a raw material for the manufacture of methanol (CH3OH), formaldehyde (CH2O), nitromethane (CH3NO2), chloroform (CH3Cl), carbon tetrachloride (CCl4), and some freons (compounds containing carbon and fluorine, and perhaps chlorine and hydrogen). The reactions of methane with chlorine and fluorine are triggered by light. When exposed to bright visible light, mixtures of methane with chlorine or fluorine react explosively. The principal use of methane is as a fuel. The combustion of methane is highly exothermic. CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) H = –891 kJ The energy released by the combustion of methane, in the form of natural gas, is used directly to heat homes and commercial buildings. It is also used in the generation of electric power. During the past decade natural gas accounted for about 1/5 of the total energy consumption worldwide, and about 1/3 in the United States. The cost of natural gas to Wisconsin consumers is regulated by the State Public Service Commission. Madison Gas Electric Company currently charges its residential consumers about $0.66 per 100 cubic feet. Natural gas occurs in reservoirs beneath the surface of the earth. It is often found in conjunction with petroleum deposits. Before it is distributed, natural gas usually undergoes some sort of processing. Usually, the heavier hydrocarbons (propane and butane) are removed and marketed separately. Non-hydrocarbon gases, such as hydrogen sulfide, must also be removed. The cleaned gas is then distributed throughout the country through thousands of miles of pipeline. Local utility companies add an odorant before delivering the gas to their customers. Some methane is manufactured by the distillation of coal. Coal is a combustible rock formed from the remains of decayed vegetation. It is the only rock containing significant amounts of carbon. The elemental composition of coal varies between 60% and 95% carbon. Coal also contains hydrogen and oxygen, with small concentrations of nitrogen, chlorine, sulfur, and several metals. Coals are classified by the amount of volatile material they contain, that is, by how much of the mass is vaporized when the coal is heated to about 900°C in the absence of air. Coal that contains more than 15% volatile material is called bituminous coal. Substances released from bituminous coal when it is distilled, in addition to methane, include water, carbon dioxide, ammonia, benzene, toluene, naphthalene, and anthracene. In addition, the distillation also yields oils, tars, and sulfur-containing products. The non- volatile component of coal, which remains after distillation, is coke. Coke is almost pure carbon and is an excellent fuel. However, it may contain metals, such as arsenic and lead, that can be serious pollutants if the combustion products are released into the atmosphere. NITROUS OXIDE – 6% Nitrous oxide, also known as dinitrogen oxide or dinitrogen monoxide, is a chemical compound with chemical General formula N2O. Under room conditions it is Name Dinitrogen oxide a colourless non-flammable gas, with a pleasant, slightly sweet odor. It is Chemical N2O commonly known as laughing gas due formula to the exhilarating effects of inhaling it, Appearance Colorless gas and because it can cause spontaneous laughter in some people; it's also known Physical as NOS or nitrous in racing and Formula weight 44.0 u motorsports, where its usage is Melting point 182 K (-91 °C) widespread. It is used in surgery and dentistry for its anaesthetic and Boiling point 185 K (-88 °C) analgesic effects. Nitrous oxide is Critical present in the atmosphere where it acts 309.6 K (36.4 °C) temperature as a powerful greenhouse gas. Critical 7.245 MPa pressure Chemistry 3 Density 1.2 g/cm (liquid) The structure of the nitrous oxide Solubility 0.112 g in 100g water molecule is a linear chain of a nitrogen Thermochemistry atom bound to a second nitrogen, which 0 in turn is bound to an oxygen atom. It ΔfH gas 82.05 kJ/mol can be considered a resonance hybrid ΔfH0liquid ? kJ/mol of 0 ΔfH solid ? kJ/mol 0 S gas, 100 kPa 219.96 J/(mol·K) 0 S liquid, 100 kPa ? J/(mol·K) 0 S solid ? J/(mol·K) Safety a n See main text. May cause Inhalation d asphyxiation without warning. Hazardous when cryogenic or Skin compressed. Hazardous when cryogenic or Eyes compressed. SI units were used where possible. Nitrous oxide [[N2O]] should not be confused with the other nitrogen oxides such as nitric oxide NO and nitrogen dioxide NO2. Note that nitrous oxide is isoelectric with carbon dioxide. Nitrous oxide can be prepared by heating ammonium nitrate in the laboratory. This is not, however, advised, since overheated ammonium nitrate can easily explode. Nitrous oxide can be used to produce nitrites by mixing it with boiling alkali metals, and to oxidize organic compounds at high temperatures. The CAS number of nitrous oxide is 10024-97-2 and its UN number is 1070. Medicine Medical grade Nitrous Oxide tanks used in dentistry Nitrous oxide is a weak general anesthetic, and is generally not used alone in anaesthesia. However, it has a very low short-term toxicity and is an excellent analgesic, so a 50/50 mixture of nitrous oxide and oxygen ("gas and air", supplied under the trade name Entonox) is commonly used during childbirth, for dental procedures, and in emergency medicine. In general anesthesia it is often used in an 2:1 ratio with oxygen in addition to more powerful general anaesthetic agents such as sevoflurane or desflurane. Its lower solubility in blood means it has a very rapid onset and offset. It has a MAC of 105% and a blood:gas partition coefficient of 0.46. Less than 0.004% is metabolised in humans. Nitrous Oxide is liquid at approximately 760 psi at room temperature, and is usually stored and shipped as a self-pressurized liquid. Aerosol propellant The gas is licensed for use as a food additive, specifically as an aerosol spray propellant. Its most common uses in this context are in aerosol whipped cream canisters and as an inert gas used to displace staleness-inducing oxygen when filling packages of potato chips and other similar snack foods. The gas is excellently soluble in fatty compounds. In aerosol whipped cream, it is dissolved in the fatty cream until it leaves the can, when it becomes gaseous and thus creates foam. One can easily obtain the propellant by slowly turning the canister upside down (NO SHAKING) and letting all the contents out, leaving you the N2O. However, if one is using the Nitrous for recreational purposes, using N2O straight from a whipped cream can is unadvisable due to the fact that it is frequently cut with certain chemicals that can cause headaches or nausea. There is also usually a negligible amount of N2O in the cans. Rocket motors Nitrous oxide can be used as an oxidizer in a rocket engine. This has the advantages over other oxidizers that it is non-toxic and, due to its stability at room temperature, easy to store and relatively safe to carry on a flight. Nitrous oxide has notably been the oxidizer of choice in several hybrid rocket designs (using solid fuel with a liquid or gaseous oxidizer). The combination of nitrous oxide with hydroxy-terminated polybutadiene fuel has been used by SpaceShipOne and others. It is also notably used in amateur and high power rocketry with various plastics as the fuel. An episode of MythBusters featured a hybrid rocket built using paraffin wax mixed with powdered carbon as its solid fuel and nitrous oxide as its oxidizer. Internal Combustion Engine In car racing, nitrous oxide (often just "nitrous" in this context) is sometimes injected into the intake manifold (or just prior to the intake manifold) to increase power: even though the gas itself is not flammable, it delivers more oxygen than atmospheric air by breaking down at elevated temperatures, thus allowing the engine to burn more fuel and air. Additionally, since nitrous oxide is stored as a liquid, the evaporation of liquid nitrous oxide in the intake manifold causes a large drop in intake charge temperature. This results in a smaller, denser charge, and can reduce detonation, as well as increase power available to the engine. The same technique was used during by World War II Luftwaffe aircraft with the GM 1 system to boost the power output of aircraft engines. Originally meant to provide the Luftwaffe standard aircraft with superior high-altitude performance, technological considerations limited its use to extremely high altitudes. Accordingly, it was only used by specialized planes like high-altitude reconnaissance aircraft, high-speed bombers and high-altitude interceptors. One of the major problems of using nitrous oxide in a reciprocating engine is that it can produce enough power to destroy the engine. Power increases of 100-300% are possible, and unless the mechanical structure of the engine is reinforced, most engines would not survive this kind of operation. There are several ways of introducing nitrous into a motor. Nitrous kits such as such as NOS, Nitrous Express, Nitrous Direct brands offer different solutions. You will find Dry kits, Wet kits & Direct port. See nitrous It is very important with nitrous oxide augmentation of internal combustion engines to maintain temperatures and fuel levels so as to prevent preignition, or detonation (sometimes referred to as knocking, pinging or pinking). Nitrous oxide in the atmosphere Nitrogen oxides, nitrous oxide included, are greenhouse gases; per kilogram, nitrous oxide has 296 times the effect of carbon dioxide for producing global warming . Therefore, nitrogen oxides are a subject of efforts to curb greenhouse gas emissions, such as the Kyoto Protocol. Behind carbon dioxide and methane, nitrous oxide is the third most important gas that contribute to global warming. Nitrous oxide is naturally emitted from soils and oceans. Human activity contributes to the release of the gas through the cultivation of soil and the production and use of nitrogen fertilizers, the production of nylon, and the burning of fossil fuels and other organic matter. Human activity is thought to account for somewhat less than 2 teragrams (this is multiplied by appx 300 when calculated as a ratio to Carbon Dioxide) of nitrogen oxides per year, nature for over 15 teragrams HFC’s, PCs, SF4s 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) 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 have been 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 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 (CFC) in 1928 as a replacement for ammonia (NH3), chloromethane (CH3Cl), and sulfur dioxide (SO2), 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 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 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 were published in the journal Nature in 1974 by Molina and Rowland (who shared the 1995 Nobel Prize for Chemistry for their work). 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 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 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 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 (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, 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 carbon tetrachloride. 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 . The specified ANSI/ASHRAE prefixes were FC (fluorocarbon), or R (refrigerant), but today most are prefixed by a more specific classification: CFC - chlorofluorocarbons HCFC - hydrochlorofluorocarbons HFC - hydrofluorocarbons 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 = eplaced 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. Common/Trivial Chem. Systematic name Code name(s) formula Halomethanes Chloromethane Methyl chloride CH3Cl Dichloromethane Methylene chloride CH2Cl2 Trichloromethane Chloroform CHCl3 Carbon tetrachloride, tet, Freon Tetrachloromethane CFC-14 CCl4 14 Trichlorofluoromethane Freon-11, R-11 CFC-11 CCl3F Dichlorodifluoromethane Freon-12, R-12 CFC-12 CCl2F2 Chlorotrifluoromethane CFC-13 CClF3 Chlorodifluoromethane 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 Halon 1211 CBrClF2 Bromotrifluoromethane Halon 1301 CBrF3 Iodotrifluoromethane 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- Dichlorotetrafluoroethane CFC-114 ClF2C-CClF2 tetrafluoroethane 1-Chloro-1,1,2,2,2- Chloropentafluoroethane CFC-115 ClF2C-CF3 pentafluoroethane 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 HFC-134a, 1,1,1,2-Tetrafluoroethane F3C-CH2F Suva-134a 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 HFC-227ea, F3C-CHF- 1,1,1,2,3,3,3-heptafluoropropane FE-227, FM- CF3 200 R610, PFB, F3C-CF2- Decafluorobutane perfluorobutane CEA-410 CF2-CF3 -[CHCl- Polychloroethene polyvinyl chloride, PVC CH2]x- Polytetrafluoroethylene, Polytetrafluoroethene -[CF2-CF2]x- PTFE, Teflon 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 CH3-(CH2)6-OH + SOCl2 → CH3-(CH2)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 (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: 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. 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.)They have now been outlawed in 500 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 (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. 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 minimum of ten (10) 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 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 (CF3I) is being considered as an alternative.  Environmental issues Ozone-depleting gas trends 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 + 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, 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 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 (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. 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.
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