Halomethane compounds are derivatives of methane with one or more of the hydrogen atoms replaced with halogen atoms. Halomethanes are both occurring in marine environments, human-made, most notably as refrigerants, solvents and fumigants. Many, including the chlorofluorocarbons, have attracted wide attention because they become active when exposed to ultraviolet light found at high altitudes and destroy the Earth's protective ozone layer. Like methane itself, halomethanes are tetrahedral molecules; the halogen atoms differ in size and charge from hydrogen and from each other. The various halomethanes deviate from the perfect tetrahedral symmetry of methane; the physical properties of halomethanes depend on the number and identity of the halogen atoms in the compound. In general, halomethanes are volatile but less so than methane because of the polarizability of the halides; the polarizability of the halides and the polarity of the molecules makes them useful as solvents. The halomethanes are far less flammable than methane.
Broadly speaking, reactivity of the compounds is greatest for the iodides and lowest for the fluorides. The halomethanes are produced on a massive scale from abundant precursors, i.e. natural gas or methanol, from halogens or halides. They are prepared by one of three methods. Free radical chlorination of methane:CH4 + Cl2 → CH3Cl + HClThis method is useful for the production of CH4−xClx; the main problems with this method are that it cogenerates HCl and it produces mixtures of different products. Using CH4 in large excess generates CH3Cl and using Cl2 in large excess generates CCl4, but mixtures of other products will still be present. Halogenation of methanol; this method is used for the production of the mono-chloride, -bromide, -iodide. CH3OH + HCl → CH3Cl + H2O 4 CH3OH + 3 Br2 + S → 4 CH3Br + H2SO4 + 2 HBr 3 CH3OH + 3 I2 + P → 3 CH3I + HPO2 + 3 HIHalogen exchange; the method is used to produce fluorinated derivatives from the chlorides. CH3Cl + HF → CH3F + HCl CH2Cl2 + HF → CH2FCl + HCl CH2Cl2 + 2 HF → CH2F2 + 2 HCl CH2Cl2 + F2 → CH2F2 + Cl2 HCCl3 + HF → HCFCl2 + HCl HCCl3 + 2 HF → HCF2Cl + 2 HCl HCCl3 + F2 → HCF2Cl + Cl2 HCCl3 + 3 HF → HCF3 + 3 HCl HCCl3 + F2 + HF → HCF3 + Cl2 + HCl CCl4 + HF → CFCl3 + HCl CCl4 + 2 HF → CF2Cl2 + 2 HCl CCl4 + F2 → CF2Cl2 + Cl2 CCl4 + 3 HF → CF3Cl + 3 HCl CCl4 + F2 + HF → CF3Cl + Cl2 + HCl CCl4 + 4 HF → CF4 + 4 HCl CCl4 + F2 + 2 HF → CF4 + Cl2 + 2 HCl CCl4 + 2 F2 → CF4 + 2 Cl2Reaction of methane with hypochlorous acid, producing water.
CH4 + ClOH → CH3Cl + H2OReaction of methanol with hypochlorous acid. CH3OH + ClOH → CH3Cl + H2O2Traces of halomethanes in the atmosphere arise through the introduction of other non-natural, industrial materials. Many marine organisms biosynthesize halomethanes bromine-containing compounds. Small amounts of chloromethanes arise from the interaction of chlorine sources with various carbon compounds; the biosyntheses of these halomethanes are catalyzed by the chloroperoxidase and bromoperoxidase enzymes, respectively. An idealized equation is: CH4 + Cl− + 1/2 O2 → CH3Cl + OH− Halons are defined as hydrocarbons where the hydrogen atoms have been replaced by bromine, along with other halogens, they are referred to by a system of code numbers similar to the system used for freons. The first digit specifies the number of carbon atoms in the molecule, the second is the number of fluorine atoms, the third is the chlorine atoms, the fourth is the number of bromine atoms. If the number includes a fifth digit, the fifth number indicates the number of iodine atoms.
Any bonds not taken up by halogen atoms are allocated to hydrogen atoms. For example, consider Halon 1211: C F Cl Br 1 2 1 1 Halon 1211 has one carbon atom, two fluorine atoms, one chlorine atom and one bromine atom. A single carbon only has four bonds, all of which are taken by the halogen atoms, so there is no hydrogen, thus its formula is CF2BrCl, its IUPAC name is therefore bromochlorodifluoromethane. The refrigerant naming system is used for fluorinated and chlorinated short alkanes used as refrigerants. In the United States, 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: CFC—list of chlorofluorocarbons HCFC—list of hydrochlorofluorocarbons HFC—list of hydrofluorocarbons FC—list of fluorocarbons 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, a, b, or c are added as the masses diverge from normal.
Other coding systems are in use as well. Hydrofluorocarbons contain no chlorine, they are composed of carbon and fluorine. They have no known effects on the ozone layer. However, HFCs and perfluorocarbons are greenhouse gases. Two groups of haloalkanes, hydrofluorocarbons and perfluorocarbons, are targets of the Kyoto Protocol. Allan Thornton, President of the Environmental Investigation Agency, a non-governmental, environmental watchdog, says that HFCs are up to 12,500 times as potent as carbon dioxide in global warming; the higher global warming potential has two causes: HFCs remain in the atmosphere for long periods of time, they have more chemical bon
Bromotrifluoromethane known as Halon 1301, R13B1, Halon 13B1 or BTM, is an organic halide with the chemical formula CBrF3. It is used for fire refrigeration. Human exposure to Halon 1301 can be toxic, affecting the central nervous system and other bodily functions. Additionally, it is known to contribute to the depletion of Earth's atmospheric ozone layer when released; as such Halon's use as a refrigerant has been eliminated and alternatives are being used for fire suppression. Halon 1301 was developed in a joint venture between the U. S. Army and DuPont in 1954, introduced as an effective gaseous fire suppression fixed systems agent in the 1960s, was used around valuable materials, such as aircraft, mainframe computers, telecommunication switching centers in total flooding systems, it was widely used in the maritime industry to add a third level of protection should the main and emergency fire pumps become inoperable or ineffective. Halon 1301 was never used in portables outside military and spacecraft applications, due to its limited range, invisible discharge.
It does not produce the characteristic white cloud like CO2 and is difficult to direct when fighting large fires. Halon 1301 is ideal for armored vehicles and spacecraft, because it produces less toxic by-products than does Halon 1211, critical for combat or space conditions where a compartment may not be able to be ventilated immediately. Halon 1301 is used by the U. S. Military and NASA in a 2-3/4 lb portable extinguisher with a sealed, disposable cylinder for quick recharging. Other agents such as CO2 and E-36 Cryotech wet chemical are replacing halon 1301, due to environmental concerns. Civilian models in 2-3/4, 3, 4 lb sizes were made, it is considered good practice to avoid all unnecessary exposure to Halon 1301, to limit exposures to concentrations of 7 percent and below to 15 minutes. Exposure to Halon 1301 in the 5 to 7 percent range produces little, if any, noticeable effect. At levels between 7 and 10 percent, mild central nervous system effects such as dizziness and tingling in the extremities have been reported.
In practice, the operators of many Halon 1301 total flooding systems evacuate the space on impending agent discharge. There is a risk of the production of toxic and irritant pyrolysis products hydrogen bromide and hydrogen fluoride. Due to lessons learned in the Vietnam War, the use of Halon 1301 began in the F-16 fighter aircraft to prevent vapors in the fuel tanks from becoming explosive. Upon entering areas with possible unfriendly fire, Halon 1301 is injected into the fuel tanks for one-time use. Due to environmental concerns, trifluoroiodomethane is being considered as an alternative; the most effective and used fire protection systems used on commercial aircraft are Halon systems. Halon 1301 is the primary agent used in commercial aviation engine, cargo compartments, auxiliary power unit fire zones. Efforts to find a suitable replacement for Halon 1301 have not produced a accepted replacement. Bromotrifluoromethane was used as a filling of the bubble chamber in the neutrino detector Gargamelle.
Before the dangers of Halon 1301 as an ozone depleter were known, many industrial chillers used it as an efficient refrigerant gas. Alternatives to Halon 1301 in fire extinguishing systems are being deployed. Many installations from which halon is removed can be protected with fire sprinklers, depending on the level of damage the equipment in the space will incur by exposure to water. In other cases, different total flooding agents can be used; the alternatives for occupied areas include, C3F8, HCFC Blend A, HFC-23, HFC-227ea, IG-01, IG-55, HFC-125, or HFC-134a. For unoccupied areas, the alternatives include carbon dioxide, powdered Aerosol C, CF3I, HCFC-22, HCFC-124, HFC-125, HFC-134a, gelled halocarbon/dry chemical suspension, blend of inert gas, high expansion foam systems and powdered aerosol, IG-541. Perfluorocarbons, i.e. PFCs such as C3F8, have long atmospheric lifetimes and high global warming potentials. Hydrochlorofluorocarbons, i.e. HCFCs including HCFC containing NAF S-III, contain chlorine and are stratospheric ozone layer depleters, although less so than Halon 1301.
Their selection for usage as Halon replacements should consider those factors, is restricted in some countries. Halon 1211 Fire extinguisher Montreal Protocol International Chemical Safety Card 0837 NIOSH Pocket Guide to Chemical Hazards. "#0634". National Institute for Occupational Safety and Health. MSDS sheet at airliquide.com FAA paper on testing cylinders used to store Halon 1301 without breaking their seals MSDS for bromotrifluoromethane MSDS at ansul.com Toxicokinetics of Inhaled Bromotrifluoromethane in Human Subjects Effect of CF3H and CF3Br on laminar diffusion flames in normal and microgravity Basic Facts about Halon
The haloalkanes are a group of chemical compounds derived from alkanes containing one or more halogens. They are a subset of the general class of halocarbons, although the distinction is not made. Haloalkanes are used commercially and are known under many chemical and commercial names, they are used as flame retardants, fire extinguishants, propellants and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes which contain chlorine and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases. Methyl iodide, a occurring substance, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, see Halomethane.
Haloalkane or alkyl halides are the compounds which have the general formula "RX" where R is an alkyl or substituted alkyl group and X is a halogen. Haloalkanes have been known for centuries. Chloroethane was produced synthetically in the 15th century; the systematic synthesis of such compounds developed in the 19th century in step with the development of organic chemistry and the understanding of the structure of alkanes. Methods were developed for the selective formation of C-halogen bonds. Versatile methods included the addition of halogens to alkenes, hydrohalogenation of alkenes, the conversion of alcohols to alkyl halides; these methods are so reliable and so implemented that haloalkanes became cheaply available for use in industrial chemistry because the halide could be further replaced by other functional groups. While most haloalkanes are human-produced, non-artificial-source haloalkanes do occur on Earth through enzyme-mediated synthesis by bacteria and sea macroalgae. More than 1600 halogenated organics have been identified, with bromoalkanes being the most common haloalkanes.
Brominated organics in biology range from biologically produced methyl bromide to non-alkane aromatics and unsaturates. Halogenated alkanes in land plants are more rare, but do occur, as for example the fluoroacetate produced as a toxin by at least 40 species of known plants. Specific dehalogenase enzymes in bacteria which remove halogens from haloalkanes, are known. From the structural perspective, haloalkanes can be classified according to the connectivity of the carbon atom to which the halogen is attached. In primary haloalkanes, the carbon that carries the halogen atom is only attached to one other alkyl group. An example is chloroethane. In secondary haloalkanes, the carbon that carries the halogen atom has two C–C bonds. In tertiary haloalkanes, the carbon that carries the halogen atom has three C–C bonds. Haloalkanes can be classified according to the type of halogen on group 7 responding to a specific halogenoalkane. Haloalkanes containing carbon bonded to fluorine, chlorine and iodine results in organofluorine, organochlorine and organoiodine compounds, respectively.
Compounds containing more than one kind of halogen are possible. Several classes of used haloalkanes are classified in this way chlorofluorocarbons, hydrochlorofluorocarbons and hydrofluorocarbons; these abbreviations are common in discussions of the environmental impact of haloalkanes. Haloalkanes resemble the parent alkanes in being colorless odorless, hydrophobic; the melting and boiling points of chloro-, bromo-, iodoalkanes are higher than the analogous alkanes, scaling with the atomic weight and number of halides. This is due to the increased strength of the intermolecular forces—from London dispersion to dipole-dipole interaction because of the increased polarizability, thus carbon tetraiodide is a solid. Many fluoroalkanes, however, go against this trend and have lower melting and boiling points than their nonfluorinated analogues due to the decreased polarizability of fluorine. For example, methane has a melting point of -182.5 °C whereas tetrafluoromethane has a melting point of -183.6 °C.
As they contain fewer C–H bonds, halocarbons are less flammable than alkanes, some are used in fire extinguishers. Haloalkanes are better solvents than the corresponding alkanes because of their increased polarity. Haloalkanes containing halogens other than fluorine are more reactive than the parent alkanes—it is this reactivity, the basis of most controversies. Many are alkylating agents, with primary haloalkanes and those containing heavier halogens being the most active; the ozone-depleting abilities of the CFCs arises from the photolability of the C–Cl bond. Haloalkanes are of wide interest because they are widespread and have diverse beneficial and detrimental impacts; the oceans are estimated to release 1-2 million tons of bromomethane annually. A large number of pharmaceuticals contain halogens fluorine. An estimated one fifth of pharmaceuticals contain fluorine, including several of the most used drugs. Examples include 5-fluorouracil, paroxetine, ciprofloxacin and fluconazole; the beneficial effects arise because the C-F bond is unreactive.
Fluorine-substituted ethers are volatile anesthetics, including the commercial product
Carbon tetrachloride known by many other names is an organic compound with the chemical formula CCl4. It is a colourless liquid with a "sweet" smell, it has no flammability at lower temperatures. It was widely used in fire extinguishers, as a precursor to refrigerants and as a cleaning agent, but has since been phased out because of toxicity and safety concerns. Exposure to high concentrations of carbon tetrachloride can affect the central nervous system, degenerate the liver and kidneys. Prolonged exposure can be fatal. Carbon tetrachloride was synthesized by the French chemist Henri Victor Regnault in 1839 by the reaction of chloroform with chlorine, but now it is produced from methane: CH4 + 4 Cl2 → CCl4 + 4 HClThe production utilizes by-products of other chlorination reactions, such as from the syntheses of dichloromethane and chloroform. Higher chlorocarbons are subjected to "chlorinolysis": C2Cl6 + Cl2 → 2 CCl4Prior to the 1950s, carbon tetrachloride was manufactured by the chlorination of carbon disulfide at 105 to 130 °C: CS2 + 3Cl2 → CCl4 + S2Cl2The production of carbon tetrachloride has steeply declined since the 1980s due to environmental concerns and the decreased demand for CFCs, which were derived from carbon tetrachloride.
In 1992, production in the U. S./Europe/Japan was estimated at 720,000 tonnes. In the carbon tetrachloride molecule, four chlorine atoms are positioned symmetrically as corners in a tetrahedral configuration joined to a central carbon atom by single covalent bonds; because of this symmetrical geometry, CCl4 is non-polar. Methane gas has the same structure, making carbon tetrachloride a halomethane; as a solvent, it is well suited to dissolving other non-polar compounds and oils. It can dissolve iodine, it is somewhat volatile, giving off vapors with a smell characteristic of other chlorinated solvents, somewhat similar to the tetrachloroethylene smell reminiscent of dry cleaners' shops. Solid tetrachloromethane has two polymorphs: crystalline II below −47.5 °C and crystalline I above −47.5 °C. At −47.3 °C it has monoclinic crystal structure with space group C2/c and lattice constants a = 20.3, b = 11.6, c = 19.9, β = 111°. With a specific gravity greater than 1, carbon tetrachloride will be present as a dense nonaqueous phase liquid if sufficient quantities are spilled in the environment.
In organic chemistry, carbon tetrachloride serves as a source of chlorine in the Appel reaction. One specialty use of carbon tetrachloride is in stamp collecting, to reveal watermarks on postage stamps without damaging them. A small amount of the liquid was placed on the back of a stamp, sitting in a black glass or obsidian tray; the letters or design of the watermark could be seen. Carbon tetrachloride was used as a dry cleaning solvent, as a refrigerant, in lava lamps. In case of the latter, carbon tetrachloride is a key ingredient that adds weight to the otherwise buoyant wax, it once was a popular solvent in organic chemistry, because of its adverse health effects, it is used today. It is sometimes useful as a solvent for infrared spectroscopy, because there are no significant absorption bands > 1600 cm−1. Because carbon tetrachloride does not have any hydrogen atoms, it was used in proton NMR spectroscopy. In addition to being toxic, its dissolving power is low, its use has been superseded by deuterated solvents.
Use of carbon tetrachloride in determination of oil has been replaced by various other solvents, such as tetrachloroethylene. Because it has no C-H bonds, carbon tetrachloride does not undergo free-radical reactions, it is a useful solvent for halogenations either by the elemental halogen or by a halogenation reagent such as N-bromosuccinimide. In 1910, the Pyrene Manufacturing Company of Delaware filed a patent to use carbon tetrachloride to extinguish fires; the liquid was vaporized by the heat of combustion and extinguished flames, an early form of gaseous fire suppression. At the time it was believed the gas displaced oxygen in the area near the fire, but research found that the gas inhibits the chemical chain reaction of the combustion process. In 1911, Pyrene patented a portable extinguisher that used the chemical; the extinguisher consisted of a brass bottle with an integrated handpump, used to expel a jet of liquid toward the fire. As the container was unpressurized, it could be refilled after use.
Carbon tetrachloride was suitable for liquid and electrical fires and the extinguishers were carried on aircraft or motor vehicles. In the first half of the 20th century, another common fire extinguisher was a single-use, sealed glass globe known as a "fire grenade," filled with either carbon tetrachloride or salt water; the bulb could be thrown at the base of the flames to quench the fire. The carbon tetrachloride type could be installed in a spring-loaded wall fixture with a solder-based restraint; when the solder melted by high heat, the spring would either break the globe or launch it out of the bracket, allowing the extinguishing agent to be automatically dispersed into the fire. A well-known brand was the "Red Comet,", variously manufactured with other fire-fighting equipment in the Denver, Colorado area by the Red Comet Manufacturing Company from its founding in 1919 until manufacturing operations were closed in the early 1980s. Prior to the Montreal Protocol, large quantities of carbon tetrachloride were used to produce the chlorofluorocarbon re