Chlorofluorocarbon
Chlorofluorocarbons and hydrochlorofluorocarbons are fully or partly halogenated hydrocarbons that contain carbon, hydrogen, chlorine, and fluorine. They are produced as volatile derivatives of methane, ethane, and propane.
The most common example of a CFC is dichlorodifluoromethane. R-12, also commonly called Freon, is used as a refrigerant. Many CFCs have been widely used as refrigerants, propellants, gaseous fire suppression systems, and solvents. As a result of CFCs contributing to ozone depletion in the upper atmosphere, the manufacture of such compounds has been phased out under the Montreal Protocol, and they are being replaced with other products such as hydrofluorocarbons and hydrofluoroolefins including R-410A, R-134a and R-1234yf.
Structure, properties and production
As in simpler alkanes, carbons in CFCs bond with tetrahedral symmetry. Because the fluorine and chlorine atoms differ greatly in size and effective charge from hydrogen and from each other, methane-derived CFCs deviate from perfect tetrahedral symmetry.The physical properties of CFCs and HCFCs can be affected by changes in the number and identity of the halogen atoms. They are generally volatile, but less so than their parent alkanes. The decreased volatility is attributed to the molecular polarity induced by the halides, which induces intermolecular interactions. Thus, methane boils at −161 °C whereas the fluoromethanes boil between −51.7 and −128 °C. CFCs still have higher boiling points because the chloride is even more polarizable than fluoride. Because of their polarity, CFCs are useful solvents, and their boiling points make them suitable as refrigerants. CFCs are far less flammable than methane, in part because they contain fewer C–H bonds and in part because, in the case of the chlorides and bromides, the released halides quench the free radicals that sustain flames.
The densities of CFCs are higher than their corresponding alkanes. In general, the density of these compounds correlates with the number of chlorides.
CFCs and HCFCs are usually produced by halogen exchange starting from chlorinated methanes and ethanes. Written below is the synthesis of chlorodifluoromethane from chloroform:
Brominated derivatives are generated by free-radical reactions of hydrochlorofluorocarbons, replacing C–H bonds with C–Br bonds. The production of the anesthetic 2-bromo-2-chloro-1,1,1-trifluoroethane is written out below:
Applications
CFCs and HCFCs are used in various applications because of their low toxicity, reactivity and flammability. Every permutation of fluorine, chlorine and hydrogen based on methane and ethane has been examined and most have been commercialized. Furthermore, many examples are known for higher numbers of carbon as well as related compounds containing bromine. Uses include refrigerants, blowing agents, aerosol propellants in medicinal applications, and degreasing solvents.Billions of kilograms of chlorodifluoromethane are produced annually as a precursor to tetrafluoroethylene, the monomer that is converted into Teflon.
Classes of compounds and Numbering System
- Chlorofluorocarbons : when derived from methane and ethane, these compounds have the formulae and, where m is nonzero.
- Hydro-chlorofluorocarbons : when derived from methane and ethane, these compounds have the formula and, where m, n, x, and y are nonzero.
- Bromofluorocarbons : have formulae similar to the CFCs and HCFCs, but also include bromine.
- Hydrofluorocarbons : when derived from methane, ethane, propane, and butane, these compounds have the respective formulae,,, and, where m is nonzero.
Numbering system
Freon-12, for example, indicates a methane derivative containing two fluorine atoms and no hydrogen. It is therefore.
Another equation that can be applied to get the correct molecular formula of the CFC/R/Freon class compounds is to take the numbering and add 90 to it. The resulting value will give the number of carbons as the first numeral, the second numeral gives the number of hydrogen atoms, and the third numeral gives the number of fluorine atoms. The rest of the unaccounted carbon bonds are occupied by chlorine atoms. The value of this equation is always a three figure number. An easy example is that of CFC-12, which gives: 90+12=102 -> 1 carbon, 0 hydrogens, 2 fluorine atoms, and hence 2 chlorine atoms resulting in. The main advantage of this method of deducing the molecular composition in comparison with the method described in the paragraph above is that it gives the number of carbon atoms of the molecule.
Freons containing bromine are signified by four numbers. Isomers, which are common for ethane and propane derivatives, are indicated by letters following the numbers:
Reactions
The reaction of the CFCs which is responsible for the depletion of ozone, is the photo-induced scission of a C-Cl bond:The chlorine atom, written often as Cl•, behaves very differently from the chlorine molecule. The radical Cl• is long-lived in the upper atmosphere, where it catalyzes the conversion of ozone into. Ozone absorbs UV-B radiation, so its depletion allows more of this high energy radiation to reach the Earth's surface. Bromine atoms are even more efficient catalysts; hence brominated CFCs are also regulated.
Impact as greenhouse gases
CFCs were phased out via the Montreal Protocol due to their part in ozone depletion.The atmospheric impacts of CFCs are not limited to their role as ozone-depleting chemicals. Infrared absorption bands prevent heat at that wavelength from escaping Earth's atmosphere. CFCs have their strongest absorption bands from C-F and C-Cl bonds in the spectral region of 7.8–15.3 μm—referred to as the "atmospheric window" due to the relative transparency of the atmosphere within this region.
The strength of CFC absorption bands and the unique susceptibility of the atmosphere at wavelengths where CFCs absorb radiation creates a "super" greenhouse effect from CFCs and other unreactive fluorine-containing gases such as perfluorocarbons, HFCs, HCFCs, bromofluorocarbons,, and. This "atmospheric window" absorption is intensified by the low concentration of each individual CFC. Because is close to saturation with high concentrations and few infrared absorption bands, the radiation budget and hence the greenhouse effect has low sensitivity to changes in concentration; the increase in temperature is roughly logarithmic. Conversely, the low concentration of CFCs allow their effects to increase linearly with mass, so that chlorofluorocarbons are greenhouse gases with a much higher potential to enhance the greenhouse effect than.
Groups are actively disposing of legacy CFCs to reduce their impact on the atmosphere.
According to NASA in 2018, the hole in the ozone layer has begun to recover as a result of CFC bans. However, research released in 2019 reported an alarming increase in CFCs, pointing to unregulated use in China.
History
Prior to, and during the 1920s, refrigerators used toxic gases as refrigerants, including ammonia, sulphur dioxide, and chloromethane. Later in the 1920s after a series of fatal accidents involving the leaking of chloromethane from refrigerators, a major collaborative effort began between American corporations Frigidaire, General Motors, and DuPont to develop a safer, non-toxic alternative. Thomas Midgley Jr. of General Motors is credited for synthesizing the first chlorofluorocarbons. The Frigidaire corporation was issued the first patent, number 1,886,339, for the formula for CFCs on December 31, 1928. 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 in 1930.By 1930, General Motors and Du Pont formed the Kinetic Chemical Company to produce Freon, and by 1935, over 8 million refrigerators utilizing R-12 were sold by Frigidaire and its competitors. In 1932, Carrier began using R-11 in the worlds first self-contained home air conditioning unit known as the "atmospheric cabinet". As a result of CFCs being largely non-toxic, they quickly became the coolant of choice in large air-conditioning systems. Public health codes in cities were revised to designate chlorofluorocarbons as the only gases that could be used as refrigerants in public buildings.
Growth in CFCs continued over the following decades leading to peak annual sales of over 1 billion USD with greater than 1 million metric tonnes being produced annually. It wasn't until 1974 that it was first discovered by two University of California chemists, Professor F. Sherwood Rowland and Dr. Mario Molina, that the use of chlorofluorocarbons were causing a significant depletion in atmospheric ozone concentrations. This initiated the environmental effort which eventually resulted in the enactment of the Montreal Protocol.
Commercial development and use in fire extinguishing
During World War II, various chloroalkanes were in standard use in military aircraft, although these early halons suffered from excessive toxicity. Nevertheless, after the war they slowly became more common in civil aviation as well. In the 1960s, fluoroalkanes and bromofluoroalkanes became available and were quickly recognized as being highly effective fire-fighting materials. 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.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 expressed about the impact of chloroalkanes and bromoalkanes on the ozone layer. The Vienna Convention for the Protection of the Ozone Layer did not cover bromofluoroalkanes under the same restrictions, because emergency discharge of extinguishing systems was thought to be too small in volume to produce a significant impact and too important to human safety for restriction. Instead, the consumption of bromofluoroalkanes was frozen at 1986 levels.