have potent ozone depletion potential due to the homolytic cleavage of the carbon-chlorine bonds; their use is largely prohibited by the montreal protocol. hydrofluorocarbons (hfcs), such as tetrafluoroethane, serve as cfc replacements because they do not catalyze ozone depletion. oxygen exhibits a high solubility in perfluorocarbon compounds, reflecting on their lipophilicity. perfluorodecalin has been demonstrated as a blood substitute transporting oxygen to the lungs. the solvent 1,1,1,2-tetrafluoroethane has been used for extraction of natural products such as taxol, evening primrose oil, and vanillin. 2,2,2-trifluoroethanol is an oxidation-resistant polar solvent. organofluorine reagents the development of organofluorine chemistry has contributed many reagents of value beyond organofluorine chemistry. triflic acid (cf3so3h) and trifluoroacetic acid (cf3co2h) are useful throughout organic synthesis. their strong acidity is attributed to the electronegativity of the trifluoromethyl group that stabilizes the negative charge. the triflate-group (the conjugate base of the triflic acid) is a good leaving group in substitution reactions. fluorous phases of topical interest in the area of "green chemistry," highly fluorinated substituents, e.g. perfluorohexyl (c6f13) confer distinctive solubility properties to molecules, which facilitates purification of products in organic synthesis. this area, described as "fluorous chemistry," exploits the concept of like-dissolves-like in the sense that fluorine-rich compounds dissolve preferentially in fluorine-rich solvents. because of the relative inertness of the c-f bond, such fluorous phases are compatible with even harsh reagents. this theme has spawned techniques of "fluorous tagging and fluorous protection. illustrative of fluorous technology is the use of fluoroalkyl-substituted tin hydrides for reductions, the products being easily separated from the spent tin reagent by extraction using fluorinated solvents. hydrophobic fluorinated ionic liquids, such as organic salts of bistriflimide or hexafluorophosphate, can form phases that are insoluble in both water and organic solvents, producing multiphasic liquids. organofluorine ligands in transition metal chemistry organofluorine ligands have long been featured in organometallic and coordination chemistry. one advantage to f-containing ligands is the convenience of 19f nmr spectroscopy for monitoring reactions. the organofluorine compounds can serve as a "sigma-donor ligand," as illustrated by the titanium(iii) derivative bph4. most often, however, fluorocarbon substituents are used to enhance the lewis acidity of metal centers. a premier example is "eufod," a coordination complex of europium(iii) that features a perfluoroheptyl modified acetylacetonate ligand. this and related species are useful in organic synthesis and as "shift reagents" in nmr spectroscopy. structure of +, a coordination complex of an organofluorine ligand. in an area where coordination chemistry and materials science overlap, the fluorination of organic ligands is used to tune the properties of component molecules. for example, the degree and regiochemistry of fluorination of metalated 2-phenylpyridine ligands in platinum(ii) complexes significantly modifies the emission properties of the complexes. the coordination chemistry of organofluorine ligands also embraces fluorous technologies. for example, triphenylphosphine has been modified by attachment of perfluoroalkyl substituents that confer solubility in perfluorohexane as well as supercritical carbon dioxide. as a specific example, c-f bond activation an active area of organometallic chemistry encompasses the scission of c-f bonds by transition metal-based reagents. both stoichiometric and catalytic reactions have been developed and are of interest from the perspectives of organic synthesis and remediation of xenochemicals. c-f bond activation has been classified as follows "(i) oxidative addition of fluorocarbon, (ii) m–c bond formation with hf elimination, (iii) m–c bond formation with fluorosilane elimination, (iv) hydrodefluorination of fluorocarbon with m–f bond formation, (v) nucleophilic attack on fluorocarbon, and (vi) defluorination of fluorocarbon". an illustrative metal-mediated c-f activation reaction is the defluorination of fluorohexane by a zirconium dihydride, an analogue of schwartz's reagent: (c5me5)2zrh2 + 1-fc6h13 → (c5me5)2zrh(f) + c6h14 fluorocarbon anions in ziegler-natta catalysis fluorine-containing compounds are often featured in noncoordinating or weakly coordinating anions. both tetrakis(pentafluorophenyl)borate, b(c6f5)4−, and the related tetrakisborate, are useful in ziegler-natta catalysis and related alkene polymerization methodologies. the fluorinated substituents render the anions weakly basic and enhance the solubility in weakly basic solvents, which are compatible with strong lewis acids. materials science organofluorine compounds enjoy many niche applications in materials science. with a low coefficient of friction, fluid fluoropolymers are used as specialty lubricants. fluorocarbon-based greases are used in demanding applications. representative products include fomblin and krytox, made by solvay solexis and dupont, respectively. certain firearm lubricants such as "tetra gun" contain fluorocarbons. capitalizing on their nonflammability, fluorocarbons are used in fire fighting foam. organofluorine compounds are components of liquid crystal displays. the polymeric analogue of triflic acid, nafion is a solid acid that is used as the membrane in most low temperature fuel cells. the bifunctional monomer 4,4'-difluorobenzophenone is a precursor to peek-class polymers. biosynthesis of organofluorine compounds in contrast to the many naturally-occurring organic compounds containing the heavier halides, chloride, bromide, and iodide, only a handful of biologically synthesized carbon-fluorine bonds are known. the most common natural organofluorine species is fluoroacetate, a toxin found in a few species of plants. others include fluorooleic acid, fluoroacetone, nucleocidin (4'-fluoro-5'-o-sulfamoyladenosine), fluorothreonine, and 2-fluorocitrate. several of these species are probably biosynthesized from fluoroacetaldehyde. the enzyme fluorinase catalyzed the synthesis of 5'-deoxy-5'-fluoroadenosine (see scheme to right). history organofluorine chemistry began in the 1800s with the development of organic chemistry. the first organofluorine compounds were prepared using antimony trifluoride as the f− source. the nonflammability and nontoxicity of the chlorofluorocarbons ccl3f and ccl2f2 attracted industrial attention in the 1920s. on april 6, 1938, roy j. plunkett a young research chemist who worked at dupont's jackson laboratory in deepwater, new jersey, accidentally discovered polytetrafluoroethylene (ptfe). subsequent major developments, especially in the us, benefited from expertise gained in the production of uranium hexafluoride. starting in the late 1940s, a series of electrophilic fluorinating methodologies were introduced, beginning with cof3. electrochemical fluorination ("electrofluorination") was announced, which joseph h. simons had developed in the 1930s to generate highly stable perfluorinated materials compatible with uranium hexafluoride. these new methodologies allowed the synthesis of c-f bonds without using elemental fluorine and without relying on metathetical methods. in 1957, the anticancer activity of 5-fluorouracil was described. this report provided one of the first examples of rational design of drugs. this discovery sparked a surge of interest in fluorinated pharmaceuticals and agrichemicals. the discovery of the noble gas compounds, e.g. xef4, provided a host of new reagents starting in the early 1960s. in the 1970s, fluorodeoxyglucose was established as a useful reagent in 18f positron emission tomography. in nobel prize-winning work, cfc's were shown to contribute to the depletion of atmospheric ozone. this discovery alerted the world to the negative consequences of organofluorine compounds and motivated the development of new routes to organofluorine compounds. in 2002, the first c-f bond-forming enzyme, fluorinase, was reported. environmental and health concerns only a few organofluorine compounds are acutely bioactive and highly toxic, such as fluoroacetate and perfluoroisobutene. some organofluorine compounds pose significant risks and dangers to health and the environment. cfcs and hcfcs (hydrochlorofluorocarbon) deplete the ozone layer and are potent greenhouse gases. hfcs are potent greenhouse gases and are facing calls for stricter international regulation and phase out schedules as a fast-acting greenhouse emission abatement measure, as are perfluorocarbons (pfcs), and sulphur hexafluoride (sf6). because of the compound's effect on climate, the g-20 major economies agreed in 2013 to support initiatives to phase out use of hcfcs. they affirmed the roles of the montreal protocol and the united nations framework convention on climate change in global hcfc accounting and reduction. the u.s. and china at the same time announced a bilateral agreement to similar effect. persistence and bioaccumulation because of the strength of the carbon–fluorine bond, many synthetic fluorocarbons and fluorocarbon-based compounds are persistent in the environment. fluorosurfactants, such as pfos and pfoa, are persistent global contaminants. fluorocarbon based cfcs and tetrafluoromethane have been reported in igneous and metamorphic rock. pfos is a persistent organic pollutant and may be harming the health of wildlife; the potential health effects of pfoa to humans are under investigation by the c8 science panel.