Problems Fluorine

B. Selenium in the form of selenocysteine is required for three enzymes that remove iodide from thyroid hormones. There are no signihcant areas in which dietary intake of sodium or potassium are problems. Fluorine deficiency is not associated with thyroid hormone metabolism. 

Direct fluorination of organic compounds using fluorine is an important methodology when considering safety, environmental and recycling problems. Fluorine can be fed into the reactor from a cylinder for the direct fluorination procedure, but this method has economic and safety disadvantages. Electrochemical generation of fluorine has recently been adopted for fluorination even in the laboratory. It can be operated easily and safely, because operational difficulties have been improved. 

Effluents and gypsum disposal pose problems. Fluorine is evolved at various steps in the process and scrubbers are required to reduce release to the atmosphere. Gypsum is frequently piled in diked areas or dumped into abandoned mines. Wastewater from these plants is heavily contaminated with fluorine, phosphates, sulfates, and other compounds. It is commonly impounded in large ponds, where a portion of the contaminants may precipitate or be lost by other processes. The cooled effluent from the... 

Many various types of industrial activities result in pollution of the atmosphere. The furnaces at the earlier mines were sources of pollution. Modern electrochemical industry brought other types of problems. Fluorine exhausted from aluminum factories has caused great damage in the vicinity. Some of these factories emit organic components that may be dangerous to health. Smoke from ferrosilicium, ferromanganese, and ferrochrome factories is really noticeable. Many factories which refine heavy metals spread polluting material. 

Fluorination is thus accompanied by degradation of hydrocarbon chains. Because of these problems, fluorination with elemental fluorine is rarely attempted in laboratory syntheses. 

What about diatomic fluorine, F-F For many years, elemental fluorine was thought to be too reactive and too dangerous for reaction with alkenes. To void such problems, fluorine is typically mixed with an inert gas such as nitrogen or argon. Diluted in this manner, fluorine does react with alkenes, but the yields are often poor and, in some cases, solvents for the alkene, such as methanol, participate in the reaction. 1-Phenylpropene (PhCH=CH2), for example, reacted with fluorine in methanol to give 51% of the corresponding difluoride, along with 49% of 2-fluoro-l-methoxy-l-phenylpropane. The problems associated with fluorine lead to a simpliflcation. In this chapter, alkene reactions are reported only for chlorine, bromine, or iodine but not fluorine. 

Kinetic as weU as thermodynamic problems are encountered in fluorination. The rate of reaction must be decelerated so that the energy Hberated may be absorbed or carried away without degrading the molecular stmcture. The most recent advances in direct fluorination ate the LaMar process (18—20) and the Exfluot process (21—24), which is practiced commercially by 3M. 

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

Manufacture. One commercial process features a three-stage saturation—rearomatization technique using benzene and fluorine gas as raw materials (73). Principal problems with this method are the complex nature of the process, its dependence on fluorine gas which is cosdy to produce, and the poor overall utilization of fluorine, because nearly one-half of the input fluorine is removed during the process. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Ha/ogenation. Heats of reaction are highly exothermic for halogens, particularly fluorine (qv), and chain reactions can result in explosions over broad concentration ranges. Halogens also present severely challenging corrosion problems (see Corrosion and corrosion control). 

Fluorination presents problems of the other extreme. Both steps in the substitution chain reaction are so exothermic that the reaction is violent if not performed under carefully controlled conditions. Furthermore, fluorine atoms are capable of cleaving carbon-carbon bonds ... 

Synthetic cryolite solved the supply problem, but synthetic cryolite requires fluorine which is actually more abundant in the Earth s crust than chlorine, but dispersed in small concentrations in rocks. Until the 1960s, fluorspar (CaFj) a mineral long known and used as a flux in various metallurgical operations was the source. A source is phosphate rock that contains fluorine i.s 3% quantity,... 

Organic fluorine as a separate industry that began in the late 1920s with the discovery by Midgle of fluorocarbons for use as refrigerants. Ammonia was unsuitable because of the hazard and unpleasant smell from minute leaks. Sulfur dioxide had similar problems. The best refrigerant was... 

The discussion of acylation reactions in this chapter is focused on fluonnated carboxylic acid derivatives and their use to build up new fluorine-containing molecules of a general preparative interest Fifteen years ago, fluonnated carboxylic acids and their derivatives were used mainly for technical applications [/] Since then, an ever growing interest for selectively fluonnated molecules for biological applications [2, 3, 4, 5] has challenged many chemists to use bulk chemicals such as tnfluoroacetic acid and chlorodifluoroacetic acid as starting materials for the solution of the inherent synthetic problems [d, 7,, 9]... 

Application of an excessive amount of ammonia solution in the precipitation of tantalum and niobium hydroxides from strip solutions usually ensures good quality of the products. Nevertheless, the method has two general problems. First, hydroxides containing low levels of fluorine contamination... 

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