Halogens, elemental chlorination

The abundant halogen element chlorine is a powerful oxidant used extensively in the chemical industry and for water sterilisation (e.g. communal swimming pools). It was first used as a weapon to incapacitate enemy troops on the battlefield during World War 1 and was recently used by Iraqi insurgents in 2007. Mixing household bleach with certain acidic agents can liberate chlorine gas. 

The first pubHshed information on the halogenation of butyl mbber was provided by B. F. Goodrich Co. (2). Brominating agents such as /V-bromosuccinimide were used the bromination occurred ia a bulk reaction. This technology was commercialized ia 1954, but withdrawn ia 1969 (3). Exxon Chemical researchers pursued the chlorination of butyl mbber ia hexane solution usiag elemental chlorine, and a continuous process was commercialized ia 1961 (4). Currentiy, both chlorination and bromination are carried out ia continuous-solution processes. 

Halobutyls. Chloro- and bromobutyls are commercially the most important butyl mbber derivatives. The halogenation reaction is carried out in hydrocarbon solution using elemental chlorine or bromine (equimolar ratio with enchained isoprene). The halogenation is fast, and proceeds mainly by an ionic mechanism. The stmctures that may form include the following ... 

Halogenated Butyl Rubber. The halogenation is carried out in hydrocarbon solution using elemental chlorine or bromine in a 1 1 molar ratio with enchained isoprene. The reactions ate fast chlorination is faster. Both chlorinated and brominated butyl mbbers can be produced in the same plant in blocked operation. However, there are some differences in equipment and reaction conditions. A longer reaction time is requited for hromination. Separate faciUties are needed to store and meter individual halogens to the reactor. Additional faciUties are requited because of the complexity of stabilising brominated butyl mbber. 

It is important to produce HCl rather than elemental chlorine, CI2, because HCl can be easily scmbbed out of the exhaust stream, whereas CI2 is very difficult to scmb from the reactor off-gas. If the halogenated hydrocarbon is deficient in hydrogen relative to that needed to produce HCl, low levels of water vapor may be needed in the entering stream (45) and an optional water injector may be utilized. For example, trichloroethylene [79-01 -6] C2HCI2, and carbon tetrachloride require some water vapor as a source of hydrogen (45). 

Halogenated compound A compound that contains one or more of the elements chlorine, bromine, fluorine, or iodine as a part of its structure. 

Of all the elements, fluorine is the most chemically reactive. It combines directly with other elements. Chlorine is slightly less reactive. Both are gases at room temperature which is an important advantage in delivery and metering. Because of their reactivity, they form halides readily, but also attack most materials which makes them difficult to handle and requires equipment designed with inert materials such as Monel or Teflon. Halogens are also toxic, fluorine more so than chlorine by an order of magnitude. 

A convenient modification of the Gassman oxindole synthesis was reported using ethyl (methylsulfinyl)acetate (101) activated by oxalyl chloride to generate the same chlorosulfonium salt 102 normally generated from ethyl (methylthio)acetate 100 and elemental chlorine <96TL4631>. Thus, treatment of the sulfoxide 101 with oxalyl chloride, followed by the addition of the desired aniline, triethylamine, and finally acid cyclization of 103 affords the oxindoles 104. This procedure is particularly convenient for reactions carried out on smaller scales and for anilines that ate susceptible to electrophiUc halogenation. 

Fig. 4 a-Halogenations with elemental chlorine Perhaps the next bold step in enamine catalysis... 

As towards hydrogen so towards sulphur the affinity of the halogen elements rapidly decreases on passing from fluorine to iodine. Sulphur hexafluoride, SF6, is a stable substance, whereas the highest chlorine derivative which has been isolated is the unstable tetrachloride, SC14 sulphur monobromide, S2Br2, is the only bromide of sulphur known, whilst sulphur and iodine do not form any definite compound. 

All the halogen elements combine with tellurium. The powdered crystalline form is inflamed by fluorine in the cold 1 and by warm chlorine, the product in the latter case being the tetrachloride.2 With bromine the product is the dibromide, whilst iodine reacts only at a higher temperature, giving a tetra-iodide. Hydrogen chloride does not affect the element. 

Hydrogen telluride is very sensitive towards the halogen elements. It not only readily reduces chlorine, bromine and iodine to the corresponding hydracids with simultaneous liberation of tellurium (which in the case of chlorine can further pass easily into the tetrachloride), but it also reduces solutions of such salts as ferric chloride and mercuric chloride to the lower chlorides, tellurium being precipitated. It also reduces tellurium chlorides, the only products being hydrogen chloride and tellurium. 

The halogenation of a wide variety of aromatic compounds proceeds readily in the presence of ferric chloride, aluminum chloride, and related Friedel-Crafts catalysts. Halogenating agents generally used are elemental chlorine, bromine, or iodine and interhalogen compounds (such as iodine monochloride, bromine monochloride, etc.). These reactions were reviewed554 and are outside the scope of the present discussion. 

The elemental halogens — fluorine, chlorine, bromine, and iodine — are all toxic. Both fluorine and chlorine are highly corrosive gases that are very damaging to exposed tissue. These elements are chemically and toxicologically similar to many of their compounds, such as the interhalogen compounds, discussed in Chapter 11. The toxicities of halogen compounds are discussed in the next two sections. 

Chlorine derivatives of siloxene are also known. The action of elemental chlorine on siloxene is much more vigorous than that of bromine or iodine. In addition to the Si—H bond, the Si—Si bond is also attacked to effect decomposition of the siloxene network. One of the decomposition products is Cl3SiOSiCli34) No fluorine derivatives of siloxene have as yet been described, but many other groups exchange with halogen or hydrogen, e.g. OR, NHR, and SR groups. 

The ability of halogens to form chemical compounds with all metals and most nonmetals has led to a wide variety of uses for these elements. Chlorine is used as a bleach and a disinfectant. Iodine has been used as a topical microbicide. Iodine and bromine are added to halogen lamps to lengthen... 

Many hydrocarbons can be halogenated with elemental chlorine or bromine while being heated and/or irradiated together ... 

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