Generation of Halogens

Numerous cases of electrochemical halogenation (chlorination, bromination, iodination) are known and have been discussed in some detail by Allen 6 Generally, these reactions appear to be indirect electrolytic processes, since in most cases the substrates are oxidized at higher potentials than chloride, bromide, and iodide ion. More work is needed, however, to clarify anodic halogenation mechanism. 

Electrochemical fluorination 168,169 is a commercial process for perfluorina-tion of aliphatic compounds. The reaction is performed in liquid hydrogen fluoride -potassium fluoride at a nickel anode. The mechanism is not known free fluorine cannot be detected during electrolysis, so it seems probable that fluorination is a direct electrochemical reaction. Theoretically, hydrogen fluoride-potassium fluoride should be a very oxidation-resistant SSE, and it might well be that the mechanism is analogous to that proposed for anodic acetamidation of aliphatic compounds in acetonitrile-tetrabutylammonium hexafluorophosphate 44 K 

There has been a great deal of commercial interest in an electrochemical process for the production of propylene oxide from propene via anodic generation of halogen 70 171). The reactions are summarized below ( X = Cl, Br)  

This process is said to compete favorably with the existing commercial procedure. 

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Reactivity is enhanced in conditions which promote the generation of halogen atoms, though this does not imply that all reactions proceed via the intermediacy of X atoms. The reversible thermal dissociation of gaseous I2 v 21 was... 

Two other hydrogen peroxide activation routes which need to be mentioned in this section are Caro s acid and the generation of halogens with hydrogen peroxide. The general schemes for their manufacture are shown in Figure 2.34. 

Figure 3.105 In situ generation of halogen for the preparation of organo-halogen compounds.

Schulz, S. and H. H. Hahn (1997). Generation of halogenated organic compounds in municipal waste water. Water Science TechnoL, Proc. 1997 2nd lAWQ Int. Conf. on the Sewer as a Physical, Chemical and Biological Reactor, May 25-28, 37,1, 303-309. Elsevier Science Ltd., Exeter, England. 

Stereospecific generation of halogenated and tosylated medium size rings (90) by ringopening of gem-dibromocyclopropanes (89) was observed on heating with silver fluoride and tosylate in refluxing acetonitrile, respectively (equation 56) . 

For generation of halogen. 2,6-Dichloroaniline is prepared by dichlorination of sulfanilamide and hydrolytic removal of the sulfonamide group. The required... 

Low-level chlorination of 0.5-1.5 mg/1, resulting in a chlorine residual of 0.1 -0.2 mg/1, is used to reduce the degree of biofouling. In sea-water, chlorine oxidizes bromide, present at about 65 mg/1, to bromine, which also contributes to the generation of halogenated by-products [176]. The by-products include hypobromous acid, hypobromite, chloramines, bromamines, trihalomethanes, haloacetonitriles,haloacetic acids, and small amounts of halophenols. However, some haloforms and bromophenols as well as other organobromine compounds are also produced naturally in coastal waters [ 181,182]. Empirical equations for the disappearance of chlorine/bromine derived oxidants from brackish water have been published [183]. 

Scheme 6.44. A generalized representation of allylic halogenation. Although photochemical generation of halogen can be effective, it is more common to use N-halosuccinimides.

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