Hypohalates

Hypohalite ion Hypohalite ions Hypohalous acid Hypokalemia... 

In contrast to reaction of ozone with nucleophilic haUde and hypohaUte ions, reaction of ozone with electrophilic hypohalous acids is very slow. 

Piddition to the double bond occurs readily with hydrogen haUdes, hypohalous, sulfuric, or formic acids (53)... 

However, as HOX is consumed, hydrolysis is retarded because of build-up of free amine. Consumption of hypohalous acid through reaction with HX can result in formation of elemental halogen HOX + + X X2 + H2 O (1—3). The tendency for halogen formation is much greater for HOBr and... 

Disinfection. The disinfection efficiency of /V-ha1amines is related to the extent of hydrolysis to hypohalous acid. For example, NH2CI (Kf 10 ) is a poor bactericide compared to HOCl (5). By contrast, monochloroisocyanurate 10 ) exhibits good bactericidal properties (6). 

Since hypohalous acid is a much more effective disinfectant than hypohaUte, pH affects the disinfection efficiency. The fraction of hypohalous acid for aqueous chlorine and bromine is HOX/(HOX + XO ) = 1 — + where the pfC s of HOCl and HOBr at 25°C are 7.54 (2) and 8.70,... 

Although pH determines the ratio of hypohalous acid to hypohaUte ion, the fraction of the total available halogen present as HOX is dependent on of the halamine as well as the concentration of excess amine. In the case of chloroisocyanurates, which are the most widely used /V-ch1oramine disinfectants in swimming pools and spas, the extent of hydrolysis at 1 ppm av CI2 (as monochloroisocyanurate) is - 34% but only - 1% when 25 ppm cyanuric acid is added (4). Nevertheless, effective disinfection can stiU occur with chloroisocyanurates if a sufficient FAC is maintained, eg, 1—3 ppm. The observed reduction in disinfection rate because of cyanuric acid (6) has been shown to be direcdy related to the concentration of HOCl formed by hydrolysis of chloroisocyanurates (10). 

Preparation. Substituted A/-halamines are usually prepared by reaction of RR NH with halogen, hypohalous acid, or hypohaUte where R is an organic substituent and R is either an organic substituent or H. 

The critical intermediate (64) for this route (Scheme 65) can be reached in several ways. The commonest is by the reaction of an alkene with hypohalous acid (Scheme 66) which proceeds with overall retention. 

The latter is an exttemely reactive species. Trifluoroacetate is a good leaving group and facilitates cleavage of the O—Br bond. The acyl hypohalites are also the active halogenating species in solutions of the hypohalous acids in carboxylic acids, where they exist in equilibrium. 

Since hypohalous acid is a much more active disinfectant than the hypohalite ion, the effect of pH on ionization becomes important. Hypobromous acid has a lower ionization value than hypochlorous acid and this contributes to the higher disinfectant activity of BrCl compared with chlorine. 

Formation of oxiranes on the sterically more hindered side of the steroid ring system is usually carried out via /raw -halohydrins which afford oxiranes on treatment with base (c -Halohydrins yield ketones on exposure to base). Two general methods are available for the synthesis of tm s-halohydrins (1) the reduction of a-halo ketones and (2) the addition of a hypohalous acid to unsaturated steroids. 

The manner and rate of decomposition of hypohalous acids (and hypohalite ions) in solution are much influenced by the concentration, pH and temperature of the solutions, by the presence or absence of salts which can act as catalysts, promotors or activators, and by light quanta. The main competing modes of decomposition are ... 

Other uses of hypohalous acids and hypohalites are described in the Panel. 

This section concludes with a reminder that, in addition to the hypohalous acids HOX and metal hypohalites M(OX) , various covalent (molecular) hypohalites are known. Hypochlorites are summarized in Table 17.22. All are volatile liquids or gases at room temperature and are discussed elsewhere (see Index). Organic hypohalites are unstable and rapidly expel HX or RX to form the corresponding aldehyde or ketone ... 

In addition to the applications indicated on p. 858. hypohalous acids are useful halogenating agents for Ixjth aromatic and aliphatic compounds. HOBr and HOI are usually generated in silii. The ease of aromatic halogenation increa.ses in the sequence OCl < OBr < Ol and is facilitated by salts of Pb or Ag. Another well-known reaction of hypohalites is their cleavage of methyl ketones to form carboxylates and haloform ... 

Addition of Hypohalous Acids to Alkenes Halohydrin Formation... 

ThomsorWOW Click Organic Interactive to use a web-based palette to predict products of the addition of hypohalous acid to alkenes. 

Yet another example of an electrophilic addition is the reaction of alkenes with the hypohalous acids HO—Cl or HO-Br to yield 1,2-halo alcohols, called halohydrins. Halohydrin formation doesn t take place by direct reaction of an alkene with HOBr or HOC1, however. Rather, the addition is done indirectly by reaction of the alkene with either Br2 or Cl2 in the presence of water. 

The addition of mineral acids to hypohalous acids produces a large increase in the rate at which these latter acids halogenate and reaction under these conditions is usually referred to as positive haiogenation which has been subjected to intensive kinetic studies. Whilst there is ample evidence supporting the existence... 

The acid strengths and oxidizing abilities of the halogen oxoacids increase with the oxidation number of the halogen. The hypohalous acids, HXO (halogen oxidation number +1), are prepared by direct reaction of the halogen with water. For example, chlorine gas disproportionates in water to produce hypochlorous acid and hydrochloric acid ... 

Addition of Hypohalous Acids and Hypohalites (Addition of Halogen, Oxygen)... 

Cleavage of epoxides with hydrogen halides Addition of hypohalous acids to alkenes... 

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