Generation of hypochlorous acid

Generation of hypochloric acid at the anode allows allyl substitution via an... 

Another important neutrophil function that can influence the protease-antiprotease balance is the generation of hypochlorous acid (HOCl) and N-chloramines via the myeloperoxidase-H202-halide system. HOCl and N-chloramines efficiently oxidize or chlorinate a wide number of molecules including both a2-macroglobulin and a 1-antiprotease (43,44). Up to 2500 to 5000 nmol of HOCl can be generated by as few as 25 x 106 neutrophils (45). Furthermore, the ability of HOCl and N-chloramines to activate latent MMPs may shift the protease-anti-protease equilibrium further towards degradation (46,47). 

In two proposed alternative processes, the chlorine is replaced in the hypochlorination reaction by hypochlorous acid [7790-92-3] HOCl, or tert-huty hypochlorite. In the first, a concentrated (>10% by weight) aqueous solution of hypochlorous acid, substantially free of chloride, chlorate, and alkah metal ions, is contacted with propylene to produce propylene chlorohydrin (113). The likely mechanism of reaction is the same as that for chlorine, as chlorine is generated in situ through the equiUbrium of chlorine and hypochlorous acid (109). 

Emerging technologies for the commercial manufacture of chloric acid fall into three categories (/) generation of high purity chloric acid by thermal decomposition of pure solutions of hypochlorous acid [7790-92-3], HCIO (5). 

Figure 17.27 Summary of the mechanisms used by phagocytes to generate agents that kill phagocytosed pathogens. These are (i) formation of the superoxide radicals, (ii) formation of per-oxynitrite by interaction between nitric oxide and the superoxide radical, (iii) formation of hypochlorous acid by interaction between the superoxide radical and the chloride ion.

In cooling water applications, great importance is placed on activity against Legionella pneumophila, the causative agent of Legionnaires disease. Bromochloro dimethylhydantoin has been shown to rapidly hydrolyze in water with the formation of hypobromous acid (21). The pKa of hypobromous acid is 8.8, whereas the pK of hypochlorous acid is 7.4. Because the undissociated hypohalous acid is the active biocide, the hypobromous-generating chemical is more active in alkaline systems. 

An on-site generation technique for hypochlorite solutions has recently been developed. This system consists of a two-cell unit, in which a brine solution (salt in water) is electrolyzed, producing a solution of hypochlorous acid in one cell and a solution of caustic (sodium hydroxide) in the other, according to the following equation ... 

Below pH 5, hypochlorous acid disproportionates to chloride and chlorate faster than hypochlorite above pH 11, but slower than between pH 5-9. In addition, the chloride that forms rapidly reacts with hypochlorous acid to generate chlorine, and chlorine becomes the major product of the decomposition of hypochlorous acid. 

Electrolytic cells are also used in a variety of devices - and appliances " to produce dilute solutions of electrolyzed water for cleaning and sanitization. Miniature, battery-powered cells are used to generate hypochlorite in handheld sprayers and small portable water disinfection devices. " " The brine may be saturated or have 1-5 g L of salt. An acidic solution of chlorine, hypochlorous acid, and possibly chlorine dioxide is obtained from the anode of a divided cell. It typically has 10-100 mg L of available chlorine and pH values of 2-4. Its stability is poor, and the volatile oxidants are rapidly lost from open solutions. A neutral solution of hypochlorous acid and sodium hypochlorite is dispensed from undivided cells or by combining effluents from the anode and cathode. It typically has 80-100 mg L of available chlorine and pH 5-8. Devices to make 500-1000 mg L" of available chlorine are also available. Neutral solutions made using larger versions of these devices are bottled and sold in some regions. 

In the second proposed alternative process, tert-huty hypochlorite, formed from the reaction of chlorine and tert-huty alcohol, reacts with propylene and water to produce the chlorohydrin. The alcohol is a coproduct and is recycled to generate the hypochlorite (114—116). No commercialisation of the hypochlorous acid and tert-huty hypochlorite processes for chlorohydrin production is known. 

Chlorine gas is usually used, but electrolysis of alkaline salt solutions in which chlorine is generated in situ is also possible and may become more important in the future. The final pH of solutions to be sold or stored is always adjusted above 11 to maximize stabiUty. The salt is usually not removed. However, when the starting solution contains more than 20.5% sodium hydroxide some salt precipitates as it is formed. This precipitate is removed by filtration to make 12—15% NaOCl solutions with about one-half of the normal amount of salt. Small amounts of such solutions are sold for special purposes. Solutions with practically no salt can be made by reaction of high purity hypochlorous acid with metal hydroxides. 

Hypochlorous acid undergoes a variety of reactions with organic substances including addition, oxidation, C- and iV-chlorination, and ester formation. On an industrial scale, hypochlorous acid, generated m situ via chlorine hydrolysis, reacts with propylene forming primarily the a-propjlene chlorohydrin isomer. 

Hypochlorous acid, preformed or generated in situ from chlorine and water, is employed in the manufacture of chlorohydrins (qv) from olefins, en route to epoxides, and in the production of chloramines (qv), especially chloroisocyanurates from cyanuric acid (see Cyanuric and isocyanuric acids). 

It is well established that aerobes constantly produce small amounts of oxygen-derived species, such as the superoxide radical (02 ), hydrogen peroxide (H2O2) and hypochlorous acid (HOCl), the latter being generated by... 

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