Hypochlorous acid, chlorination

The acid addition shifts the chlotine solution equihbtium to favor molecular chlorine. The hypochlorous acid—chlorine equihbria is... 

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

Cbemical/Physical. The aqueous chlorination of indole by hypochlorite/hypochlorous acid, chlorine dioxide, and chloramines produced oxindole, isatin, and possibly 3-chloroindole (Lin and Carlson, 1984). 

Hypochlorous acid - [CHLORINE OXYGEN ACIDS AND SALTS - CHLORIC ACID AND CHLORATES] (Vol 5) - [CHLOROHYDRINS] (Vol 6) - [CHLOROHYDRINS] (Vol 6)... 

The direct oxidation of propylene on silver catalysts has been intensively investigated, but has failed to provide results with commercial potential. Selectivities are generally too low and the isolation of propylene oxide is complicated by the presence of many by-products. The best reported selectivities are in the range 50-60% for less than 9% propylene conversion. The relatively low selectivity arises from the high temperature necessary for the silver catalysts, the radical nature of molecular oxygen, as well as the allylic hydrogens in propylene. Thus alternative routes have been studied based on the use of oxidants able to act heterolytically under mild conditions. Hypochlorous acid (chlorine+water) and organic hydroperoxides fulfill these requirements and their use has led to the introduction of the chlorohydrin (Box 2) and the hydroperoxide processes, both currently employed commercially. 

Typical biocides include hypochlorous acid, chlorine dioxide, hypobromus acid, hydrogen peroxide, ozone, ultraviolet-light treatment, phenolics, aldehydes, and quaternary ammonium compounds (Ref 73, 80). A brief description of each follows (Ref 73, 80). Hypochlorous acid is probably the most commonly used biocide and also one of the most powerful oxidizing agents. The sources of hypochlorous acid are chlorine gas and sodium hypochlorite. In aque-... 

Ford, D. A. 2010. Lipid oxidation by hypochlorous acid Chlorinated lipids in atherosclerosis and myocardial ischemia, 5, 835-852. 

Chlorine produces the intermediate (CI2O2] more rapidly than does hypochlorous acid, since ki is greater than k2 (48), High hydrogen ion concentration also favors the production of ICI2O2] by means of ki. Consequently, the higher concentration of [CI2O2] favors the second-order decomposition of the intermediate to form chlorine dioxide rather than the first-order process, which produces chlorate ion. This is substantiated by the fact that the chlorine(0)-chlorine(III) reaction produces less chlorate and more chlorine dioxide than the hypochlorous acid-chlorine(III) reaction (48). Indeed, some authors (34, 97, 118, 220,... 

This proton-transfer reaction is much faster than the formation of hypochlorous acid, but the equilibrium constant is significantly less. Thus, the formation of hypochlorous acid and its subsequent conversion to hypochlorite occur in two reaction planes. Hypochlorous acid can be obtained as a gas from the first reaction plane. High yields can be obtained from spray columns with high rates of gas mass transfer and low rates of liquid mass transfer. Particulate bases such as calcium hydroxide can also be used to increase the yield of hypochlorous acid. Chlorinating solutions of weak bases such as bicarbonate also yield hypochlorous acid. "... 

As shown in Figure 24.5, 50% sodium hydroxide is sprayed into a reactor to form droplets that react with recirculating chlorine gas. The resulting heat vaporizes the water to obtain a mixture of hypochlorous acid, chlorine, dichlorine monoxide, water vapor, and solid sodium chloride. The solid falls to the bottom of the reactor where it is removed. The gas is chilled to separate a solution of hypochlorous acid from the chlorine that is recycled. " " Alternatively, the hypochlorous acid can be absorbed into water instead of being condensed. 

In the chlorohydrination step, the reactants propylene and hypochlorous acid (chlorine and water) are converted into two propylene chlorohydrin isomers (90% l-chloro-2-propanol and 10% 2-chloro-l-propanol). Yields of up to 94% can be achieved in modern commercial plants. The main by-products formed in this reaction step are dichloropropane (3-10%), dichloropropanol (0.3-1.2%), and dichlorodiisopropyl ether (0.2-1.7%). In the second step (dehydrochlorination, also called epoxidation or saponification ) the aqueous propylene chlorohydrin solution is treated with slaked lime or caustic soda. Propylene oxide and calcium or sodium chloride are formed. In a commercial process 1.4-1.5 units of chlorine are consumed to produce one unit of propylene oxide. Typical by-products are monopropylene glycol, epichlorohydrin, glycerol monochlorohydrin, glycerol, propanal, and acetone. In dehydrochlorination, propylene oxide yields of up to 96% can be obtained. 

Due to chlorines deleterious effects on polyamide membranes, it [and more specifically, free chlorine (i.e., hypcochlorite, + hypochlorous acid + chlorine gas + trichloride ion)] must be removed to prevent contact with the membranes. Dechlorination is relatively simple, typically using either sodium bisulfite to chemically remove free chlorine or carbon filtration to catalytically remove chlorine (see chapter 8.2.3. and 8.1.4, respectively). 

The diagram shows that the couple Cb/Cl" becomes more oxidant than the couple HC10/Cl2(w) at pH > 1.2, wherefrom the disproportionation. Chlorine of the first couple becomes the oxidant of chlorine (the reductor) of the second couple. The disproportionation is evidenced by the appearance of two disconnected areas of chlorine (dashed lines). Another interesting result may be found from the consideration of this diagram. Normally, hypochlorous acid, chlorine, and the hypochlorous ion should oxidize water with the production of dioxygen. This means that these species are not stable in water from a thermodynamic standpoint. However, here it is the contrary. They are apparently stable. This is for kinetic reasons. Likewise, at pH < 1.1,... 

Sulphites are oxidised by chlorine water and solutions con-tainingchloric(I) (hypochlorous)acid or the chlorate(I) (hypochlorite) ion... 

A similar intramolecular oxidation, but for the methyl groups C-18 and C-19 was introduced by D.H.R. Barton (1979). Axial hydroxyl groups are converted to esters of nitrous or hypochlorous acid and irradiated. Oxyl radicals are liberated and selectively attack the neighboring axial methyl groups. Reactions of the methylene radicals formed with nitrosyl or chlorine radicals yield oximes or chlorides. 

Dry chlorine reacts with most metals combustively depending on temperature alurninum, arsenic, gold, mercury, selenium, teUerium, and tin react with dry CI2 in gaseous or Hquid form at ordinary temperatures carbon steel ignites at about 250°C depending on the physical shape and titanium reacts violendy with dry chlorine. Wet chlorine is very reactive because of the hydrochloric acid and hypochlorous acid (see eq. 37). Metals stable to wet chlorine include platinum, silver, tantalum, and titanium. Tantalum is the most stable to both dry and wet chlorine. 

Chlorine and Bromine Oxidizing Compounds. The organo chlorine compounds shown in Table 6 share chemistry with inorganic compounds, such as chlorine/77< 2-3 (9-j5y and sodium hypochlorite/7 )< /-j5 2-5 7. The fundamental action of chlorine compounds involves hydrolysis to hypochlorous acid (see Cm ORiNE oxygen acids and salts). 

Lithium Hypochlorite. Lithium hypochlorite [13840-33-0], LiOCl, is obtained from reaction of chlorine and an aqueous solution of lithium hydroxide. The soHd is usually obtained as a dry stable product containing other alkaH haHdes and sulfates (64). A product containing 35% available chlorine is used for sanitizing appHcations in swimming pools and in food preparation areas where its rapid and complete dissolution is important. The salt can also be obtained in higher purity by reaction of lithium hydroxide and hypochlorous acid (65). 

In the reaction of aEyl alcohol with an aqueous chlorine solution, addition of hypochlorous acid to the double bond of aEyl alcohol yields glycerol monochlorohydrin and as a by-product, glycerol dichlorohydrin. Thus, a poor yield of glycerol monochlorohydrin is obtained (8). To improve the yield of glycerol monochlorohydrin, addition of sodium carbonate in an amount equivalent to that of the hydrogen chloride in the aqueous chlorine solution, has been proposed (9). 

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). 

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. 

V-Chlorosuccinimide [128-09-6] mp 150—151°C, forms orthorhombic crystals and has a chlorine-like odor it is prepared from succinimide and hypochlorous acid (114,115). Because of its powerhil germicide properties, it is used ia disiafectants for drinking water. Like its bromine derivative, it is also a halogenating agent. 

Chlorine Vehicle ndStabilizer. Sulfamic acid reacts with hypochlorous acid to produce /V-ch1orosu1famic acids, compounds in which the chlorine is stiU active but more stable than in hypochlorite form. The commercial interest in this area is for chlorinated water systems in paper mills, ie, for slimicides, cooling towers, and similar appHcations (54) (see INDUSTRIALANTIMICROBIALAGENTS). 

Cooling water pH affects oxidizing antimicrobial efficacy. The pH determines the relative proportions of hypochlorous acid and hypochlorite ion or, in systems treated with bromine donors, hypobromous acid and hypobromite ion. The acid forms of the halogens are usually more effective antimicrobials than the dissociated forms. Under some conditions, hypochlorous acid is 80 times more effective in controlling bacteria than the hypochlorite ion. Hypochlorous acid predominates below a pH of 7.6. Hypobromous acid predominates below pH 8.7, making bromine donors more effective than chlorine donors in alkaline cooling waters, especially where contact time is limited. 

Antimicrobial efficacy is also affected by demand in the cooling water system, specifically demand exerted by ammonia. Chlorine reacts with ammonia to form chloramines, which are not as efficacious as hypochlorous acid or the hypochlorite ion in microbiological control. Bromine reacts with ammonia to form bromamines. Unlike chloramines, bromamines are unstable and reform hypobromous acid. 

Sodium hypochlorite and calcium hypochlorite are chlorine derivatives formed by the reaction of chlorine with hydroxides. The appHcation of hypochlorite to water systems produces the hypochlorite ion and hypochlorous acid, just as the appHcation of chlorine gas does. 

The dissociation of hypochlorous acid depends upon pH and, to a much lesser extent, temperature (6). At 25°C, it is - 0% at pH 5, about 50% at pH 7.5, and - 100% at pH 10, see Figure 1. Because of the acidity formed by chlorine gas, addition of soda ash (Na2C02) or sodium sesquicarbonate (Na2C03-NaHC03) is necessary to maintain the proper pH and to replenish alkalinity. 

Calcium Hypochlorite. This chemical, marketed since 1928, is one of the most widely used swimming-pool water sanitizers. Calcium hypochlorite, a crystalline sofld, is a convenient source of available chlorine and is sold in granular or tablet form for use in home, semiprivate, and commercial pools. When dissolved in water, Ca(OCl)2 forms hypochlorous acid and hypochlorite ion similar to NaOCl. It contains small amounts of stabilizing Ca(OH)2, which has a very small effect on pool pH (7). Calcium hypochlorite has superior storage stabiUty and much higher available CI2 concentration than Hquid bleach, which reduces storage requirements and purchasing frequency. 

In solutions, the concentration of available chlorine in the form of hypochlorite or hypochlorous acid is called free-available chlorine. The available chlorine in the form of undissociated A/-chloro compounds is called combined-available chlorine. Several analytical methods can be used to distinguish between free- and combined-available chlorine (8). Bleaches that do not form hypochlorite in solution like chlorine dioxide and nonchlorine bleaches can be characterized by thek equivalent available chlorine content. This can be calculated from equation 5 by substituting the number of electrons accepted divided by two for the number of active chlorine atoms. It can also be measured by iodomettic titration. 

Solutions of available chlorine bleaches decompose on standing at a rate that depends on the conditions described below. Hypochlorous acid [7790-92-3] and hypochlorite anions decompose according to equations 6 and 7 (20,21) ... 

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. Hypochlorous acid [7790-92-3] solutions are made for immediate use as chemical intermediates from chlorine monoxide or in bleaching and water disinfection by adjusting the pH of hypochlorite solutions. Salt-free hypochlorous acid solutions have been economically made... 

Hypochlorous acid can also be used, but the reaction is slower. Chlorine dioxide is also made by adding acid to sodium chlorite solutions by the overall reaction in equation 11 ... 

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