Hydrogen peroxide reaction with bromide ions

In the absence of a catalyst, this reaction occurs extremely slowly. Many different substances are capable of catalyzing flie reaction, including bromide ion, Br (aq), as shown in Figure 14.19(a) T. The bromide ion reacts with hydrogen peroxide in acidic solution, forming aqueous bromine and water ... 

During the induced reactions involving peroxydisulphate in the presence of oxygen, the induced oxidation of iron(II) by oxygen can be observed just as with the hydrogen peroxide-iron(ll) system. Chloride and particularly bromide ions are effective inhibitors in the iron(II)-peroxydisulphate system. 

Other cationic surfactants such as TTAB, DTAB, DODAB, STAC, CEDAB, and DDDAB have been used in CL reactions with less frequency. Thus, tetradecyltrimethylammonium bromide [TTAB] has been used to increase the sensitivity of the method to determine Fe(II) and total Fe based on the catalytic action of Fe(II) in the oxidation of luminol with hydrogen peroxide in an alkaline medium [47], While other surfactants such as HTAB, hexadecylpiridinium bromide (HPB), Brij-35, and SDS do not enhance the CL intensity, TTAB shows a maximum enhancement at a concentration of 2.7 X 10 2 M (Fig. 11). At the same time it was found that the catalytic effect of Fe(II) is extremely efficient in the presence of citric acid. With regard to the mechanism of the reaction, it is thought that Fe(II) forms an anionic complex with citric acid, being later concentrated on the surface of the TTAB cationic micelle. The complex reacts with the hydrogen peroxide to form hydroxy radical or superoxide ion on the... 

Amongst other inorganic catalysts are to be included the iodides 9 (also bromides and chlorides but less active) and chromates or dichromates. The agent in the former case appears to be the iodide ion, the mechanism of the reaction probably involving oxidation to hypo-iodite which then reacts with more hydrogen peroxide with formation... 

Chromates, vanadates, and cerium salts give colour reactions with the reagent and should therefore be absent. Iron salts give a yellow colour with hydrogen peroxide, but this is eliminated by the addition of syrupy phosphoric acid. Fluorides bleach the colour (stable [TiF6]2 ions are formed), and large amounts of nitrates, chlorides, bromides, and acetates as well as coloured ions... 

The bromide ion does not appear to react with one form of the enzyme-hydrogen peroxide complex. It is clear that at least two pH-dependent intermediates are present, which react with bromide to yield the oxidized bromine species. The second-order rate constant for the reaction between bromide and these peroxo-intermediates w is estimated to be 1.7 X 10 s" Bromide also acts as an inhibitor of the enzyme in a... 

Another example of homogeneous catalysis is that of the decomposition of hydrogen peroxide by bromide ion. Here hydrogen peroxide acts as both an oxidant and a reductant, with two different reactions proceeding at characteristic rates under a given set of conditions ... 

Several reports exist on the effect of metal cations on satin hydrolysis (Wagner-Jauregg et al., 1955 Gustafson and Martell, 1962 Epstein and Mosher, 1968). Some of the metal ions that appear to be effective include Cu(ll), Au(ll), Ag(l), Ni(ll), and Zn(ll). The effect of hypochlorite on sarin decomposition has been studied (Epstein et al., 1956), including the effect of surfactant micelles on the process (Dubey et al., 2002). It was shown that the use of cetyltrimethylammonium bromide substantially increased the speed with which satin was destroyed by hypochlorite. The effect of surfactant on the oxidation of sarin (and other OPs) with o-iodosobenzoic acid has been examined (Hammond et al., 1989). As part of an extensive study of the reactivity of nerve agents, Larsson (1958a) conducted a study of hydrogen peroxide s reaction with sarin. 

Decomposition of hydroxyperoxides to free radicals is also accelerated by bromide ions [90], The reaction of hydroperoxide with Br" is second order, in contrast to that of hydrogen peroxide, i.e. 

If this were the complete reaction, bromide ion would not be a catalyst because it undergoes chemical change during the reaction. However, hydrogen peroxide also reacts with the Br2(aq) generated in Equation 14.30 ... 

Alternatively reaction of an olefinic alcohol or acid can furnish a dimer through interaction of the bromonium ion with a second molecule of reactant acting as nucleophile. Methoxy bromo adducts are easily made by reaction of the alkene with t-butoxy bromide in methanol solution. Another interesting reaction of this kind is the selective iodination of arachidonic acid and docosahexaenoic acid by iodine and hydrogen peroxide in the presence of lactoperoxidase to give iodolactones. 

Excess fluoride ions prevent the reaction through the formation of stable [TiFe] ions. However, by demasking, it is possible to detect titanium in the presence of even large amounts of fluoride (see below). Large eimounts of acetates, nitrates, chlorides, bromides, and colored ions reduce the sensitivity. Chromates, vanadates, and molybdates, should not be present because they too give color reactions with hydrogen peroxide. Iron salts yield... 

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