Bromide ion oxidation

Displacement reactions involve one element displacing another element from solution The element that dissolves in the solution is more active than the element supplanted from solution. Within the halogen group the activity decreases from top to bottom. Thus, each halogen is able to displace the members of the group below it, but not those above it. For instance, molecular bromine can oxidize aqueous iodide ion but molecular iodine is incapable of oxidizing bromide ion ... 

Brv does not have the power to oxidize bromide ion beyond the elementary stage. The reaction with bromide ion is ... 

In TEMPO-mediated oxidations, bromide ions are key to the efficient catalytic cycle (Figure 9.10) [29, 30]. Rychnovsky found thatTEMPO-catalyzed oxidation of alcohols using m-CPBA was also dependent on bromide ions for rapid reaction [31]. Any compounds that sequester bromide ions in these catalyzed reactions should be avoided. 

One of the most common bimolecular reactions of radicals is their association with other nonradical molecules. We have used the term hemicolligation to describe this reaction type elsewhere.11 This mode of reaction is particularly important because many radical precursors can react in this way. For example, bromine atoms are often generated by oxidizing bromide ions, so the reaction Br + Br- = Br2 is an unavoidable component in such systems. Association of radicals with 02 is another common process that can be important when atmospheric oxygen is not completely excluded from the reaction mixture. When the radical is the hydrated electron, the association reaction is simply a reduction and is treated separately (Table 9.5). 

A halogen higher in the periodic table is a stronger oxidizing agent than one lower down. Thus, chlorine can oxidize bromide ions or iodide ions from solution, and bromine can oxidize iodide ions. Here, chlorine displaces bromine ... 

Silver bromide is a semiconductor. Absorbed light excites electrons in the conduction band and electron holes in the valence band. The holes oxidize bromide ions to traces of bromine that are dissolved in the gelatine layer. The free electrons reduce silver ions to silver atoms that form the catalytically active clusters in the halide crystallites. The reactions are ... 

Bromide and iodide ions can be determined successively in one water sample using a method developed by P. Hofer hypochlorite ions are used to oxidize bromide ions to bromate ions and iodide ions to iodate ions ... 

The free radicals that form upon decomposition also possess great oxidizing power and are very reactive with organics present in the pool or spa environment. Ozone will oxidize bromide ion to bromine and bromate, chloride ion to chlorine and hydrogen peroxide forming hydroxyl radicals. 

The pH range in which only the iodide ions are oxidized is 1.1 < pH < 4.3. In this range, the preponderance areas of dichromate and iodide ions are disconnected. This is not the case for the areas of dichromate and bromide ions, which are only disconnected in the range 0 < pH < 1.1. In the latter range, the areas of dichromate and iodide ions are also disconnected. As a result, it is possible to oxidize iodide ions only by buffering the medium at about pH = 4 and by allowing the dichromate ions to react as long as the total oxidation of iodides lasts. It then becomes possible to oxidize bromide ions of the same sample with potassium dichromate, after acidification at pH < 1.1 (Fig. 15.15). 

Bromide ion acts as an inliibitor through step (9) which competes for HBr02 with the rate detennining step for the autocatalytic process described previously, step (4) and step (5). Step (8) and Step (9) constitute a pseudo-first-order removal of Br with HBr02 maintained in a low steady-state concentration. Only once [Br ] < [Br ] = /fo[Br07]//r2 does step (3) become effective, initiating the autocatalytic growth and oxidation. 

The basie features of the oseillatory meehanism of the BZ reaetion are given by the Field-Koros-Noyes (FKN) model [14]- This involves three proeesses —A, B and C. Proeess A involves step (8) and step (9) from seetion A3,14.2.1. leading to removal of uiliibitor bromide ion. Proeess B involves step (3) and step (4) from Seetion A3.14.1.1 and gives the autoeatalytie oxidation of the eatalyst. This growth is limited partly by the disproportionation reaetion... 

In the laboratory, bromine is prepared by oxidation of bromide ion the oxidation is carried out by mixing solid potassium bromide with manganese(IV) oxide and distilling with concentrated sulphuric acid ... 

Furan can be catalyticaHy oxidized in the vapor phase with oxygen-containing gases to maleic anhydride (93). Oxidation with bromine or in an electrochemical process using bromide ion gives 2,5-dimethoxy-2,5-dihydrofuran [332-77-4] (19) which is a cycHc acetal of maleic dialdehyde (94—96). 

The halogen displacement polymerization proceeds by a combination of the redistribution steps described for oxidative coupling polymerization and a sequence in which a phenoxide ion couples with a phenoxy radical (eq. 11) and then expels a bromide ion. The resultant phenoxy radical can couple with another phenoxide in a manner that is analogous to equation 11 or it can redistribute with other aryloxy radicals in a process analogous to equations 7 and 8. 

Electrochemical Process. Applying an electrical current to a brine solution containing propylene results in oxidation of propylene to propylene oxide. The chemistry is essentially the same as for the halohydrin process. AH of the chemistry takes place in one reactor. Most of the reported work uses sodium or potassium bromide as the electrolyte. Bromine, generated from bromide ions at the anode, reacts with propylene and water to form propylene bromohydrin. Hydroxide generated at the cathode then reacts with the bromohydrin to yield propylene oxide (217—219). The net reaction involves transfer of two electrons ... 

Sanitizers. Spa and hot-tub sanitation is dominated by chlorine- and bromine-based disinfectants. Public spas and tubs usually employ automatic feeders, eg, CI2 gas feeders, to maintain a disinfectant residual. Private or residential spas and tubs can use automatic chemical feeding or generating devices, or they can be sanitized manually with granular or liquid products. The most widely used products for private spa and tub sanitation are sodium dichloroisocyanurate and bromochlorodimethylhydantoin. Granular products are normally added before and after use, whereas solids, eg, stick-bromine, are placed in skimmers or feeders. Bromine generating systems can also be used and are based on oxidation of bromide ions (added to the water as sodium bromide) by peroxymonosulfate, chloroisocyanurates, hypochlorites, or ozone to generate the disinfectant HOBr. 

One of the most required methods of determination of iodide-ions in praetiee of ehemieal analysis is photometrie determination of produets of iodination of organie eompounds. The oxidation of iodide to iodine ean be earned out suffieiently seleetively. But in ease of presenee of great abundanee of bromide-ions the seleetive oxidation of iodide-ions is problematie. The variants of determination of iodide-ions with different organie reagents are known, but the absenee of bromide-ions in a system is supposed in most of them. In natural objeets these halides are present simultaneously. 

The eleetroehemieal oxidation of solution eontaining iodide- and bromide-ions in HCIO is earned out. Work and auxiliary eleetrodes are platinum. At this potential sueh reaetions are running ... 

When the eleetroehemieal oxidation has been finished the analyzable solution is boiled till almost removal of bromine. Then the halogenation of fluoreseein by two aliquots of determined solution at pH 5.5 is earned out. Abundanee of iodide-ions is added to one of the aliquots and abundanee of bromide-ions - to the other. The dependenee A(X) is measured for every solution. C(T) is ealeulated from reeeived data supposing that two lightabsorbing forms - fluoreseein and dihalide-fluoreseein are present at this eondition. 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

The ortho indirect deactivating effect of the two methyl groups in 2,6-dimethyl-4-nitropyridine 1-oxide (163) necessitates a much higher temperature (about 195°, 24 hr) for nucleophilic displacement of the nitro group by chloride (12iV HCl) or bromide ions N HBr) than is required for the same reaction with 4-nitropyridine 1-oxide (110°). With 5-, 6-, or 8-methyl-4-chloroquinolines, Badey observed 2-7-fold decreases in the rate of piperidino-dechlorination relative to that of the des-methyl parent (cf. Tables VII and XI, pp. 276 and 338, respectively). 

An excess of a standard solution of iron(II) must therefore be added and the excess back-titrated with standard cerium(IV) sulphate solution. Erratic results are obtained, depending upon the exact experimental conditions, because of induced reactions leading to oxidation by air of iron(II) ion or to decomposition of the persulphate these induced reactions are inhibited by bromide ion in concentrations not exceeding 1M and, under these conditions, the determination may be carried out in the presence of organic matter. 

Cations forming insoluble chromates, such as those of silver, barium, mercury (I), mercury(II), and bismuth, do not interfere because the acidity is sufficiently high to prevent their precipitation. Bromide ion from the generation may be expected to form insoluble silver bromide, and so it is preferable to separate silver prior to the precipitation. Ammonium salts interfere, owing to competitive oxidation by bromate, and should be removed by treatment with sodium hydroxide. 

Discussion. Bromine may be electro-generated with 100 per cent current efficiency by the oxidation of bromide ion at a platinum anode. Bromination of oxine proceeds according to the equation ... 

D.12 Write the formula for the ionic compound formed from (a) strontium and bromide ions (b) aluminum and sulfate ions (c) lithium and oxide ions (d) ammonium and sulfide ions ... 

FIGURE K.2 When chlorine is bubbled through a solution of bromide ions, it oxidizes the ions to bromine, which colors the solution reddish brown. 

We can often recognize loss of electrons by noting the increase in the charge of a species. This rule also applies to anions, as in the oxidation of bromide ions (charge —1) to bromine (charge 0) in a reaction such as the one used commercially to make bromine (Fig. K.2) ... 

Here, the bromide ion (as sodium bromide) is oxidized to bromine by the chlorine gas. 

We have seen that oxidation is electron loss and reduction is electron gain. Electrons are real particles and cannot just be "lost. Therefore, whenever a species is oxidized, another species must be reduced. Oxidation or reduction considered separately is like one hand clapping one transfer must occur in conjunction with the other for reaction to take place. For instance, in the reaction between chlorine and sodium bromide, the bromide ions are oxidized and the chlorine... 

The products of the reaction between bromide ions and permanganate ions, Mn04, in basic aqueous solution are solid manganese(IV) oxide, MnO>, and bromate ions. Balance the net ionic equation for the reaction. 

HGURE 15.16 Sulfuric acid is an oxidizing agent. When some concentrated acid is poured on solid sodium bromide, NaBr, the bromide ions are oxidized to elemental bromine, which colors the mixture red-brown. 

Use Appendix 2B to determine whether an acidic sodium dichromate solution can oxidize (a) bromide ions to bromine and (b) silver(I) ions to silver(II) ions under standard conditions. 

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