Bromine, dissociation

Separate experiments on the iodine-catalysed bromination of these compounds revealed a rate maximum at [I2]/[Br2] = 0.35, from which it follows that the concentrations of molecular bromine and iodine monobromide are equal, i.e. the latter catalyses bond-breaking in the former in the intermediate. Since iodine monobromide is dissociated into iodine and bromine, dissociation constant K, [Br2]VAT is proportional to [IBr] and hence equation (152) may be rewritten in the form... 

Stabilized by resonance delocalization indeed, they are even more stable than tertiary radicals. In the presence of a suitable initiator, bromine dissociates to bromine atoms that will selectively abstract an allylic or a benzylic hydrogen from a suitable substrate, generating the corresponding allyl and benzyl radicals. 

The high temperature experiments [19, 27, 90—94] were performed with all the inert gases as diluents, and a set of precise data has been compared [27] with several theories [95—100] dealing with the collision partner effect. The experimental results showed little variation with regard to the identity of the inert gas and corresponded most closely to the predictions of the Keck—Carrier model [100]. The efficiencies were expressed on a per collision basis, fe,/Z (Kr as reference), at 1500°K He (0.354), Ne (0.830), Ar (0.869), Kr (1.00), Xe (0.870). The efficiency of CO2 in promoting bromine dissociation [101] has been determined to be... 

During discharge, bromine dissociates from the QBr complex and is reduced to the anionic bromide form, as shown by Eqs. 2.7 and 2.8 ... 

The first stage (a) of the reaction represents the dissociation of bromine into bromine atoms. Both steps (b) and (c) lead to production of HBr, and since bromine atoms are... 

Bromination of methane is exothermic but less exothermic than chlorination The value calculated from bond dissociation energies is AH° = -30 kJ Al though bromination of methane is energetically fa vorable economic considerations cause most of the methyl bromide prepared commercially to be made from methanol by reaction with hydrogen bromide... 

Unlike the situation regarding the crossing between the Vq and Fj potentials for Nal (see Figure 9.41), that for NaBr results in very efficient and rapid dissociation to give Na + Br when it is excited to Fj. Flow would you expect the fluorescence intensity from the neutral bromine atoms to vary with time compared with that for iodine atoms from Nal in Figure 9.42 ... 

Iron(III) bromide [10031-26-2], FeBr, is obtained by reaction of iron or inon(II) bromide with bromine at 170—200°C. The material is purified by sublimation ia a bromine atmosphere. The stmcture of inoa(III) bromide is analogous to that of inon(III) chloride. FeBr is less stable thermally than FeCl, as would be expected from the observation that Br is a stronger reductant than CF. Dissociation to inon(II) bromide and bromine is complete at ca 200°C. The hygroscopic, dark red, rhombic crystals of inon(III) bromide are readily soluble ia water, alcohol, ether, and acetic acid and are slightly soluble ia Hquid ammonia. Several hydrated species and a large number of adducts are known. Solutions of inon(III) bromide decompose to inon(II) bromide and bromine on boiling. Iron(III) bromide is used as a catalyst for the bromination of aromatic compounds. 

The decomposition of sulfuryl chloride is accelerated by light and catalyzed by aluminum chloride and charcoal. Many of the reactions of sulfuryl chloride are explainable on the basis of its dissociation products. Sulfuryl chloride reacts with sulfur at 200°C or at ambient temperature in the presence of aluminum chloride producing sulfur monochloride. It hberates bromine or iodine from bromides or iodides. Sulfuryl chloride does not mix readily with water and hydrolyzes rather slowly. 

Tellurium Tetrabromide. Tellurium tetrabromide [10031-27-3] TeBr, forms yellow hygroscopic crystals which decompose above 280°C and melt at 363°C under bromine vapor. It boils at 414—427°C, dissociating into TeBr2 and bromine. It is soluble in ether and chloroform but not in CCl, and is readily hydroly2ed in water. 

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. 

Arsenic pentafluoride (arsenic(V) fluoride), AsF, is a colorless gas that condenses to a yellow Hquid its dielectric constant is 12.8 at 20 °C. It is formed by reaction of a mixture of bromine and antimony pentafluoride with arsenic trifluoride. The molecule is a trigonal bipyramid and is somewhat dissociated as indicated by vapor density measurements. 

Reactions in Water. The ionization potential for bromine is 11.8 eV and the electron affinity is 3.78 eV. The heat of dissociation of the Br2 molecule is 192 kj (46 kcal). The reduction potentials for bromine and oxybromide anions in aqueous acid solutions at 25°C are (21) ... 

A more concentrated solution of HOBr can be prepared by filtration of one of the above solutions and distillation in vacuum. Or the mercuric oxide reaction can be carried out in Freon 11 without water, yielding a solution of bromine monoxide which is filtered and hydrolyzed. Hypobromous acid is slightly ionized its dissociation constant at 25°C is 2 x 10 . ... 

Dyes and Indicators. The effects of bromine ia dye or iadicator molecules, ia place of hydrogen, iaclude a shift of light absorption to longer wavelengths, iacreased dissociation of phenoHc hydroxyl groups, and lower solubiHty (see Dyes and dye intermediates). The first two effects probably result from iacreased polarizatioa caused by bromine s electroaegativity compared to that of hydrogea. 

Oxidation of azole anions can give neutral azole radicals which could, in principle, be tt (139) or a- (140) in nature. ESR spectra indicate structure (141 hyperfine splittings in G) for imidazolyl radicals, but both tt- and cr-character have been observed for pyrazolyl radicals. Tetrazolyl radicals (142 4 143) are also well known (79AHC(25)205). Oxidation of 2,4,5-triarylimidazole anions with bromine gives l,l -diimidazolyls (144) which are in equilibrium with the dissociated free radical (145) (70AHQ 12)103). 

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

It should be noted that although BrCl is mainly a brominating agent that is eompetitive with bromine, its ehemieal reaetivity makes its action similar to that of ehlorine (that is, disinfection, oxidation, and a bleaching agent). BrCl hydrolyzes exelusively to hypobromous aeid, and if any hydrobromie aeid (HBr) is formed by hydrolysis of the dissociated bromine, it quickly oxidizes to hydrobromous acid via hypochlorous acid. 

The last example represents a fairly rare elimination of hydrogen fluoride in preference to hydrogen chloride, a reaction that deserves a more detailed discussion A comparison of bond dissociation energies of carbon-halogen bonds shows that the carbon-fluorine bond is much stronger than the carbon-chlorine, carbon-bromine, and carbon-iodme bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

All six possible diatomic compounds between F, Cl, Br and I are known. Indeed, ICl was first made (independently) by J. L. Gay Lussac and H. Davy in 1813-4 soon after the isolation of the parent halogens themselves, and its existence led J. von Liebig to miss the discovery of the new element bromine, which has similar properties (p. 794). The compounds vary considerably in thermal stability CIF is extremely robust ICl and IBr are moderately stable and can be obtained in very pure crystalline form at room temperature BrCl readily dissociates reversibly into its... 

The same ideas may be applied to the other processes of Fig. 1. The work required to dissociate a diatomic molecule into two electricallt/ neutral atoms may he quite small the dissociation energy of the bromine molecule Br2 in a vacuum, for example, is only 1.915 electron-volts. On the other hand, the work to dissociate a molecule into two atomic ions in a vacuum cannot be as small as this, since work must be done to set up the full electrostatic field of the positive ion, and the full electrostatic field of the negative ion and this must amount to at least a few electron-volts.1 In addition, the non-electrostatic forces may make a small or large contribution. 

Why does bromination with NBS occur exclusively at an allylic position rather than elsewhere in the molecule The answer, once again, is found by looking at bond dissociation energies to see the relative stabilities of various kinds of radicals. 

Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzyiic position). Explain, based on the bond dissociation energies in Table 5.3 on page 156. 

In its liquid form, bromine trifluoride dissociates according to Equation (6) ... 

There is convincing experimental evidence for the following important statement. To a degree of approximation satisfactory for most analytical work, the mass absorption coefficient of an element is independent of chemical or physical state. This means, for example, that an atom of bromine has the same chance of absorbing an x-ray quantum incident upon it in bromine vapor, completely or partially dissociated in potassium bromide or sodium bromate in liquid or solid bromine. X-ray absorption is predominantly an atomic property. This simplicity is without parallel in absorptiometry. 

The physical properties of the interhalogens are intermediate between those of their parent halogens. Trends in the chemistry of the interhalogen fluorides can be related to the decrease in bond dissociation energy as the central halogen atom becomes heavier. The fluorides of the heavier halogens are all very reactive bromine trifluoride gas is so reactive that even asbestos burns in it. 

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