Bromine complexes

Iron hahdes react with haHde salts to afford anionic haHde complexes. Because kon(III) is a hard acid, the complexes that it forms are most stable with F and decrease ki both coordination number and stabiHty with heavier haHdes. No stable F complexes are known. [FeF (H20)] is the predominant kon fluoride species ki aqueous solution. The [FeF ] ion can be prepared ki fused salts. Whereas six-coordinate [FeCy is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeCfy] can be isolated if large cations such as tetraphenfyarsonium or tetra alkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to kon(II) and bromine. Complex anions of kon(II) hahdes are less common. [FeCfy] has been obtained from FeCfy by reaction with alkaH metal chlorides ki the melt or with tetraethyl ammonium chloride ki deoxygenated ethanol. 

Zn + Bt2 ZnBr2 2mc—bromine 1.85 75 65 capabihty bromine complex with quaternary ammonium salt. 

The product distribution can be shifted to favor the 1 -product by use of such milder brominating agents as the pyridine-bromine complex or the tribromide ion, Br3. It is believed that molecular bromine reacts through a cationic intermediate, whereas the less reactive brominating agents involve a process more like the AdgS and-addition mechanism. 

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. 

Bromination is catalyzed by Lewis acids, and a study of the kinetics of bromination of benzene and toluene in the presence of aluminum chloride has been reported. Toluene is found to be about 35 times more reactive than benzene under these conditions. The catalyzed reaction thus shows a good deal less substrate selectivity than the uncatalyzed reaction, as would be expected on the basis of the greater reactivity of the aluminum chloride-bromine complex. 

Fig. 20. Increase of the intensity of the ESR signal during the irradiation and its decay in darkness of a bromine complex of a poly(schiff base) prepared by polycondensation of 4,4 -diacetyldiphenyl sulfide and p-phenyienediamine mole ratio 1 S.8...

The products of the thermal transformation of PAN were used for the examination of the effect of stereoregularity of the initial polymer on the CTC properties. At equal transformation degrees, the concentration of complete charge transfer states in a bromine-complexed PCS, obtained on the basis of PAN-c (t e., a polymer with elevated content of isotactic sequences), exceeds by two orders of magnitude this parameter in the polymer obtained on the basis of PAN-r. 

Figure 2.1 Relationship between rate of reaction, Rp, and initiator concentration [I], for isoquinoline-bromine complex in methyl methacrylate (J. W. Nicholson, unpublished results, 1977)...

Bromo-6-chloro-2-methyl-imidazo-( 1,2b)-pyridazine-bromine complex dichloromethane 20°C, 2 min. 75 % (40 %) (refs. 13,31)... 

Since the discovery of the first organic semiconductor perylene-bromine complex in 1954 [1], a large number of molecular conductors, including more than 100 molecular superconductors, have been prepared. Conducting molecular materials are characterized by the following features ... 

As an example of Mulliken CT complexation, let us consider the benzene bromine complex, whose optimized structure and leading intermolecular NBO interaction are displayed in Fig. 5.41. NBO charge analysis of the C6H6- -B complex reveals the presence of slight CT (0.(X)21 e) from C6H6 to Br2, which is consistent with the Mulliken description. Both monomers are altered by complex formation, with the Br—Br bond NBO becoming noticeably polarized away from the benzene ring,... 

Subsequent kinetic and product distribution data on the reactions of 1,3-butadiene with molecular bromine, pyridine-bromine complex and tetra-n-butylammonium tribromide in chlorinated solvents have shown that pyridine-Br2 and tribromide ion act as independent electrophiles, rather than as sources of molecular bromine75. 

Dioxane-bromine complex Same as for pyridinium hydrotribromide... 

Bromination of pyridine is much easier than chlorination. Vapour phase bromination over pumice or charcoal has been studied extensively (B-67MI20500) and, as with chlorination, orientation varies with change in temperature. At 300 °C, pyridine yields chiefly 3-bromo-and 3,5-dibromo-pyridine (electrophilic attack), whilst at 500 °C 2-bromo- and 2,6-dibromo-pyridine predominate (free radical attack). At intermediate temperatures, mixtures of these products are found. Similarly, bromination of quinoline over pumice at 300 °C affords the 3-bromo product, but at higher temperatures (450 °C) the 2-bromo isomer is obtained (77HC(32-1)319). Mixtures of 3-bromo- and 3,5-dibromo-pyridine may be produced by heating a pyridine-bromine complex at 200 °C, by addition of bromine to pyridine hydrochloride under reflux, and by heating pyridine hydrochloride perbromide at 160-170 °C (B-67MI20500). 

When sodium lignosulfonate or sulfur lignin are compounded, for instance, with iodine or bromine, complexes supposedly form (16-17). These systems are conductors with mixed ionic and electronic nature. Presumably they are charge transfer complexes, since the electronic conductivity predominates (18-19). These compounded materials form charge transfer structures (20). Water is supposed to introduce ionic conductivity to the system. Impurities affect conductivity, too (21). In any case, the main models of conductivity are probably based on the band model and/or the hopping model. 

Tellurium tetrabromide is amphoteric in fused arsenic tribromide, from which complex bromides such as [(Et)4N]2TeBr6 and ammine adducts such as TeBr4-2PhNMe2 have been obtained. Addition compounds with pyridine, tetramethylthiourea and piperazine have also been reported.38,39 Alkali metal Salts such as TeBr4-2CsCl and the full brominated complexes of the type M2TeBr6 and M2PoBrs are also known. 

Although there are a few examples of iodine bonded only to carbon ligands, most complexes contain other, more electronegative ligands in the coordination sphere. There are few examples of chlorine and bromine complexes, and, as yet, no examples even with iodine of oxidation state VII. Most of these complexes, and especially the more stable examples, have aromatic... 

The addition of bromine to alkenes is a rapid, exothermic reaction usually taking place at room temperature. In contrast to chlorination, the rate law in bromination depends on the solvent used. On passing from hydroxylic to nonpolar aprotic solvents, the overall second-order changes to a rate law that is first-order in alkene and second-order in bromine.226 Alkene-bromine complexes with varying compositions were shown to form under reaction conditions3,218,227 228(Scheme 6.5). At low bromine concentration in protic solvents the reaction proceeds via a 1 1 complex (23). A 1 2 alkene-bromine complex (25) is involved at high bromine concentration in nonprotic solvents. The ionic intermediates (24, 26) were shown to exist as contact ion pairs, solvent-separated ions, or dissociated ions. 

Bromine complexes of ethylene and cyclopropene have been isolated in argon matrices and studied by IR spectroscopy and ab initio methods. Two methods, namely MP2 and BLYP, were compared and found suitable for the calculation of IR spectra as they exhibited only minor differences.25... 

Bromination.2 This bromine-crown ether complex, like dioxane-bromine (5, 58), can brominate alkenes, but the stereoselectivity is greater than that with free bromine and is less sensitive to solvent effects. Thus, bromination of trans-ifi-methylstyrene with DBC Br2 occurs exclusively by anti-addition and bromination of dr-/J-methylstyrene occurs by anti-addition to the extent of 95-100%. The bromine complex of polydibenzo-18-crown-63 is a particularly useful reagent because it can be packed as a slurry in a chromatography column. The alkene is then placed on the column and eluted with CC14. 

Akamatsu, H., H. Inokuchi, and Y. Matsunaga. 1954. Electrical conductivity of the perylene-bromine complex. Nature 173 168. 

If cyclopentadienyliron dicarbonyl halides are allowed to react with themselves in the presence of Lewis acids, cations are formed in which the new substituent is the cyclopentadienyliron dicarbonyl halide itself, for example, [CjH6Fe(CO)i-X-(CO)2-FeC8H8]+ (X = Cl, Br, I). All three cations can be prepared best by treatment of the corresponding halides with boron trifluoride diethyl etherate all are isolated as tetrafluoro-borates.18 The bromine complex can also be obtained by a more complicated procedure by the reaction between C8H8Fe-(CO)2Br and AlBr3 in liquid sulfur dioxide 16 the iodine cation can be isolated from a melt of cyclopentadienyliron dicarbonyl iodide and aluminum chloride.17 In the latter two cases the hexafluorophosphate salts can be obtained. These binuclear cations are of special interest, because they are cleaved by electron donors,16-17 e.g., aniline, pyridine, benzonitrile, acetonitrile, acrylonitrile, with the formation of the corresponding [C8H8Fe(CO)2L]+ cations and the parent halide. Equations for preparation of the tetrafluoroborate are ... 

In the presence of bromide ion there is apparently no direct reaction of Co(III) with the hydrocarbon substrate, in contrast to cobalt-catalyzed autoxidations carried out in the absence of bromide. That different mechanisms are operating is illustrated by the relative rates of oxidation of alkylbenzenes catalyzed by cobalt acetate alone compared to those obtained in the presence of added bromide ion (Table VIII). In the presence of bromide ion, the relative reactivities are consistent with a mechanism involving attack by bromine atoms but not one involving electron transfer. Individual discrepancies in selectivities between bromine atom and the species active in the Co(0 Ac)2-NaBr system (Table VIII) were attributed to a bromine complex,... 

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