Br2+ ion

Br21 Ion. The Br2+ cation 329 can be prepared797 by oxidation of bromine by S206F2 in the superacid HSO3F—SbF5—3S03 however, even in this very weakly basic medium, the Br2+ ion is not completely stable because it undergoes appreciable disproportionation [Eq. (4.198)]. 

Iodine has long been known to dissolve in highly acidic, oxidizing media, for example, oleum, to give bright blue paramagnetic solutions these solutions contain the I2 ion and salts with anions of extremely low basicity can be isolated. The Br2 ion may be similarly generated, but Cl2+ is still unknown except in the gas phase. These... 

Figure 17 (a) Section of the polymeric [SeCls ] chain (b) Section of the polymeric [SeBr6 Br2] ion... 

The reaction in which an equilibrium between Br03, Br and Br2 ions is established, is a complex reaction. The mechanism of this reaction was... 

Consequences of OH Radical Reaction in Sea Water Framation and Decay of Br2 Ion Radical... 

Tellurium letrabromide, TeBr4, m.p. 380 "C, b.p. 420 0. Red solid, Te plus excess Brj. The vapour is dissociated to TeBr2 and Br2- Forms many complexes, including (TeBr p ions. 

Figure A3.10.8 Depiction of etching on a Si(lOO) surface, (a) A surface exposed to Br2 as well as electrons, ions and photons. Following etching, the surface either becomes highly anisotropic with deep etch pits (b), or more regular (c), depending on the relative desorption energies for different surface sites [28].

Bromine is moderately soluble in water, 33.6 g/L at 25°C. It gives a crystalline hydrate having a formula of Br2 <7.9H2 O (6). The solubiUties of bromine in water at several temperatures are given in Table 2. Aqueous bromine solubiUty increases in the presence of bromides or chlorides because of complex ion formation. This increase in the presence of bromides is illustrated in Figure 1. Kquilibrium constants for the formation of the tribromide and pentabromide ions at 25°C have been reported (11). 

ForCb, Bt2 and h, K. c is 4.2 x 10- 7.2 x 10 and 2.0 x 10 mol 1 respectively, thereby favouring the free halogens, whereas Kaik is 7.5 X lO, 2 X lO and 30mol I respectively, indicating a tendency to disproportionation which is overwhelming for CI2 but progressively less pronounced for Br2 and I2. In actuality the situation is somewhat more complicated because of the tendency of the hypohalite ions themselves to disproportionate further to produce the corresponding halite ions ... 

This type of isomerism occurs when isomers produce different ions in solution, and is possible in compounds which consists of a complex ion with a counter ion which is itself a potential ligand. The pairs [Co(NH3)5(N03)]S04, [Co(NH3)5(S04)]N03 and [PtCl2(NH3)4]Br2, [PtBr2(NH3)4]Cl2, and the series [CoCl(en)2-(N02)]SCN, [CoCl(en)2(SCN)]N02, [Co(en)2-(N02)(SCN)]C1 are examples of ionization isomers. 

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. 

We saw in the previous section that when Br2 reacts with an alkene, th cyclic bromonium ion intermediate reacts with the only nucleophile presen Br- ion. If the reaction is carried out in the presence of an additional nuclec phile, however, the intermediate bromonium ion can be intercepted by th added nucleophile and diverted to a different product. In the presence of watei for instance, water competes with Br- ion as nucleophile and reacts with th bromonium ion intermediate to yield a broinohydrin. The net effect is additioi of HO-Br to the alkene by the pathway shown in Figure 7.1. 

Mechanism of bromohydrin formation by reaction of an alkene with Br2 in the presence of water. Water acts as a nucleophile to react with the intermediate bromonium ion. 

Reaction of the alkene with Br2 yields a bromonium ion intermediate, as previously discussed. 

Halonium ion (Section 7.2) A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of Br2 to alkenes. 

There are many half-reactions below this one in Appendix 3, so there are quite a few substances that will oxidize I-. For example, iodide ion can be quantitatively oxidized to Is by Fe+3, Br2, Mn02, Cr2Of2, Cl2, and MnO. On the other hand, there are many half-reactions above E° = —0.53 volt in Appendix 3. For example, I2 can be quantitatively reduced to I- by Sn+2, H2S03, and Cr+S. The usefulness of the I -h reaction derives from the fact that all of the substances mentioned react rapidly and without side reactions. 

Using E° values, predict what will happen if, in turn, each halogen beginning at chlorine is added to a 1 M solution of ions of the next lower halogen Cl2 to Br-, Br2 to I-. Which halogen is oxidized and which is reduced in each case ... 

Mixed haloplatinate(IV) ions have been synthesized [34] by use of substitution reactions on PtCl - and PtBr using the stronger trans-influence of Br, the ci s-isomers can be made by treating PtBr with Cl- (in the presence of Br2) ... 

In 75 % aqueous acetic acid, the bromination of fluorene at 25 °C obeys second-order kinetics in the presence of bromide ion and higher orders in its absence287, with Ea (17.85-44.85 °C) = 17.4, log A = 10.5 and AS = —12.4 however, these values were not corrected for the bromine-tribromide ion equilibrium, the constant for which is not known in this medium, and so they are not directly comparable with the proceeding values. In the absence of bromide ion the order with respect to bromine was 2.7-2.0, being lowest when [Br2]initial was least. Second- and third-order rate coefficients were determined for reaction in 90 and 75 wt. % aqueous acetic acid as 0.0026 and 1.61 (k3/k2 = 619), 0.115 and 12.2 (k3/k2 = 106) respectively, confirming the earlier observation that the second-order reaction becomes more important as the water content is increased. A value of 7.25 x 106 was determined for f3 6 (i.e. the 2 position of fluorene). 

The rates of bromination of 3-nitrophenol in aqueous solution at 25 °C have been measured at various concentrations of perchloric acid and sodium bromide294. An increase in both caused a decrease in rate the latter again shows that Br is much less reactive than Br2, whilst the former shows that reaction occurs principally on the 3-nitrophenoxide ion and the difference from the observation for phenol in acetic acid (above p. 117) is undoubtedly partly due to the greater stability of the 3-nitrophenoxide relative to the phenoxide ion. The... 

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