Bromide anion

Step 2 Nucleophilic attack of bromide anion on the bromonium ion... 

O A hydrogen atom on the electrophile HBr is attacked by tt electrons from the nucleophilic double bond, forming a new C-H bond. This leaves the other carbon atom with a + charge and a vacant p orbital. Simultaneously, two electrons from the H-Br bond move onto bromine, giving bromide anion. 

We call the carbocation, which exists only transiently during the course of the multistep reaction, a reaction intermediate. As soon as the intermediate is formed in the first step by reaction of ethylene with H+, it reacts further with Br in a second step to give the final product, bromoethane. This second step has its own activation energy (AG ), its own transition state, and its own energy change (AG°). We can picture the second transition state as an activated complex between the electrophilic carbocation intermediate and the nucleophilic bromide anion, in which Br- donates a pair of electrons to the positively charged carbon atom as the new C-Br bond starts to form. 

It is evident that the shapes and relative stabilities of the poly bromide anions depend to a large extent on the nature of the countercations. The tendency to form... 

These reactions are postulated to proceed by electro-transfer to give the radical cation of alkoxynaphtalene, which either undergoes reaction with copper(II) bromide or dimerizes (ref. 15). That is, one-electron transfer from the electron-rich alkoxynaphtalene to Cu(II) results in generation of the corresponding radical cation. The radical cation reacts with bromide anion leading to the brominated compound, whereas the radical cation undergoes reaction with another alkoxynaphtalene leading to the binaphtyl (eqns. 2-4). 

When electron-withdrawing groups are attached to the double bond, the reaction is strongly inhibited and may fail completely. In such cases, the bromide anion, produced by the reaction of dimethyl sulfoxide with N-bromosuccinimide, competes with the dimethyl sulfoxide for the bromonium (or bromo carbonium) ion, an intermediate of the reaction. Thus, dibromide may accompany recovered alkene or any bromohydrin formed. Similarly, exogenous anions often compete with dimethyl sulfoxide for the cation. ... 

Apart from silyl shifts, other reactions that are also characteristic of this class of compounds or their derivatives are due to the easy formation of halogen-silicon bonds. Phosphonium salt 34, resulting from the addition of bromine to 33, undergoes spontaneous desilylation by the action of the bromide anion to give the P-bromophosphazene 35 [138,139] (Scheme 33). 

Halogens, the elements in Group 17 of the periodic table, have the largest electron affinities of all the elements, so halogen atoms (a n readily accept electrons to produce halide anions (a a. This allows halogens to react with many metals to form binary compounds, called halides, which contain metal cations and halide anions. Examples include NaCl (chloride anion), Cap2 (fluoride anion), AgBr (bromide anion), and KI (iodide anion). 

The linear appearance of the plot shows that this reaction obeys a first-order rate law. Additional mechanistic studies suggest that alkene formation proceeds in a two-step sequence. In the first step, which is rate-determining, the C — Br bond breaks to generate a bromide anion and an unstable cationic intermediate, hi the second step, the intermediate transfers a proton to a water molecule, forming the alkene and H3 ... 

Fig. 9 Honeycomb-like architectures formed on self-assembly of halide anions (which work as tridentate XB acceptors and sit at the networks nodes) with 1,4-DITFB (which works as bidentate donor and forms network sides) (A). The angles formed by the XBs around the halide anions determine the corrugation of the honeycomb architecture, a more planar arrangement around the halide anions (as is the case of the iodide anions in adduct l,4-DITFB/Me4P+r (B) with respect to the bromide anions in adduct l,4-DITFB/Ph4P+Br (C)) results in a less corrugated honeycomb architecture...

T-shaped tridentate nodes, the diiodobenzenes as linear bidentate modules that space the nodes and ribbons compounded of consecutive rectangles are formed [155] (Fig. 11). A similar topology is present in the co-crystal CBr4/Ph4P+Br where bromide anions and carbon tetrabromide both work as tridentate notes that alternate in the ribbon [121]. 

Vahrenkamp and co-workers formed a number of chelate-stabilized ketone complexes with pyridylphenyl ketone. Octahedral complexes formed with a 2 1 ratio of ligand to zinc were formed with trifluoromethanesulfonate, chloride, and bromide anions. 1 1 reactions resulted in the formation of square pyramidal [ZnL2X]+ with X = Br, I, and a trigonal-bipyramidal dimer [L2Zn(NCS)2]2 with thiocyanate bridges.350... 

The group ofWalborsky probably has described one of the first true anionic/radi-cal domino process in their synthesis of the spirocyclopropyl ether 2-733 starting from the tertiary allylic bromide 2-730 (Scheme 2.161) [369]. The first step is a Michael addition with methoxide which led to the malonate anion 2-731. It follows a displacement of the tertiary bromide and a subsequent ring closure which is thought to involve a SET from the anionic center to the carbon-bromine anti bonding orbital to produce the diradical 2-732 and a bromide anion. An obvious alternative Sn2 halide displacement was excluded due to steric reasons and the ease with which the reaction proceeded. 

The mechanism may involve a nucleophilic attack of the double-bond of the phosphole (17) on the phosphorus atom of phosphorus tribromide to provide intermediates that are stabilized by the loss of proton, pseudorotation, and finally the departure of a bromide anion [48, 49],... 

Nucleophilic substitution at the a-carbon atom does not occur in the case of the most studied and stable bicyclic disulfonium dications.96 Although the reaction of dication 34 with bromide ions formally leads to the S C bond cleavage, the reaction mechanism involves initial nucleophilic attack at the sulfonium atom by the bromide anion. The bromosulfonium salt intermediate... 

Fig. 6 Stabilization of oxidized [Cp Co(dithiolene)]+ complexes through (a) dimerization in [Cp Co(dddt)]22+ and (b) coordination by bromide anion in Cp Co(dmit)Br...

Free radicals are produced by the reactions of HBr and the bromide anion with hydroperoxide [198] ... 

The active alkoxyl radicals formed by this reaction start new chains. Apparently, the hydroperoxide group penetrates in the polar layer of the micelle and reacts with the bromide anion. The formed hydroxyl ion remains in the aqueous phase, and the MePhCHO radical diffuses into the hydrocarbon phase and reacts with ethylbenzene. The inverse emulsion of CTAB accelerates the decay of hydroperoxide MePhCHOOH. The decomposition of hydroperoxide occurs with the rate constant k = 7.2 x 1011 exp(-91.0/R7) L mol-1 s-1 (T = 323-353 K, CTAB, ethylbenzene [28]). The decay of hydroperoxide occurs more rapidly in an 02 atmosphere, than in an N2 atmosphere. 

Presumably, isoxazoline (175) is thermodynamically more favorable than overcrowded aziridine (176) due to jt,jt conjugation. Elimination of the bromide anion from intermediate (175) is also hindered due to instability of the carbocation that formed. 

To account for the formation of the final products, a pathway involving the initial generation of cycloadducts (405) followed by elimination of the bromide anion to give cations A was proposed. The cations can stabilize by deprotonation of the C-4 atom followed by retro-[4 + 2]-cycloaddition of intermediates B to give isolable vinylisoxazolines (403). 

FIGURE 3.20. Correlation between the fragmentation rate constant (in s 1) and the standard potential and the fragmentation standard free energy (in V vs. SCE) (top) and activation/-driving force relationship (free enthalpies in eV) (bottom) for aryl bromide anion radicals. Data from Table 3.8. Adapted from Figure 4 of reference 29, with permission from the American Chemical Society. 

Fig. 9 A projection of the structure of [Cu(dmca)2]Br2-2bu-2 MeOH-O.8 MeCN onto the (0 0 1) plane showing the layers at z/c=0.00 or 0.50 comprising bu molecules and bromide anions (atom identification as for Fig. 1 plus copper large light grey circles bromine large dark grey circles) [16]...

The crystal structure of the bromide complex of [78] has been elucidated and clearly shows bromide anions hydrogen-bonded to the amide N—H groups and, interestingly, also to the cyclopentadienyl hydrogen atoms (Fig. 40). 

The value of (feobsd caic) at a given concentration of bromide anion depends on the association constant for formation of the ion pair from free ions (Ai s = and on the relative reactivity of the ion-pair and free carbocation toward addition of solvent For example, if is small, then the concentration of the ion-pair... 

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