Anti bromination

Bromine is used in the manufacture of many important organic compounds including 1,2-dibromoethane (ethylene dibromide), added to petrol to prevent lead deposition which occurs by decomposition of the anti-knock —lead tetraethyl bromomethane (methyl bromide), a fumigating agent, and several compounds used to reduce flammability of polyester plastics and epoxide resins. Silver(I) bromide is used extensively in the photographic industry... 

The reaction of chlorine and bromine with cycloalkenes illustrates an important stereo chemical feature of halogen addition Anti addition is observed the two bromine atoms of Br2 or the two chlorines of CI2 add to opposite faces of the double bond... 

Addition of halogens (Sections 6 14-6 16) Bromine and chlorine add to alkenes to form vicinal dihalides A cy clic halonium ion is an intermediate Stereospecific anti addition is observed... 

When treated with bromine or chlorine in aqueous solution alkenes are con verted to vicinal halohydrins A haloni um ion IS an intermediate The halogen adds to the carbon that has the greater number of hydrogens Addition is anti... 

Figures 7 13 and 7 14 depict the stereochemical relationships associated with anti addition of bromine to (E) and (Z) 2 butene respectively The trans alkene (E) 2 butene yields only meso 2 3 dibromobutane but the cis alkene (Z) 2 butene gives a racemic mixture of 2R 3R) and 2S 3S) 2 3 dibromobutane...

Construct a molecular model corresponding to the Fischer projection of rneso 2 3 dibro mobutane Convert this molecular model to a staggered conformation in which the bromines are anti to one another Are the methyl groups anti or gauche to one another in this staggered con formation" ... 

Bromination. 1-Bromoalkanes are produced commercially by the anti-Markovnikow free-radical addition of HBr to a-olefins. These are further reacted with dimethyl amine to produce alkyldimethyl amines, which ultimately are converted to amine products for household cleaning and personal care. 

Addition of chlorine or bromine in the presence of water can yield compounds containing haUde and hydroxyl on adjacent carbon atoms (haloalcohols or halohydrins). The same products can be obtained in the presence of methanol (13) or acetic acid (14). As expected from the halonium ion intermediate, the addition is anti. As expected from Markovnikov s rule, the positive halogen goes to the same carbon that the hydrogen of a protic reagent would. 

Entries 1 and 2 in Scheme 2.9 are typical of concerted syn addition to alkene double bonds. On treatment with peroxyacetic acid, the Z-alkene affords the cis-oxirane, whereas the -alkene affords only the iraws-oxirane. Similarly, addition of dibromocarbene to Z-2-butene yields exclusively l,l-dibromo-cw-2,3-dimethylcyclopropane, whereas only 1,1-dibromo-/ra 5-2,3-dimethylcyclopropane is formed from -2-butene. There are also numerous stereospecific anti additions. Entiy 3 shows the anti stereochemistry typical of bromination of simple alkenes. 

The stereochemistry of chlorination can be explained in similar terms. Chlorine would be expected to be a somewhat poorer bridging group than bromine because it is less polarizable and more resistant to becoming positively charged. Comparison of the data for bromination and chlorination of E- and Z-l-phenylpropene confirms this trend (see Table 6.2). Although anti addition is dominant in bromination, syn addition is slightly preferred... 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

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. 

When indene (4) is brominated in carbon tetrachloride, it gives some syn addition ( 15%), but indenone (5) gives only anti addition under the same conditions. 

In the bromination of styrene, a po-+ plot is noticeably curved. If the extremes of the curves are taken to represent straight lines, the curve can be resolved into two Hammett relationships with p = —2.8 for electron-attracting substituents and p = —4.4 for electron-releasing substituents. When the corresponding -methylstyrenes are examined, a similarly curved ap plot is obtained. Furthermore, the stereospecificity of the reaction in the case of the -methylstyrenes varies with the aryl substituents. The reaction is a stereoespecific anti addition for strongly electron-attracting substituents but becomes only weakly stereoselective for electron-releasing substituents, e.g., 63% anti, 37% syn, forp-methoxy. Discuss the possible mechanistic basis for the Hammett plot curvature and its relationship to the stereochemical results. 

The reactions of bromine with E- or Z-1 -fluoropropene under ionic conditions result in stereospecific anti additions to yield the 15,26 and 1R,25 products, respectively [ftl] (equation 4)... 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

Fluor-, of or combined with fluorine, fluoro-, fluo-, fluoride of (see instances following),. ammonium, n. ammonium fluoride, -an-thenchinon, n. fluoranthenequinone. -anti-mon, n. antimony fluoride, -arson, n. arsenic fluoride, -baryum, n. barium fluoride, -benzol, n. fluorobensene, fluobenaene. -bor, n. boron fluoride. -borsMure, /. fluo-boric acid. -brom, n. bromine fluoride, -chrom, n. chromium fluoride, -eisen, n. iron fluoride. 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene If a bromonium ion is formed as an intermediate, we can imagine that the large bromine atom might "shield" one side of the molecule. Reaction with Br ion in the second step could then occur only from the opposite, unshielded side to give trans product. 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

The influence of 1,2-asymmctric induction on the exchange of diastereotopic bromine atoms has also been investigated22,23. Thus, treatment of the / -silyloxydibromo compound 15 with butyllithium at — 110°C in the presence of 2-methylpropana led to products 17-19 after the reaction mixture was warmed to 20 °C. The distribution of the products indicates that the diastereomeric lithium compounds 16 A and 16B were formed in a ratio of 84 16, with 16A being kinetically favored by 1,2-asymmetric induction. Formation of the m-configurated epoxide (cis,anti-18) was slowed to such an extent that its formation was incomplete and a substantial amount of the parent bromohydrin 17 remained. The analogous m.yyn-configurat-ed epoxide was not observed. Presumably for sterie reasons, the parent bromohydrin did not cyclize to the epoxide but instead led to the ketone 1923. 

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