Bromine addition

For chemical processes, some examples are the elimination of aromatics by sulfonation, the elimination of olefins by bromine addition on the double bond (bromine number), the elimination of conjugated diolefins as in the case of the maleic anhydride value (MAV), and the extraction of bases or acids by contact with aqueous acidic or basic solutions. 

The reactivity of polyo efins to bromine addition may not be complete or it can depend on the relative positions of the double bonds and the branching within the molecules provoking steric hindrance to addition. 

Bromine addition to alkenes is an example of a stereospecific reaction A stereospecific reaction is one m which stereoisomeric starting materials yield products... 

Writing the bromine addition step m this way emphasizes the increased nucleophilicity of the enol double bond and identifies the source of that increased nucleophilicity as the enolic oxygen... 

Brominated Additive Flame Retardants. Additive flame retardants are those that do not react in the appHcation designated. There are a few compounds that can be used as an additive in one appHcation and as a reactive in another. Tetrabromobisphenol A [79-94-7] (TBBPA) is the most notable example. Tables 5 and 6 Hst the properties of most commercially available bromine-containing additive flame retardants. 

Table 6. Polymeric and Oligomeric Brominated Additive Flame Retardants...

Table 3. Stereochemistry of Free-Radical Bromine Addition [5] 4-H02CCgH4CF=CFX 4-H02CCgH4CFBtCFXBr...

Repeat this analysis for the bromine addition and substitution reactions of benzene leading to trans-5,6-dibromo-l,3-cyclohexadiene and phenyl bromide, respectively. Do your thermochemical results account for the experimental observations ... 

The oxidation of 3-benzothiepin to its S -oxide 9 (R = H) was unsuccessful with 3-chloroper-oxybenzoic acid or sodium periodate.68 Also, the route which uses the formation of the di-bromosulfuranes 8, known to be sensitive to hydrolysis (vide infra), was ultimately unsuccessful because 8 is only available if the C — C double bonds of the thiepin ring are deactivated towards bromine addition by ester substituents (R = C02Me), concurrently providing the compound with stability towards hydrolysis.65... 

Faster ort/zo-bromination than bromine addition is the reason why bromine adduct was not obtained in the bromination of 2-allylphenol with NBS (ref. 15). 

In the first step bromocriptine 2 is isomerized to 2a, followed by an attack on proline ring in aminocyclol moiety of the molecule (formation of a new double bound on lO -ll, and bromination). This dibromo-compound 5 is brominated additionally on C-2 -propyl group. Tribromo-compound fi is very lipophilic and practically devoid of pharmacological activity. Hydroxy group and amide groups remain intact after all these reactions. 

Systematic studies of the selectivity of electrophilic bromine addition to ethylenic bonds are almost inexistent whereas the selectivity of electrophilic bromination of aromatic compounds has been extensively investigated (ref. 1). This surprising difference arises probably from particular features of their reaction mechanisms. Aromatic substitution exhibits only regioselectivity, which is determined by the bromine attack itself, i.e. the selectivity- and rate-determining steps are identical. 

Whereas the three possible selectivities, stereo-, regio- and chemo-selectivity, of bromine addition are determined in steps posterior to the formation of the ionic intermediate. Bromine addition is, therefore, more complex than bromine substitution, as regards the variety of the selectivities and as regards the mechanistic aspects which determine the product formation. 

The exocyclic a,P-unsaturated ketones la-h used for the preparation of dibromides 2a-h were E isomers synthesized by known procedures (refs. 7,8). Previously we reported the synthesis of some dibromides (2a,d,g) used here as starting materials, by the bromine addition of the appropriate a,P-unsaturated ketones (la,d,g) (ref. 6). In the case of our present study, compounds lb,c,e,f,h were allowed to react with a small excess of bromine in carbon tetrachloride solution at room temperature for approx. 20 min. to afford the dibromides 2b,c,e,f,h (Eqn. 1) (Table 1). 

The adducts containing di- and tribromomethyl groups can react with another molecule of monomer, i.e. the secondary reactions occur including step-by-step telomerization in this case. The presence of the bromine atom adjacent to the radical center makes it probable the fragmentation with ejection of the bromine atom, which starts the further reaction, and as a result, the products of bromine addition to a double bond, not adducts, are formed. 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

Firstly, if bromine addition is carried out in the presence of added nucleophiles Ye or Y (e.g. Cl , N03 , H20 ) then, in addition to the expected 1,2-dibromide (3), products are also obtained in which one bromine atom and one Y atom, or group, have been added to the double bond (4) ... 

The intermediate in bromine addition, whether bromonium ion or carbocation, is indeed positively charged. In so far as its formation is... 

It is noteworthy that in protic solvents most of the bromination rate constants used in mechanistic studies are k, the rate constant for free bromine addition only, that is, for a pathway from which any contribution of bromide is excluded since the involvement of this ion is taken into account in the kBrj- term. 

Data on molecular structure of bromonium ions are sometimes extrapolated from that of the tribromide-adamantylideneadamantane bromonium ion pair [6] (Slebocka-Tilk et ai, 1985), the only stable ionic bromination intermediate that can be isolated and whose crystal structure has been determined. Since the first observation by Strating et al. (1969), it has been established that bromine addition to adamantylideneadamantane [5] in... 

Ruasse (1990). bk in m-1s-1 for free bromine addition at 25 °C. This pathway occurs but its magnitude cannot be calculated exactly. 

---