Bonding bromination

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. 

Substituted and benzo-annulated oxepins readily undergo addition of bromine across the nonaromatic double bond. Bromination of 3,6-bridged oxepins can occur in two different ways, either as a 1,2-addition164 or as formal 1,4-addition to the diene system of the corresponding benzene oxide to give products i.129 138-140 164... 

A somewhat unusual sequence to generate azepanones 80 involved the intramolecular addition of hydroxylamines to alkynes 76 to form cyclic nitrones 77. A vinyl magnesium bromide addition at low temperatures and a reduction with TiCls followed by N-Boc protection led to the azepane 78. Double bond bromination and subsequent RUO4 oxidation gave the lactam 79. Several further steps allowed the generation of the lactam structure 80 proposed for d,/-aca-cialactam, but the spectral data of the synthetic material differed from that of the natural product (Scheme 16)] [23 a, b]. 

The X-ray structure of the dibromine complex with toluene (measured at 123 K) is more complicated, and shows multiple crystallographically independent donor/acceptor moieties [68]. Most important, however, is the fact that in all cases the acceptor shows an over-the-rim location that is similar to that in the benzene complex. In both systems, the acceptor is shifted by 1.4 A from the main symmetry axis, the shortest Br C distances of 3.1 A being significantly less than the sum of the van der Waals radii of 3.55 A [20]. Furthermore, the calculated hapticity in the benzene/Br2 complex (x] = 1.52) is midway between the over-atom (rj = 1.0) and over-bond (rj = 2.0) coordination. In the toluene complex, the latter varies from rj = 1.70 to 1.86 (in four non-equivalent coordination modes) and thus lies closer to the over-bond coordination model. Moreover, the over-bond bromine is remarkably shifted toward the ortho- and para-carbons that correspond to the positions of highest electron density (and lead to the transition states for electrophilic aromatic bromination [12]). Such an experimental location of bromine is in good agreement with the results of high level theoretical... 

At temperatures above 500 °C more than 90 % of the organic bonded bromine was converted into bromide. By adding oxygen complete breakdown to carbon dioxide, bromide and water is possible. Under supercritical water oxidation conditions more than 99 % of the organic bonded bromine was found in the aqueous phase. A formation of bromine, hydrogen bromide and dioxines as in thermal decomposition was not observed. 

Generally, in bromine addition to carbon-carbon double bonds, bromine bridging, solvent dependent dissociation of the ionic intermediates, steric interactions between the counteranion and the first bonded halogen during the nucleophilic step, the possibility of carbon-carbon rotation in the carbenium ion intermediate, preassociation phenomena and nucleophilic assistance determine the stereochemical behavior of the reaction . Several of these factors have been invoked to explain the stereochemistry of bromine addition to dienes, although others have been completely ignored or neglected. Bromine addition to cyclopentadiene, 1,3-cyclohexadiene, 2,4-hexadienes and 1,3-pentadienes has been examined repeatedly by Heasley and coworkers and the product distribution has been... 

All the chemical methods for the determination of the amount of unsaturated compounds are based on the addition reaction of halogens such as bromine or iodine to double bonds. Bromine number or iodine number is determined by this method. The bromine number or iodine number shows the amount of bromine or iodine needed to saturate all the unsaturated bonds in a hundred grams of the sample. 

An examination of the effect of meta- and para-bromination on the quantum yields of isomerization and the photostationary-state ratios has shown that, while a para-bonded bromine enhances Sx - Tn intersystem crossing, bromination at... 

Bromine. Of all the elements in the periodic table, only two are liquid at room temperature. One of them is mercury, and the other is bromine. Bromine is a dark brown liquid that volatilizes readily. Chemists use it as a reagent to analyze for compounds that may contain carbon-carbon double bonds. Bromine reacts with these rapidly, and, if its characteristic color disappears, it means that a double bond is present. 

As a bromine molecule approaches an alkene, the electron density of the alkene it bond repels electron density in the closer bromine, polarizing the bromine molecule and making the closer bromine atom electrophilic. The alkene donates a pair of electrons to the closer bromine, causing displacement of the distant bromine atom. As this occurs, the newly bonded bromine atom, due to its size and polarizability, donates an electron pair to the carbon that would otherwise be a carbocation, thereby stabilizing the positive charge by delocalization. The result is a bridged bromonium ion intermediate. 

Make sure to avoid selecting a quaternary position, as quaternary positions do not possess a C—H bond. Bromination cannot occur at a quaternary site. The major product is expected to result from bromination at the tertiary position ... 

Bromine molecules are polarizable a partial positive charge builds up on the bromine atom closest to the double bond. Bromine acts as an electrophile. The partial charge becomes a full charge and a bromine cation attaches to the double bond, leaving a Br ion behind. The cyclic intermediate is called a bromonium ion. 

A The covalent radius of bromine is one-half the distance between two bonded bromine atoms. 

The Jt electrons of the alkene act as a nucleophile to displace bromide ion from bromine. The resulting cychc bromonium ion can be viewed as the addition of Br to the double bond. Bromine has two covalent bonds and a formal 1+ charge in this intermediate. Attack of the nucleophilic bromide ion occurs from the opposite face because the bromine atom that is already there blocks approach from the same face. 

A detailed analysis of the cross-polarisation behaviour showed that quantitative results can be obtained. The amount of unreacted units, typically 0-15%, was found to depend on the polymerisation parameters. Conditions favouring mobility, i.e., higher temperatures or increased solvent quality, resulted in lower content of residual double bonds. Bromine addition values are 2-3% higher than the NMR data. The reactivity toward bromine further indicates that the mobility is reasonably high. This has also been confirmed by measurements of the rotating-frame relaxation time constant, Tj ( C) Most likely, Tjp is dominated by spin-lattice processes i.e., it can be interpreted in terms of molecular dynamics. The values obtained for C=0 and >C =CH2 in unreacted units are about twice that of C=0 in reacted units, indicating increased mobility. The reactivity of the remaining double bonds in a radical polymerisation with a chiral monomer was also demonstrated. 

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