Bromine addition reactions

According to K. H. Meyer, this surprising reactivity—which is also encountered amongst the enols—is explained by the fact that the OH-group activates the double bond adjacent to it and the two neighbouring double bonds of the conjugated system (Thiele s theory) are also involved in this activation. Phenol, therefore, can yield 1 2- and 1 4-substitution products with bromine, addition reactions first taking place ... 

The second piece of evidence against the mechanism of Figure 10-7 is that bromine addition reactions carried out in the presence of more than one nucleophilic reagent usually give mixtures of products. Thus the addition of bromine to an alkene in methanol solution containing lithium chloride leads not only to the expected dibromoalkane, but also to products resulting from attack by chloride ions and by the solvent ... 

Bromine addition to cyclopropanes is slow in the absence of light unless the ring is heavily substituted . In the presence of light, reaction is rapid since halogen radical attack on substituted cyclopropanes is fast even at — 78°C. In the presence of light, HBr is produced and cyclopropane reacts more competitively with proton acids than with bromine. Addition reactions to cyclopropane are generally slow in the absence of acid catalysts and therefore the conjugate acid is probably involved in reaction. The more facile reaction of cyclopropane with HBr compared with bromine is in contrast with alkene chemistry where bromine addition is the more rapid ... 

The steric effects of alkyl substituents can retard bromine addition reactions, however, and this effect can be large enough to mask the electronic effect of alkyl substituents. Ruasse, M.-F. Motallebi, S. Gal, B. Lomas, J. S. /. Org. Chem. 1990, 55, 2298. 

Role of Charge Transfer Complexes In Bromine Addition Reactions... 

One of the earliest observations about bromine addition reactions is that mixing bromine and alkenes leads to a new, transient UV-vis absorption band, suggesting the formation of a bromine-olefin n complex (6). The stabilities of these charge transfer complexes depend significantly on the ionization potentials of the alkenes, although the polarizabilities of the alkenes can be a significant factor also. ... 

Not only does the experimental evidence support the role of fl CT complex as an intermediate in bromine addition reactions, but it appears that many different CT complexes may occur in mixtures of alkenes and bromines. 

The mechanism in Figure 9.5 also provides a rationalization for some observations of solvent effects in bromine addition reactions. The major product of the reaction of bromine with cyclohexene in methanol is trans-2-bromo-l-methoxycyclohexane. traMS-l,2-Dibromocyclohexane is observed if Br is added to the solution, but the yield of the dibromo adduct approaches 0% as [Br ] approaches 0 M. This result stands in contrast to the 27% of 1,2-dibromo adduct obtained from the corresponding reaction in acetic acid. It appears that the greater polarity of methanol accelerates the dissociation of IIP to DI, and the greater nucleophiliciW of methanol enhances the reaction of solvent with IIP, SSIP, DI, and SSIP. ... 

This method follows the ASTM D 1159 and D 2710 procedures and the AFNOR M 07-017 standard. It exploits the capacity of the double olefinic bond to attach two bromine atoms by the addition reaction. Expressed as grams of fixed bromine per hundred grams of sample, the bromine number, BrN, enables the calculation of olefinic hydrocarbons to be made if the average molecular weight of a sufficiently narrow cut is known. 

The basic premise for making bromosafrole has been to mix sa-frole with Hydrobromic Acid (a.k.a. hydrogen bromide, HBr). That s it. The HBr does what is called a Markovnikov addition reaction whereby the HBr sees the allyl double bond of safrole and preferentially attaches its hydrogen to the gamma carbon and its bromine to the middle beta carbon (don t ask). 

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

Poly(phenylene oxide)s undergo many substitution reactions (25). Reactions involving the aromatic rings and the methyl groups of DMPPO include bromination (26), displacement of the resultant bromine with phosphoms or amines (27), lithiation (28), and maleic anhydride grafting (29). Additional reactions at the open 3-position on the ring include nitration, alkylation (30), and amidation with isocyanates (31). 

Addition Reactions. The C=C double bond of aEyl alcohol undergoes addition reactions typical of olefinic double bonds. For example, when bromine is added, a good yield of 2,3-dibromopropanol is obtained although 1,2,3-tribromopropane is obtained as a by-product. 

Halogen-substituted succinimides are a class of products with important appHcations. /V-Bromosuccinimide [128-08-5] mp 176—177°C, is the most important product ia this group, and is prepared by addition of bromine to a cold aqueous solution of succinimide (110,111) or by reaction of succinimide with NaBr02 iu the presence of HBr (112). It is used as a bromination and oxidation agent ia the synthesis of cortisone and other hormones. By its use it is possible to obtain selective bromine substitution at methylene groups adjacent to double bonds without addition reactions to the double bond (113). 

Purines, N-alkyl-N-phenyl-synthesis, 5, 576 Purines, alkylthio-hydrolysis, 5, 560 Mannich reaction, 5, 536 Michael addition reactions, 5, 536 Purines, S-alkylthio-hydrolysis, 5, 560 Purines, amino-alkylation, 5, 530, 551 IR spectra, 5, 518 reactions, 5, 551-553 with diazonium ions, 5, 538 reduction, 5, 541 UV spectra, 5, 517 Purines, N-amino-synthesis, 5, 595 Purines, aminohydroxy-hydrogenation, 5, 555 reactions, 5, 555 Purines, aminooxo-reactions, 5, 557 thiation, 5, 557 Purines, bromo-synthesis, 5, 557 Purines, chloro-synthesis, 5, 573 Purines, cyano-reactions, 5, 550 Purines, dialkoxy-rearrangement, 5, 558 Purines, diazoreactions, 5, 96 Purines, dioxo-alkylation, 5, 532 Purines, N-glycosyl-, 5, 536 Purines, halo-N-alkylation, 5, 529 hydrogenolysis, 5, 562 reactions, 5, 561-562, 564 with alkoxides, 5, 563 synthesis, 5, 556 Purines, hydrazino-reactions, 5, 553 Purines, hydroxyamino-reactions, 5, 556 Purines, 8-lithiotrimethylsilyl-nucleosides alkylation, 5, 537 Purines, N-methyl-magnetic circular dichroism, 5, 523 Purines, methylthio-bromination, 5, 559 Purines, nitro-reactions, 5, 550, 551 Purines, oxo-alkylation, 5, 532 amination, 5, 557 dipole moments, 5, 522 H NMR, 5, 512 pJfa, 5, 524 reactions, 5, 556-557 with diazonium ions, 5, 538 reduction, 5, 541 thiation, 5, 557 Purines, oxohydro-IR spectra, 5, 518 Purines, selenoxo-synthesis, 5, 597 Purines, thio-acylation, 5, 559 alkylation, 5, 559 Purines, thioxo-acetylation, 5, 559... 

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. 

Bromotrichloromethane can also be used effectively in the addition reaction. Because of the preferential abstraction of bromine, a trichloromethyl unit is added to the less substituted carbon atom of the alkene ... 

Preparation of 3a-Hydroxy-5) -pregn-17(20)-en-21-oic Acid . A solution of 15 g of 3a-acetoxy-5jS-pregnan-20-one in 290 ml of glacial acetic acid is treated with 13 g of bromine at room temperature. After complete addition of bromine the reaction mixture is heated at 40-50° for 30 min, and the product precipitated with water and filtered. The product is taken up in ethyl acetate (500-600 ml) and the resulting solution washed with dilute aqueous potassium bicarbonate. The solvent is concentrated in vacuo and the product crystallized from acetone to give 16g of dibromide mp, 173-175°. 

The classification of hydrocar bons as aliphatic or ar omatic took place in the 1860s when it was aheady apparent that there was something special about benzene, toluene, and their- derivatives. Their molecular- for-mulas (benzene is CgHg, toluene is CyKj ) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reactions. Under conditions in which bromine, for example, reacts rapidly with alkenes and alkynes, however, benzene proved to be inert. Benzene does react with Br-2 in the presence of iron(III) bromide as a catalyst, but even then addition isn t observed. Substitution occurs instead ... 

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 ... 

As in the case of the steroids, introduction of additional nuclear substituents yields morphine analogs of increased potency. The more important of these are derived from one of the minor alkaloids that occur in opium. Thebaine (14), present in crude opium in about one-tenth the amount of morphine, exhibits a reactive internal diene system that is well known to undergo various addition reactions in a 1,4 manner (e.g., bromination). Thus, reaction with hydrogen peroxide in acid may be visualized to afford first the 14-hydroxy-6-hemiketal (15). Hydrolysis yields the isolated unsaturated ketone (16). Catalytic reduction... 

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. 

When the allylic cation reacts with Br to complete the electrophilic addition, reaction can occur either at Cl or at C3 because both carbons share the positive charge (Figure 14.4). Thus, a mixture of 1,2- and 1,4-addition products results. (Recall that a similar product mixture was seen for NBS bromination of alkenes in Section 10.4, a reaction that proceeds through an allylic radical.)... 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Benzene shows neither the typical reactivity nor the usual addition reaction of ethylene. Benzene does react with bromine, Brs, but in a different type of reaction ... 

In this reaction, called bromination, one of the hydrogen atoms has been replaced by a bromine atom. Notice that the double bond structure is not affected—this is not an addition reaction. Nitric acid causes a similar reaction, called nitration ... 

Arenes are unsaturated but, unlike the alkenes, they are not very reactive. Whereas alkenes commonly take part in addition reactions, arenes undergo predominantly substitution reactions, with the TT-bonds of the ring left intact. For example, bromine immediately adds to a double bond of an alkene but reacts with benzene only in the presence of a catalyst—typically, iron(III) bromide—and it does not affect the bonding in the ring. Instead, one of the bromine atoms replaces a hydrogen atom to give bromobenzene, C H Br ... 

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