Bromination examples

Bromination of fatty acids in the a-position can be effected quite readily in the presence of phosphorus trichloride, red phosphorus or pyridine as catalysts or halogen carriers with acetic acid, the addition of acetic anhydride (to ensure the absence of water) improves the yield and facilitates the bromination. Examples are —... 

A number of N-brominated and N-chlorinated heterocycles also provide sources of electrophilic bromine. Examples include 1-chlorobenzotriazole (82JOC4895 87JOC173 88CHE36) and various HBr and Br2 adducts of pyridines, or pyridine perbromides [84SC939 85JAP(K)60/87264], Polymer-supported reagents of this type include 1-cyclohexylpyridinium perbromide linked to polystyrene, effective for the bromination of 1-methylindole, benzo[fc]furan, and benzo[6]thiophene (89T7869). 

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. 

Like bromine, iodine is soluble in organic solvents, for example chloroform, which can be used to extract it from an aqueous solution. The iodine imparts a characteristic purple colour to the organic layer this is used as a test for iodine (p. 349). NB Brown solutions are formed when iodine dissolves in ether, alcohol, and acetone. In chloroform and benzene a purple solution is formed, whilst a violet solution is produced in carbon disulphide and some hydrocarbons. These colours arise due to charge transfer (p. 60) to and from the iodine and the solvent organic molecules. 

Many of the reactions of halogens can be considered as either oxidation or displacement reactions the redox potentials (Table 11.2) give a clear indication of their relative oxidising power in aqueous solution. Fluorine, chlorine and bromine have the ability to displace hydrogen from hydrocarbons, but in addition each halogen is able to displace other elements which are less electronegative than itself. Thus fluorine can displace all the other halogens from both ionic and covalent compounds, for example... 

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

A -Bromosuccinimide (prepared by the action of bromine on succinimide at o in the presence of sodium hydroxide) is a valuable specific reagent for brominating olefines in the a-methylene position to the double bond without simultaneously adding bromine to this bond. For example, if A -bromosuccini-mide is represented by (C4H40t)NBr —... 

The reaction is applicable to the preparation of amines from amides of aliphatic aromatic, aryl-aliphatic and heterocyclic acids. A further example is given in Section IV,170 in connexion with the preparation of anthranilic acid from phthal-imide. It may be mentioned that for aliphatic monoamides containing more than eight carbon atoms aqueous alkaline hypohalite gives poor yields of the amines. Good results are obtained by treatment of the amide (C > 8) in methanol with sodium methoxide and bromine, followed by hydrolysis of the resulting N-alkyl methyl carbamate ... 

In the absence of catalysts, toluene when treated with chlorine (or bromine) at the boiling point, preferably with exposure to sunlight or other bright light source, undergoes halogenation in the side chain. The entrance of the first chlorine atom, for example, proceeds at a much faster rate than the entrance of the second chlorine atom so that in practice the major portion of the toluene is converted into benzyl chloride before appreciable chlorination of benzyl chloride occurs ... 

Mono-substitution products of primary amines cannot easUy be prepared by direct action of the appropriate reagent for example, bromination of aniline yields largely the 2 4 6-tribomo derivative and nitration results in much oxidation. If, however, the amino group is protected as in acetanilide, smooth substitution occurs. Thus with bromine, />-bromoacetanilide is the main product the small quantity of the ortlio isomeride simultaneously formed can be easily eliminated by crystallisation. Hydrolysis of p-bromoacetanilide gives/ -bromoaniline ... 

Phenylpropiolic acid. This is an example of an aromatic acetylenic acid, and is made by adding bromine to the ethylenio linkage in ethyl cinnamate, and treating the resulting dibromide with alcohobc potassium hydroxide which eliminates two molecules of hydrogen bromide ... 

Fester [18] has made this modification to his recipe by promoting the idea that bubbling HCI gas into the safrole/48% aq HBr reaction mix one can affect dehydration allowing dominant bromination to occur. Strike does not know if that actually works. Could be. But what Strike and Eleusis draw on are the examples given by Vogel [37 p277]. 

The high nucleophilicity of sulfur atoms is preserved, even if it is bound to electron withdrawing carbonyl groups. Thiocarboxylales, for example, substitute bromine, e.g. of a-bromo ketones. In the presence of bases the or-acylthio ketones deprotonate and rearrange to episulfides. After desulfurization with triphenylphosphine, 1,3-diketones are formed in good yield. Thiolactams react in the same way, and A. Eschenmoser (1970) has used this sequence in his vitamin B]2 synthesis (p. 261). 

A classical reaction leading to 1,4-difunctional compounds is the nucleophilic substitution of the bromine of cf-bromo carbonyl compounds (a -synthons) with enolate type anions (d -synthons). Regio- and stereoselectivities, which can be achieved by an appropiate choice of the enol component, are similar to those described in the previous section. Just one example of a highly functionalized product (W.L. Meyer, 1963) is given. 

Another method for the hydrogenoiysis of aryl bromides and iodides is to use MeONa[696], The removal of chlorine and bromine from benzene rings is possible with MeOH under basic conditions by use of dippp as a ligand[697]. The reduction is explained by the formation of the phenylpalladium methoxide 812, which undergoes elimination of /i-hydrogen to form benzene, and MeOH is oxidized to formaldehyde. Based on this mechanistic consideration, reaction of alcohols with aryl halides has another application. For example, cyclohex-anol (813) is oxidized smoothly to cyclohexanone with bromobenzene under basic conditions[698]. 

As a further electrophilic substitution the bromination of selenazoles has been investigated. This is not as complicated as nitration. Bromination was carried out in several solvents and with various amounts of bromine. In spite of the great variation in conditions, monobromo derivatives containing the bromine atom in the 5-position are always formed. This could be established, for example, by the bromination of the 2-amino-4-p-nitrophenylselenazole (Scheme 34) and its 2-benzamino compound (98). The 2-benzamido bromo compound gives the same bromo... 

This simplified method gives 2-aminothiazole in good yield (50 to 70%) (311, 330), Other reactants can replace iodine, for example, chlorine, bromine, sulfuryl chloride, chlorosulfonic acid, or sulfur monochloride also give good results. 

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

Because vicinal dihalides are prepared by addition of chlorine or bromine to alkenes (Section 6 14) alkenes especially terminal alkenes can serve as starting mate rials for the preparation of alkynes as shown m the following example... 

In the two examples cited so far the chlorination of propene and the bromination of cyclohexene both criteria are met... 

The classification of hydrocarbons as aliphatic or aromatic took place m the 1860s when It was already apparent that there was something special about benzene toluene and their derivatives Their molecular formulas (benzene is CgHg toluene is C7Hj ) indicate that like alkenes and alkynes they are unsaturated and should undergo addition reac tions Under conditions m which bromine for example reacts rapidly with alkenes and alkynes however benzene proved to be inert Benzene does react with Bi2 m the pres ence of iron(III) bromide as a catalyst but even then addition isn t observed Substitu tion occurs instead ... 

Aluminum chloride is a stronger Lewis acid than iron(lll) bromide and has been used as a catalyst in electrophilic bromination when as in the example shown the aromatic ring bears a strongly deactivating substituent... 

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

Compounds that contain chlorine, bromine, sulfur, or silicon are usually apparent from prominent peaks at masses 2, 4, 6, and so on, units larger than the nominal mass of the parent or fragment ion. Eor example, when one chlorine atom is present, the P + 2 mass peak will be about one-third the intensity of the parent peak. When one bromine atom is present, the P + 2 mass peak will be about the same intensity as the parent peak. The abundance of heavy isotopes is treated in terms of the binomial expansion (a -I- h) , where a is the relative abundance of the light isotope, b is the relative abundance of the heavy isotope, and m is the number of atoms of the particular element present in the molecule. If two bromine atoms are present, the binomial expansion is... 

For other elements that occur with major relative abundances of more than one isotope in the natural state, the isotope pattern becomes much more complex. For example, with chlorine and bromine, the presence of these elements is clearly apparent from the isotopes Cl and for chlorine and Br and Br for bromine. Figure 47.2a shows the molecular ion region for the compound chlorodecane. Now, there are new situations in that C, C, C1, and Cl isotopes all have probabilities of occurring together. Thus, there are molecular ion peaks for + Cl, C + Cl, + Cl, and so on. Even so, the isotopic ratio of 3 1 for Cl to Cl is very clear... 

Loaded Adsorbents. Where highly efficient removal of a trace impurity is required it is sometimes effective to use an adsorbent preloaded with a reactant rather than rely on the forces of adsorption. Examples include the use of 2eohtes preloaded with bromine to trap traces of olefins as their more easily condensible bromides 2eohtes preloaded with iodine to trap mercury vapor, and activated carbon loaded with cupric chloride for removal of mercaptans. 

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