A-Bromination

A white solid, m.p. 178 C. Primarily of interest as a brominaling agent which will replace activated hydrogen atoms in benzylic or allylic positions, and also those on a carbon atom a to a carbonyl group. Activating influences can produce nuclear substitution in a benzene ring and certain heterocyclic compounds also used in the oxidation of secondary alcohols to ketones. 

Copper 1) chloride, bromide and cyanide were used by Sandmeyer to introduce a chlorine, a bromine atom and a cyanide group respectively into a benzene ring by addition to the phenyl diazonium salt. 

And in the recipe above, Vogel want s to get rid of a secondary alcohol just like the one on MD-P2Pol and replace it with a bromine. Wait a minuter you may say, That isn t a double bond like... 

Addition of a bromine atom to C 1 gives a secondary alkyl radical... 

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

Begin by asking the question What kind of compound is the target molecule and what methods can I use to prepare that kind of compound The desired product has a bromine and a hydroxyl on adjacent carbons it is a vicinal bromohydrin The only method we have learned so far for the preparation of vicinal bromohydrms involves the reaction of alkenes with Bi2 m water Thus a reasonable last step is... 

Because the position of electrophilic attack on an aromatic nng is controlled by the direct ing effects of substituents already present the preparation of disubstituted aromatic com pounds requires that careful thought be given to the order of introduction of the two groups Compare the independent preparations of m bromoacetophenone and p bromoace tophenone from benzene Both syntheses require a Friedel-Crafts acylation step and a bromination step but the major product is determined by the order m which the two steps are carried out When the meta directing acetyl group is introduced first the final product IS m bromoacetophenone... 

This method of a bromination of carboxylic acids is called the Hell-Volhard-Zelinsky reaction This reaction is sometimes carried out by using a small amount of phosphorus instead of phosphorus trichloride Phosphorus reacts with bromine to yield phosphorus tribromide as the active catalyst under these conditions... 

Suggested formulations for various polymers using hexabromocyclododecane (HBCD), a brominated aHphatic a chlorinated paraffin, ie, a chlorinated aHphatic and decabromodiphenyl oxide, a brominated aromatic, are shown in Tables 2—4. These suggested formulations may not be strictiy comparable because of differences in the nature of the base resins. However, the suggestions are specific to a given UL-94 rating. 

Another approach to durable press—flame retardancy uses a combination of a cross-linking system, antimony(III) oxide, and a bromine-containing reactive additive, namely dibromoneopentyl glycol, to achieve dual properties (120,121). 

Iron(III) bromide [10031-26-2], FeBr, is obtained by reaction of iron or inon(II) bromide with bromine at 170—200°C. The material is purified by sublimation ia a bromine atmosphere. The stmcture of inoa(III) bromide is analogous to that of inon(III) chloride. FeBr is less stable thermally than FeCl, as would be expected from the observation that Br is a stronger reductant than CF. Dissociation to inon(II) bromide and bromine is complete at ca 200°C. The hygroscopic, dark red, rhombic crystals of inon(III) bromide are readily soluble ia water, alcohol, ether, and acetic acid and are slightly soluble ia Hquid ammonia. Several hydrated species and a large number of adducts are known. Solutions of inon(III) bromide decompose to inon(II) bromide and bromine on boiling. Iron(III) bromide is used as a catalyst for the bromination of aromatic compounds. 

Acetaldehyde can be used as an oxidation-promoter in place of bromine. The absence of bromine means that titanium metallurgy is not required. Eastman Chemical Co. has used such a process, with cobalt as the only catalyst metal. In that process, acetaldehyde is converted to acetic acid at the rate of 0.55—1.1 kg/kg of terephthahc acid produced. The acetic acid is recycled as the solvent and can be isolated as a by-product. Reaction temperatures can be low, 120—140°C, and residence times tend to be high, with values of two hours or more (55). Recovery of dry terephthahc acid follows steps similar to those in the Amoco process. Eastman has abandoned this process in favor of a bromine promoter (56). Another oxidation promoter which has been used is paraldehyde (57), employed by Toray Industries. This leads to the coproduction of acetic acid. 2-Butanone has been used by Mobil Chemical Co. (58). 

When the sample contains <0.1% selenium or if interfering substances are present, selenium may be preconcentrated by distillation from a bromine—hydrobromic acid mixture ... 

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). 

Introduction of a 3-bromosubstituent onto thiophene is accompHshed by initial tribromination, followed by reduction of the a-bromines by treatment with zinc/acetic acid, thereby utilizing only one of three bromines introduced. The so-called halogen dance sequence of reactions, whereby bromothiophenes are treated with base, causing proton abstraction and rearrangement of bromine to the produce the most-stable anion, has also been used to introduce a bromine atom at position 3. The formation of 3-bromotbiopbene [872-31-1] from this sequence of reactions (17) is an efficient use of bromine. Vapor-phase techniques have also been proposed to achieve this halogen migration (18), but with less specificity. Table 3 summarizes properties of some brominated thiophenes. 

Hydrogen haHde addition to vinyl chloride in general yields the 1,1-adduct (50—52). The reactions of HCl and hydrogen iodide [10034-85-2], HI, with vinyl chloride proceed by an ionic mechanism, while the addition of hydrogen bromide [10035-10-6], HBr, involves a chain reaction in which a bromine atom [10097-32-2] is the chain carrier (52). In the absence of a transition-metal catalyst or antioxidants, HBr forms the 1,2-adduct with vinyl chloride (52). HF reacts with vinyl chloride in the presence of stannic chloride [7646-78-8], SnCl, to form 1,1-difluoroethane [75-37-6] (53). 

The costs of building and maintaining a bromine plant are high because of the corrosiveness of brine solutions which contain chlorine and bromine and require special materials of constmction. The principal operating expenses are for pumping, steam, environmental costs, energy, and chlorine. The plants are very capital intensive. 

Impurities in bromine may be deterrnined quantitatively (54). Weighing the residue after evaporation of a bromine sample yields the total nonvolatile matter. After removing the bromine, chloride ion may be deterrnined by titration with mercuric nitrate, and iodide ion by titration with thiosulfate water and organic compounds may be detected by infrared spectroscopy sulfur may be deterrnined turbidimetricaHy as barium sulfate and heavy metals may be deterrnined colorimetricaHy after conversion to sulfides. 

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