Bromine, elemental

Halogenation (Section 11 12) Free radical halo genation of alkylbenzenes is highly selective for substitution at the benzylic position In the exam pie shown elemental bromine was used Alterna Lively N bromosuccinimide is a convenient re agent for benzylic bromination... 

The heat of formation of ammonium chloride from the elements is 317 kJ /mol (75.8 kcal/mol) it is 175 kJ /mol (41.9 kcal/mol) from gaseous ammonia and gaseous hydrogen chloride. The heat of formation of ammonium bromide from the elements, bromine in the Hquid form, is 273 kJ /mol (65.3 kcal/mol) for ammonium iodide, the corresponding heat of formation is 206 kJ /mol (49.3 kcal/mol). Iodine is in the soHd state. 

Manufacture. Ammonium bromide and Ammonium iodide are manufactured either by the reaction of ammonia with the corresponding hydrohahc acid or, more economically, by the reaction of ammonia with elemental bromine or iodine. In the latter reaction, an excess of ammonia must be used. 

Rhenium Halides and Halide Complexes. Rhenium reacts with chlorine at ca 600°C to produce rheniumpentachloride [39368-69-9], Re2Cl2Q, a volatile species that is dimeric via bridging hahde groups. Rhenium reacts with elemental bromine in a similar fashion, but the metal is unreactive toward iodine. The compounds ReCl, ReBr [36753-03-4], and Rel [59301-47-2] can be prepared by careful evaporation of a solution of HReO and HX. Substantiation in a modem laboratory would be desirable. Lower oxidation state hahdes (Re X ) are also prepared from the pentavalent or tetravalent compounds by thermal decomposition or chemical reduction. 

Steam stripping or steaming-out (Eig. 3), is the only method currently used in the United States to remove elemental bromine from the brine. This method is economical for brines containing bromine above 1000 ppm. 

All six possible diatomic compounds between F, Cl, Br and I are known. Indeed, ICl was first made (independently) by J. L. Gay Lussac and H. Davy in 1813-4 soon after the isolation of the parent halogens themselves, and its existence led J. von Liebig to miss the discovery of the new element bromine, which has similar properties (p. 794). The compounds vary considerably in thermal stability CIF is extremely robust ICl and IBr are moderately stable and can be obtained in very pure crystalline form at room temperature BrCl readily dissociates reversibly into its... 

The low concentration of elemental bromine required for the chain propagation step is generated from NBS 4 by reaction with the hydrogen bromide that has been formed in the first step ... 

By this reaction a constantly low concentration of elemental bromine is supplied. With higher concentrations of free bromine, an addition to the carbon-carbon double bond is to be expected. 

In order to induce the free-radical chain reaction, a starter compound such as dibenzoyl diperoxide, azo-Zj -(isobutyronitrile) or tcrt-butyl hydroperoxide or UV-light is used. The commercially available, technical grade N-bromosuccinimide contains traces of bromine, and therefore is of slight red-brown color. Since a small amount of elemental bromine is necessary for the radical... 

Figure 2 demonstrates that the bromine vapor pressure over a complex phase remains remarkably low with increasing temperature and is not a critical factor restricting battery operation. Even at -60 °C, vapor pressures of Br2 reaching only a few percent of the atmospheric pressure and that of elemental bromine are obtained. 

Elemental bromine is a readily evaporating liquid (pBr at 1 °C = 0.23 bar) with high reactivity. Because of the good solubility of Br2 in lipids, its aggressive and toxic properties affect skin and mucous membranes (bronchi). The MAK value of elemental Br2 is defined as 0.1 ppm (0.7 mg m 3), while the sense of smell is affected at a value of 0.01 ppm. The lethal concentration (around 100-200 ppm) is reached for example, by twice the MAK value, 5 min, eight times per working unit [91, 92]. 

HGURE 15.16 Sulfuric acid is an oxidizing agent. When some concentrated acid is poured on solid sodium bromide, NaBr, the bromide ions are oxidized to elemental bromine, which colors the mixture red-brown. 

C02-0002. Elemental bromine, chlorine, and iodine exist as diatomic molecules. Chlorine is a gas at room temperature, bromine is a liquid, and iodine is a solid. Draw molecular pictures that show the molecular distributions in samples of chlorine, bromine, and iodine. 

C02-0097. The element bromine exists as diatomic molecules and is a liquid under normal conditions. Bromine evaporates easily, however, giving a red-brown color to the gas phase above liquid bromine, as shown in the photo. Draw molecular pictures showing liquid bromine, the gas above it, and the dynamic equilibrium between the phases. 

Treatment of methano-dimer 28 with elemental bromine revealed a remarkable reactivity at low temperatures it proceeded quantitatively to the furano-spiro dimer 29, by analogy with the ethano-dimer 12 giving spiro dimer 9 upon oxidation. With increasing temperatures, the reaction mechanism changed, however, now affording a mixture of 5-bromo-y-tocopherol (30) and spiro dimer 9 (Fig. 6.24). Thus, the methano-dimer 28 fragmented into an a-tocopherol part, in the form of o-QM 3 that dimerized into 9, and a /-tocopherol part, which was present as the 5-bromo derivative 30 after the reaction. Thus, the overall reaction can be regarded as oxidative dealkylation. 

Treatment of a-tocopherol (1) with elemental bromine provided quantitative yields of 5a-bromo-a-tocopherol (46). The reaction was assumed to proceed according to a radical mechanism, but later a nonradical oxidation-addition mechanism was proven (Fig. 6.33). Bromine oxidized a-tocopherol (1) to the intermediate ortho-qainone methide (3), which in turn added the HBr produced in the oxidation step.60 If the HBr was removed by flushing with nitrogen, the spiro dimer (9) became the main product, and if it was purged by HC1 gas, mainly 5a-chloro-a-tocopherol was produced. 

With regard to metallic element bromine itself acts almost exclusively in the vapour phase (1,16)> (Reproduced with permission from Ref. 34, Copyright 1989 John Wiley Sons Ltd.)... 

Ispra Mark 13A A flue-gas desulfurization process developed at the Joint Research Centre of the European Community at Ispra, Italy, from 1979. It uses a novel electrochemical method to regenerate the solution used for absorbing the sulfur dioxide. The products are concentrated sulfuric acid and hydrogen. The absorbent is a dilute aqueous solution of sulfuric and hydrobromic acids, containing a small amount of elemental bromine. Sulfur dioxide reacts with the bromine thus ... 

This procedure is described by Lock 2 a modification using a small amount of phenol has been published.3 The patent literature discloses the use of a tertiary amine, such as pyridine, and its combination with other solvents for the monobromination of pyrene with elemental bromine.4 Brominating agents, such as N-bromosuccinimide5 and N-bromohydantoins,6 have also been used. 

Figure 8.4 The red colour of elemental bromine is lost during addition across an alkenic double bond the brominated compound is usually colourless...

Another example consider the adsorption of bromine on silica. We start by placing elemental bromine and silica at either end of a long sealed vessel. Having a relatively high vapour pressure, bromine volatilizes and molecules of bromine soon chemisorp onto the surface of the silica. When the chemisorbed monolayer is complete, successive layers of bromine form by physisorption. The first physisorbed layer is unique,... 

A. (Z)-2,3-Dibromo-2-propenoic acid ethyl ester.2 Ethyl propiolate (9.01 g.91.8 mmol) and carbon tetrachloride (75 mL) are added in sequence to a 250-mL, round-bottomed flask equipped with a Teflon-coated magnetic stirring bar (Note 1). The flask is fitted with a 30-mL pressure equalizing addition funnel to which elemental bromine... 

Elemental bromine was used as received from EM Science. 

With Bcomine.— The element bromine will combine with hydrogen to form hydrobromic acid, in accordance with the following equation —... 

Among the less attractive elements is bromine—a reddish-brown liquid that evolves a toxic brown vapor that one must absolutely avoid inhaling. The liquid itself is nasty stuff, causing bums that are slow to heal. Elemental bromine is foul stuff. It has the personality of an ill-treated pit bull. 

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