Active bromine

Incorporation of a vinylic bromide at the 2 position also gives a compound with good activity. Bromination of fluorohydrin under carefully controlled conditions... 

The two basic requirements for efficient bromine storage in zinc-bromine batteries, which need to be met in order to ensure low self-discharge and more over a substantial reduction of equilibrium vapor pressure of Br2 of the polybromide phase in association with low solubillity of active bromine in the aqueous phase. As mentioned by Schnittke [4] the use of aromatic /V-substitucnts for battery applications is highly problematic due to their tendency to undergo bromination. Based on Bajpai s... 

Safety risks and the environmental impact are of major importance for the practical success of bromine storage system. The nonaqueous polybromide complexes in general show excellent physical properties, such as good ionic conductivity (0.1-0.05 Qcirf1), oxidation stability (depending on the nature of the ammonium ion), and a low bromine vapor pressure. The concentration of active bromine in the aqueous solution is reduced by formation of the complex phase up to 0.01-0.05 mol/L, hence ensuring a decisive decrease of selfdischarge. 

Protocol for Labeling Nucleic Acids by N-Bromosuccinimide Activation Bromination... 

Theory Active bromine is liberated from the standard solution of bromine in an acidic medium (HC1) that subsequently attacks the double bond present in the side chain of the ethacrynic acid molecule thereby resulting into the formation of the corresponding dibromo derivative. This particular reaction takes place quantitatively. Hence, the reactions involved in this assay may be expressed as follows ... 

A double mediatory system consisting of A-oxoammonium salts and active bromine species, generated from 2,2,6,6-tetramethylpiperidine-1 -oxyl derivatives... 

The type and reactivity of the products formed in the bromide oxidation process may differ. Vojinovic etal. [112] have investigated the formation of bromine in propylene carbonate. The tribromide anion, Br3, was identified as the main product and an unwanted reaction of the solvent with the active bromine was observed. 

In addition, mechanisms for regeneration of photo-chemically active bromine that involve aerosol particles or reactions on the snowpack have been proposed. For example, McConnell et al. (1992) and Tang and McConnell (1996) proposed that HBr and organobromine compounds could be converted to Br2 through adsorption and reaction on ice and aerosol particles. Fan and Jacob (1992) suggested that HOBr, formed by the reaction of BrO with H02,... 

Aranda et al. (1997) have shown in laboratory studies that CH302 also reacts with BrO in a manner analogous to the H02 reaction (123), with a rate constant at 298 K of 5.7 X 10"12 cm3 molecule 1 s 1. About 80% of the reaction generates HOBr + CH202, with the remainder forming Br + CH30 + 02. In either case, photochemically active bromine species are regenerated. 

The central point is that photochemically active bromine, and perhaps chlorine (see following), compounds are generated that lead to the chain destruction of gaseous O, at ground level at polar sunrise. 

Arctic at polar sunrise. The mechanism likely involves regeneration of photochemically active bromine via heterogeneous reactions on aerosol particles, the snow-pack, and/or frozen seawater. The source of the bromine is likely sea salt, but the nature of the reactions initiating this ozone loss remains to be identified. For a review, see the volume edited by Niki and Becker (1993) and an issue of Tellus (Barrie and Platt, 1997). 

The hydrolysis of BrONOz does not generate active bromine directly, but rather HOBr, which must then photolyze to OH + Br. Calculate the lifetime of HOBr with respect to photolysis at a solar zenith angle of 86° and an altitude of 25 km. Assume the earth-sun distance correction factor is 1.0. Is the hydrolysis of BrONOz or the photolysis of HOBr the rate-determining step ... 

Positively polarized selenation reagents for oxyselenation reactions can be substituted by an indirect electrochemical procedure in which the bromide ion acts as the redox catalyst. The active bromine species generates the phenylselenyl cation from the added diphenyl diselenide (Eq. (48) Table 4, No. 21-26)... 

Bromide and iodide ions are not only effective as redox catalyst in the indirect electrochemical formation of carbon-hetero atom bonds but also of hetero-hetero atom bonds. Thus, the nitrogen-sulfur bond in sulfenimides (Eq. (53) Table 4, No. 33) and sulfenimines (Eq. (54) Table 4, No. 34) can be generated. The electrogenerated active bromine species forms a positively polarized sulfenyl intermediate from disulfides. 

Antimony(V) chloride activates bromine trifluoride to such an extent that it can replace chlorine with fluorine in CFC-112, even at room temperature, to give CFC-113 in 90% yield. 

The activated bromine atom in bromomethyl phenyl ketone (1) can be replaced by fluorine by refluxing with sodium fluoride in methanol for several hours,12 but besides fluoromethyl phenyl ketone (2), the product of nucleophilic displacement by methoxide ion, methoxymethyl phenyl ketone (3) is also formed. 

Reactions by polar mechanisms are very slow with bromine alone, but apparently are accelerated by electrophilic agents, which activate bromine by facilitating formation of Br by complexing with Bre (e.g., HBr, FeBr3, etc.). Cyclopropane reacts rather rapidly with bromine by a radical-chain mechanism, even at—78°, if bromine atoms are formed by light irradiation. 

The fate of CHBrO produced is also unknown. It is, in particular, not known whether bromine wul be released in the form of active bromine atom or as inactive HBr molecules. 

Fig. Mechanism by which a Lewis acid activates bromine towards electrophilic substitution.

LDH-WC>4 has also been applied for oxidative bromination of unsaturated hydrocarbons (367). In a first step, W on LDH catalyzes the oxidation of Br with H202. Subsequently, the resultant bromonium species (Br+, which could be equilibrated with species such as HOBr or Br2) react rapidly with the target molecules. An advantage of the oxidative electrophilic bromination is that the active bromine is formed in situ in a controlled way there is no direct contact with Br2. Moreover, the bromination is performed under mild conditions such as room temperature, atmospheric pressure, and near neutral pH. Indeed, the FI ) 11-WO -catalyzed bromination mimics the enzymatic activity of bromoperoxidases not only formally but also organizationally and mechanistically. For example, indene is bromohydroxylated almost quantitatively with NH4Br and H2O2 in a water-2-methyltetrahydrofuran solvent mixture (368) ... 

The succinimide recovered corresponded to 97-98% of the theoretical amount and analyzed for 0.4% active bromine. 

Bthyl bromide Is Irradiated with thermal neutrons and the active bromine la separated ea HBr by Sallord-Chelmera process. Hen, by saturating with tetro-butyl alcohol to the HBr, tetrabutyl bronlde (3 g) le synthesised. The tetrabutyl hramide Is dissolved In 200 nl of aqueous alcohol (73 1 ). Tha radioactive... 

Interestingly, a study of the bromide-dependent chloroperoxidase bromination of tyrosine reveals that the active brominating agent is free bromine and not a bromine-enzyme complex [122]. The situation is very different for bromoperoxidase- and vanadium peroxidase-catalyzed brominations [106-108]. 

Photodilorination has been recommended for the preparation of choice of cyclodecene finom cyclodecane via the intermediate chloride. Bromine is a rather weak brominating agent for alkanes a only unusually favorable substrates, like adamantane, react at a reasonable rate. The mixture HgO/Brz is much more reactive than bromine itself, as shown by the facile tHomination of 1,1,3,3-tetramethylbutane. Silver hexafluoroantimonate has dso been used to activate bromine for this type of reaction. Iodine is normally ineffective in functionalizing alkanes, but use of y-itradiation of a solution of I2 in alkanes leads to unselective formation of all possible iodoalkanes. ... 

TROPOSPHERIC OZONE DEPEETION AT POLAR SUNRISE. 3.1 Main Features of Polar ODEs. 3.2 Satellite Observations. 3.3 Sources of Active Bromine... 

Note that in these reactions, for each molecule of active bromine (Br 4- BrO 4- HOBr 4- Br2) that is created, one HO2 radical is lost to produce H2O leading to the loss HO c and a reduction of the HO2/OH concentration ratio (already discussed for iodine by Jenkin (1993)). Therefore, a significant factor that drives these chains of reactions may be the production of HOj radicals. 

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