Hydrogen bromide bromine free

From the purely chemical aspect, it is at first glance a very surprising result that hydrogen atoms (" nascent hydrogen ) should oxidize hydrogen bromide to free bromine. 

A compound, CgH5.PCl2.Br4, has also been described as a red mass decomposed by water into phenylphosphinous acid, hydrogen chloride, hydrogen bromide and free bromine. 

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... 

One possible interpretation is a change to a free radical chain mechanism. Bromine radical is first produced which then adds to the alkene. The resulting free radical reacts with hydrogen bromide to yield the final alkyl bromide and regenerate bromine radical. 

Although direct nitration was not possible, 2-amino-4-methylselena-zole can be directly brominated by treatment with bromine in carbon tetrachloride, the hydrogen bromide salt of 2-amino-4-methyl-5-bromoselenazole, mp 180°C (decomp.) is formed. However, all attempts to obtain the free base from this salt failed and led to complete decomposition. In this bromination, an equivalent quantity of bromine must be used excess also leads to complete destruction of the molecule. From the decomposition products an oily compound can be detected similar to bromoacetone. ... 

The allylic bromination of an olefin with NBS proceeds by a free-radical chain mechanism. The chain reaction initiated by thermal decomposition of a free-radical initiator substance that is added to the reaction mixture in small amounts. The decomposing free-radical initiator generates reactive bromine radicals by reaction with the N-bromosuccinimide. A bromine radical abstracts an allylic hydrogen atom from the olefinic subsfrate to give hydrogen bromide and an allylic radical 3 ... 

G. N. Lewis and H. Storch, "The Potential of the Bromine Electrode the Free Energy of Dilution of Hydrogen Bromide the Distribution of Bromine Between Several Phases". J. Am. Chem. Soc., 39, 2544-2554 (1917). 

Bradfield et al.21g first studied the kinetics of molecular bromination using aromatic ethers in 50% aqueous acetic acid at 18 °C. They showed that the kinetics are complicated by the hydrogen bromide produced in the reaction which reacts with free bromine to give the tribromide in BrJ, a very unreactive electrophile. To avoid this complication, reactions were carried out in the presence of 5-10 molar excess of hydrogen bromide, and under these conditions second-order rate coefficients (believed to be I02k2 by comparison with later data) were obtained as follows after making allowance for the equilibrium Br2 + Br7 Bn, for which K = 50 at 18 °C 4-chloroanisole (1.12), 4-bromoanisole (1.20), 4-... 

Because concentrated sulphuric acid is also an oxidising agent, hydrogen bromide and hydrogen iodide are oxidised to the free halogens bromine and iodine, respectively. The results are shown in the table below. 

Hydrogen bromide gas may be produced by combustion of hydrogen in bromine vapor at 37.5°C using a catalyst such as platinized asbestos or platinized silica gel. Unreacted free bromine is removed from the product by passing the gaseous product mixture over hot activated charcoal. Hydrogen bromide formed may be absorbed in water to obtain the acid or may be cooled and liquefied for shipment in cylinders. 

The chain reaction is initiated by the interaction of a free radical [formed according to Eq. (6.12)] with hydrogen bromide to form a bromine atom [Eq. (6.13)], which, after reacting with the alkene, yields a bromoalkyl radical [Eq. (6.14)]. The reaction of this radical with HBr yields the alkyl bromide and regenerates the bromine atom [Eq. (6.15)] ... 

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... 

In the following method a slight excess of an alcoholic solution of bromine is added to an alcoholic solution of the tautomeric mixture the excess of bromine is immediately removed by the addition of a few drops of alcoholic /3-naphthoI solution potassium iodide solution is next added, and the hydrogen iodide formed by interaction with the hydrogen bromide present reduces the bromo-ketone with liberation of free iodine, which is estimated by titration with standard thiosulphate (in absence of starch). One molecule of iodine = one molecule of enolic compound. For criticism of this method, see Ann. Rep., 1930, 100. 

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