Radicals bromine

Further evidence for a bromine-bridged radical comes from radical substitution of optically active 2-bromobutane. Most of the 2,3-dibromobutane which is formed is racemic, indicating that the stereogenic center is involved in the reaction. A bridged intermediate that can react at either carbon can explain the racemization. When the 3-deuterated reagent is used, it can be shown that the hydrogen (or deuterium) that is abstracted is replaced by bromine with retention of stereochemistry These results are also consistent with a bridged bromine radical. 

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. 

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 ... 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

As mentioned in an earlier section (cf. Chapter 1, Section III), allylic positions are subject to attack by free radicals resulting in the formation of stable allyl radicals. A-Bromosuccinimide (NBS) in the presence of free-radical initiators liberates bromine radicals and initiates a chain reaction bromination sequence by the abstraction of allylic or benzylic hydrogens. Since NBS is also conveniently handled, and since it is unreactive toward a variety of other functional groups, it is usually the reagent of choice for allylic or benzylic brominations (7). 

Whereas bromine radicals (133) and succinimidyl radicals (134) react by the Sh2 mechanism at the tin center in tetraalkyltins, but not in alkyltin halides, alkoxyl radicals (135) and ketone triplets (136) react with alkyltin halides, but not tetraalkyltins this may reflect the conflicting, electronic demands of the radical reagents which, as electrophilic species, should be more reactive towards tetraalkyltins than alkyltin halides, but which would also tend to make use of a 5d orbital... 

Bodner and Domin (2000) demonstrated the inability of many university students to interpret abbreviated structural portrayals with some atoms implied, rather than shown. The students were asked to predict the major products of the reaction of bromine with methylcyclopentane portrayed as in Fig. 1.2, and to estimate the ratio of the products if bromine radicals were just as likely to attack one hydrogen atom as another. Most of the 200 students predicted three products, with a relative abundance 3 2 2 (Fig. 1.4). 

This radical (2) captures molecular bromine to give the product and a new bromine radical so that the cycle continues. 

Why does the presence of peroxides cause the addition to be anb -Markovnikov In order to understand the answer to this question, we will need to explore the mechanism in detail. This reaction follows a mechanism that involves radical intermediates (such as Br ), rather than ionic intermediates (such as Br ). Peroxides are used to generate bromine radicals, in the following way ... 

Answer In this problem, we are starting with an alkane. There are no leaving groups, so we cannot do a substitution or an elimination reaction. There are also no double bonds, so we cannot do an addition. It seems that we are stuck, with nothing to do. Clearly, our only way out of this situation is to introduce a functional group into the compound, via radical bromination. Radical bromination will place a Br at the most substituted position (the tertiary position), and then we can eliminate ... 

Strongly acidic vanadium(V) oxidises bromide in a sulphate ion medium . The reaction is first-order in both oxidant and sulphuric acid. The dependence of the rate on bromide ion concentration is complex and a maximum is exhibited at certain acidities. A more satisfactory examination is that of Julian and Waters who employed a perchlorate ion medium and controlled the ionic strength. They used several organic substrates which acted as captors for bromine radical species. The rate of reduction of V(V) is independent of the substrate employed and almost independent of substrate concentration. At a given acidity the kinetics are... 

Assume that the steady state of (Br) is formally equivalent to partial equilibrium for the bromine radical chain-initiating step and recalculate the form of Eq. (2.37) on this basis. 

Example From the tribromomethane spectrum (Fig. 3.9) a mass difference of 79 u is calculated between m/z 250 and m/z 171, which belongs to Br, thus identifying the process as a loss of a bromine radical. Starting from the CH r2 Br isotopic ion at m/z 252 would yield the same information if Br was used for the calculation. Use of Br would be misleading and suggest the loss of H2Br. 

Thus, abstraction of a hydrogen atom from HBr generates a bromine radical. Note that, for convenience, we tend not to put in all of the electron movement arrows. This simplifies the representation, but is more prone to errors if we do not count electrons. Our attacking radical has an unpaired electron, and it abstracts the proton plus one of the electrons comprising the H-Br a bond, i.e. a hydrogen atom, and the... 

The relationships between rate of cleavage, bond strength and radical-anion redox potential can be combined in one concept. In this, cleavage rate is dependent on a reaction driving force, defined as the difference between the redox potential of the substrate radical-anion and the redox potential of the product anion in equ-librium with the coiresponding radical (E° for bromine ion, bromine radical as an example). 

Bierbach, A., I. Barnes, and K. H. Becker, Rate Coefficients for the Gas-Phase Reactions of Bromine Radicals with a Series of Alkenes, Dienes, and Aromatic Hydrocarbons at 298 + 2 K, lnt.. J. Chem. Kinet., 28, 565-577 (1996). 

Bromination of alkanes follows the same mechanism as chlorination. The only difference is the reactivity of the radical i.e., the chlorine radical is much more reactive than the bromine radical. Thus, the chlorine radical is much less selective than the bromine radical, and it is a useful reaction when there is only one kind of hydrogen in the molecule. If a radical substitution reaction yields a product with a chiral centre, the major product is a racemic mixture. For example, radical chlorination of n-butane produces a 71% racemic mixture of 2-chlorobutane, and bromination of n-butane produces a 98% racemic mixture of 2-bromobutane. 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

The bromine radical abstracts an allylic hydrogen atom of the cyclohexene, and forms a resonance stabilized allylic radical and hydrogen bromide. 

Hydrogen bromide reacts with NBS to produce a Br2 molecule, which reacts with the allylic radical to form 3-bromocyclohexene, and a bromine radical is produced to continue the chain. 

Initiation The oxygen-oxygen bond is weak, and is easily homolyticaUy cleaved to generate two afkoxy radicals, which in turn abstract hydrogen to generate bromine radicals. 

Propagation The bromine radical is electron deficient and electrophilic. The radical adds to the double bond, generating a carbon-centred radical. This radical abstracts hydrogen from HBr, giving the product and another bromine radical. The orientation of this reaction is anfi-Markovnikov. The reversal of regiochemistry through the use of peroxides is called the peroxide effect. 

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+ ... 

Stratospheric Ozone depletion is largely due to chlorine and bromine radicals released from halogenated hydrocarbons. This paper describes properties, emission histories and budgets of relevant substances and outlines the pertinent photochemical processes, along with a comprehensive presentation of halocarbon measurements and global distributions. 

Hydroxyl groups are always protected prior to reaction with bromine radicals, and derived esters have proved suitable. Benzoates are particularly useful and are preferable to acetates, which are susceptible to methyl-group bromination, particularly when the acetoxyl groups are bonded to carbon atoms in the -relationship to carbon radicals. Conceivably, this susceptibility can be accounted for as follows. 

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