Bromination radical chain

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. 

Because radical brominations are so selective, they can be used successfully in the lab to make alkyl bromides. There are relatively few ways of functionalizing an unfunctionalized centre, but radical allylic bromination is one of these. Just as tertiary radicals are more stable than primary ones, so allylic radicals are even more stable than tertiary ones (see the table on p. 1026). In the presence of a suitable initiator, bromine will therefore selectively abstract an allylic hydrogen atom to give an allylic radical that can then be trapped by a molecule of bromine to regenerate a bromine radical (chain propagation) and produce the allylic bromide, initiation Br2 ----------- 2 x Br ... 

The first propagation step of the bromination radical chain is significantly endothermic. A small change in product radical stability is reflected in the barrier for reaction and consequently in the rate of reaction. Thus the first propagation step (and the chain reaction) that goes the fastest forms the most stable product radical. Bromination will select for allylic and benzylic > tertiary > secondary > primary > vinyl and phenyl. 

Bromine reacts with alkanes by a free radical chain mechanism analogous to that of chlorine There is an important difference between chlorination and brommation how ever Brommation is highly selective for substitution of tertiary hydrogens The spread m reactivity among pnmary secondary and tertiary hydrogens is greater than 10 ... 

The reaction with fluorine occurs spontaneously and explosively, even in the dark at low temperatures. This hydrogen—fluorine reaction is of interest in rocket propellant systems (99—102) (see Explosives and propellants, propellants). The reactions with chlorine and bromine are radical-chain reactions initiated by heat or radiation (103—105). The hydrogen-iodine reaction can be carried out thermally or catalyticaHy (106). 

The present method offers several advantages over earlier methods. The use of carbon tetrachloride instead of diethyl ether as solvent avoids the intrusion of certain radical-chain reactions with solvent which are observed with bromine and to a lesser degree with chlorine. In addition, the potassium bromide has a reduced solubility in carbon tetrachloride compared to diethyl ether, thus providing additional driving force for the reaction and ease of purification of product. The selection of bro-... 

Because the bromine adds to the less substituted carbon atom of the double bond, generating the more stable radical intermediate, the regioselectivity of radical-chain hydrobromination is opposite to that of ionic addition. The early work on the radical mechanism of addition of hydrogen bromide was undertaken to understand why Maikow-nikofF s rule was violated under certain circumstances. The cause was found to be conditions that initiated the radical-chain process, such as peroxide impurities or light. 

Methylcyclopropane shows strikingly different reactivity toward chlorination and bromination under radical-chain conditions. With chlorine, cyclopropyl chloride (56%) is the major product, along with small amounts of 1,3-dichlorobutane (7%). Bromine gives a quantitative yield of 1,3-dibromobutane. Offer an explanation for the difference. 

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 reaction is likely to proceed by a radical-chain mechanism, involving intermediate formation of carboxyl radicals, as in the related Kolbe electrolytic synthesis. Initially the bromine reacts with the silver carboxylate 1 to give an acyl hypobromite species 3 together with insoluble silver bromide, which precipitates from the reaction mixture. The unstable acyl hypobromite decomposes by homolytic cleavage of the O-Br bond, to give a bromo radical and the carboxyl radical 4. The latter decomposes further to carbon dioxide and the alkyl radical 5, which subsequently reacts with hypobromite 3 to yield the alkyl bromide 2 and the new carboxyl radical 4Z... 

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. 

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... 

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). 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

Fluorine reacts explosively by a radical chain reaction as soon as the gases are mixed. A mixture of hydrogen and chlorine explodes when exposed to light. Bromine and iodine react with hydrogen much more slowly. A less hazardous laboratory source of the hydrogen halides is the action of a nonvolatile acid on a metal halide, as in... 

Generated from diacetyl peroxide, methyl radicals attack 2-methylfuran at position 5 preferentially if both 2- and 5-positions are occupied as in 2,5-dimethylfuran there is still little or no attack at the 3(4)-position. If there is a choice of 2(5)-positions, as in 3-methylfuran, then that adjacent to the methyl substituent is selected.249 These orientation rules are very like those for electrophilic substitution, but are predicted for radical attack by calculations of superdelocalizability (Sr) by the simple HMO method. Radical bromination by IV-bromsuccinimide follows theory less closely, presumably because it does not occur through a pure radical-chain mechanism.249... 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

In 1937, three important publications concerning the role of short-lived radicals appeared first, the review of Hey and Waters,""" second, Kharasch s formulation of a bromine atom chain mechanism for the addition of hydrogen... 

Chlorination (and bromination) of alkanes such as methane, CH., has a radical-chain mechanism, as follows INITIATION STEP... 

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. 

Primary aliphatic alcohols are air-oxidized to the corresponding esters in the presence of a bromine-nitric acid catalyst. Evidence supporting a mechanism, which is not a free radical chain process, is presented. 

Our observations are summarized as follows (1) no induction period, (2) fast alcohol oxidation in an oxygen-poor liquid phase, (3) no carboxylic acids from the higher alcohols, (4) slow oxidation of lauryl aldehyde to lauric acid in the presence of water, and (5) recovery of bromine in the organic phase on reaction completion. These data show that the reaction is not a radical chain process but rather a bromine oxidation in which the halogen is continuously regenerated, as shown in Reactions 1 through 7. 

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+ ... 

Ellis K. Fields We have found o-methylbenzyl bromide among the oxidation products of o-xylene. Even though this bromide persists during the oxidation period and appears to solvolyze somewhat more slowly than other benzyl bromides, it releases its bromine eventually to initiate and propagate free radical chains. 

Write initiation, propagation, and termination steps for this radical-chain reaction. Estimate a AH0 for the overall reaction using the bond-dissociation energies of Table 4-6. Would you expect bromotrichloromethane to be a selective or nonselective brominating agent Explain. 

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