Bromination free radical chain

Know the meaning of substitution reaction, halogenation, chlorination, bromination, free-radical chain reaction, chain initiation, propagation, termination, combustion. 

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 ... 

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 ... 

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... 

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... 

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. 

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. 

Schuchmann H-P, von Sonntag C (1988) The oxidation of methanol and 2-propanol by potassium peroxodisulphate in aqueous solution free-radical chain mechanisms elucidated by radiation-chemical techniques. Radiat Phys Chem 32 149-156 Schwarz HA, Bielski BHJ (1986) Reactions of H02 and 02 with iodine and bromine and l2 and I atom reduction potentials. J Phys Chem 90 1445-1448... 

The bromination of propane C3H8 proceeds via a free radical chain reaction. The dominant major product is HBr, with 2-bromopropane produced in substantial amounts. 1-bromopropane, though produced in much smaller amounts, can still be classified as a major product. 2 3-dimethylbutane is a minor product, and hexane and 2-methylpentane are trace products. 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

The templates can be simply coordinated rather than attached. For example, complex 100 directed the radical relay chlorination to C-9, although the process was not as clean as with the attached templates [173]. We also used template-directed chlorina-tions to determine the conformations of flexible chains, just as we had previously with the benzophenone probes [174]. Also, by use of a set of tandem free radical chain reactions we could direct the formation of carbon-bromine and carbon-sulfur bonds, again with geometric control by the attached template [175]. 

A free-radical initiator such as chlorine, bromine, or a peroxide (with heat or light) suggests that a free-radical chain reaction is most likely. Free-radical reactions were discussed in detail in Chapter 4. 

Bodenstein and Lind have studied the thermal reaction between H2 and Br2 to form HBr. Their careful study showed this reaction to be of apparently much greater complexity than the H2-I2 reaction. As was pointed out later in the interpretations of this data by Christiansen, Herzfeld, and Polanyi the reaction proceeds by a free radical chain mechanism involving bromine and hydrogen atoms. Assuming the bromine atom concentration to be governed by the Br2 = 2Br thermal equilibrium, the mechanism of HBr formation is... 

Assuming the free radical chain mechanism and a steady-state thermal equilibrium concentration of bromine atoms, Levy finds... 

In the Hunsdiecker reaction, the silver salt of a carboxylic acid (RC02Ag) is treated with Br2 to give an alkyl bromide RBr with one fewer C atoms. The reaction does not work well with aromatic acids, suggesting that a free-radical mechanism is involved. The carboxylate and bromine react to give an acyl hy-pobromite, which decomposes by a free-radical chain mechanism. 

Bromine Carriers. Z gler and Spaith showed that a hydrogen atom on a methylene group adjacent to an ethylenic double bond may be easily replaced with bromine by means of N-bromosuccinimide. This reaction proceeds by a free radical chain mechanism, and substitution may occur on either side of the double bond,... 

In this case a free radical chain reaction is used, in which the tin radical abstracts the bromine atom. The resulting phenyl radical induces a 1,5-hydrogen shift. The resulting radical fragments, producing the ketone and a benzyl radical that carries the chain. The reaction sequence is remarkable because no free alcohol is generated as an intermediate, which in certain... 

Experimentally, you will allow an excess of a hydrocarbon to react with molecular bromine and will measure the time required for disappearance of the color of the bromine. The reactions are performed in dichloromethane solution using two different conditions (a) room temperature with no illumination, and (b) room temperature with illumination. The period of time required for the decoloration depends upon the reaction conditions and the relative reactivities of the various hydrocarbons some of the substrates may not react at all under the specified conditions. The ratios of the different reaction times thus represent the relative rates of the free-radical chain bromination reaction. 

Relative Rates of Free-Radical Chain Bromination... 

Arrange the six hydrocarbons 8-13 in increasing order of reactivity in free-radical chain bromination. 

A classic example of how reactivity is related to selectivity is concerned with the free radical halogenation of alkanes by CI2 and Br2. In this free radical chain reaction, the step that sets the position of the halogen in the alkane is a hydrogen atom abstraction step. The carbon based radical created in this first propagation step then abstracts a halogen atom from the CI2 or Br2, giving the alkyl halide (see below). In free radical halogenation by either CI2 or Br2, tertiary alkyl halides are created preferentially to secondary, which in turn are formed preferentially to primary alkyl halides. This reflects the fact that the order of radical stability decreases from tertiary to secondary to primary. Yet, the extent of the selectivity for tertiary over secondary over primary is quite different for chlorination and bromination. 

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