Radical chlorination bromination

Lewis acid catalyst is normally required when ammonium polyhalides are used, although recourse does not have to be made to strong acids, such as aluminium trichloride. Bromination and iodination reactions are normally conducted in acetic acid in the presence of zinc chloride [32], but chlorination using the ammonium tetrachloroiodate in acetic acid does not require the additional presence of a Lewis acid [33]. Radical chlorination of toluenes by benzyltrimethylammonium tetrachloroiodate in the presence of AIBN gives mixtures of the mono-and dichloromethylbenzenes [34], Photo-catalysed side-chain chlorination is less successful [35], Radical bromination using the tribromide with AIBN or benzoyl peroxide has also been reported [36, 37],... 

A significant observation concerning bromine addition is that it and many of the other reactions listed on page 360 proceed in the dark and are not influenced by radical inhibitors. This is evidence against a radical-chain mechanism of the type involved in the halogenation of alkanes (Section 4-4D). However, it does not preclude the operation of radical-addition reactions under other conditions, and, as we shall see later in this chapter, bromine, chlorine, and many other reagents that commonly add to alkenes by ionic mechanisms also can add by radical mechanisms. 

Table 9.8 illustrates the second main feature of selectivity in abstraction of hydrogen. Polar substituents affect the product distribution, but do so differently for different abstracting radicals.142 Chlorine atoms attack a hydrogen on a carbon already bearing chlorine less rapidly than on one that does not, whereas phenyl radicals behave in just the opposite way and bromine atoms are intermediate. The explanation is that the chlorine atom is electrophilic and its attack will be hindered by electron withdrawal. In resonance terms, there is an im-... 

Also, according to Equation 1.9, the overall reaction radical chlorination takes place on a given substrate considerably faster than the overall reaction radical bromination. If we consider this and the observation from Section 1.7.3, which states that radical chlorinations on a given substrate proceed with considerably lower regioselectivity than radical brominations, we have a good example of the so-called reactivity/selectivity principle. This states that more reactive reagents and reactants are less selective than less reactive ones. So selectivity becomes a measure of reactivity and vice versa. However, the selectivity-determining step of radical chlorination reactions of hydrocarbons takes place near the diffusion-controlled limit. Bromination is considerably slower. Read on. 

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]. 

Free-radical bromination is highly selective, chlorination is moderately selective, and fluorination is nearly nonselective. 

All the hydrogen atoms in cyclohexane are equivalent, and free-radical chlorination gives a usable yield of chlorocyclohexane. Formation of dichlorides and trichlorides is possible, but these side reactions are controlled by using only a small amount of chlorine and an excess of cyclohexane. Free-radical bromination is highly selective (Section 4-14), and it gives good yields of products that have one type of hydrogen atom... 

Selective radical bromination of the p-methyl group by elemental bromine is performed in solution either thermally, photolytically, or in the presence of radical initiators. The reaction does not lead to any change in molar mass or distribution, and the only potential side reaction, which has to be controlled by adjusting the reaction conditions, is debromination between two p-bromobenzyl moieties. Under similar conditions radical chlorination leads to substitution on the benzylic site as well as on the methylene and methyl groups of isobutene units, with changes in molar mass. 

Perfluorinated alkanes, alkenes or cycloalkanes show a high thermal stability, heating to temperatures over 1000 C results in radical cleavage to give short chains or alkenes.However, C C bonds which contain quaternary and tertiary carbons are cleaved at lower temperatures than those between secondary carbons. In the presence of bromine, chlorine or toluene, cleav age products are isolated, e.g.Rp—Rp - 2 RpBr Other examples involve pyrolyses. ... 

Most double bonds are easily halogenated ° with bromine, chlorine, or inter-halogen compounds.Substitution can compete with addition in some cases.lodination has also been accomplished, but the reaction is slower. Under free-radical conditions, iodination proceeds more easily. " However, vic-diiodides are generally unstable and tend to revert to iodine and the alkene. 

Thus, research efforts in different industrial laboratories have been directed toward the preparation of 1-bro-moalkyl alkyl carbonates assumed to be more stable than the 1-iodo derivatives, and more reactive than the parent chloro compounds. For example, 1-bromoethyl ethyl carbonate was made by the halide exchange of 1-chloroethyl ethyl carbonate with LiBr or NaBr, or by a radical type bro-mination of diethyl carbonate (Ref. 82). However, in the case of halide exchange, the conversion is low and a mixture results. Even with a large excess of bromide salt, this problem remains. Radical bromination was found to give unsatisfactory results for the same reasons than the chlorination, and failed in the case of unsymmetrical dialkyl carbonates because of its non-regioselectivity. 

Predict which you would expect to be more selective radical chain chlorination or bromination of an alkane. Explain. 

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