Allylic carbon radical halogenation

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. 

While carbon and oxygen radicals add irreversibly to carbon-carbon double bonds, the fragmentation reaction is rapid (and often reversible) for elements like tin, sulfur, selenium and the halogens (Scheme 36). This elimination reaction can be very useful in synthesis if the eliminated radical Y- can either directly or indirectly react with a radical precursor to propagate a chain. Given this prerequisite, an addition chain can be devised with either an allylic or a vinylic precursor, as illustrated in Scheme 37. Carbon radicals are generated by the direct or indirect reaction with Y- and are removed by the -elimination of Y-. Selectivity is determined by the concentration of the alkene acceptor and the rate of -elimination... 

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

Now let s examine radical halogenation at an aUylic carbon— the carbon adjacent to a double bond. Homolysis of the allylic C-H bond of propene generates the allyl radical, which has an unpaired electron on the carbon adjacent to the double bond. 

Halogenation at an allylic carbon often results in a mixture of products. For example, bromination of 1-butene under radical conditions forms a mixture of 3-bromo-1 -butene and 1-bromo-2-butene. 

Radical halogenation of alkanes was discussed in Chapter 15. The mechanism of radical halogenation at an allylic carbon was given in Section 15.10. 

Allyl Chloride.—The halogen substitution products of propene and the higher hydrocarbons of the ethene series, when the substitution is in a carbon group not doubly linked, are of importance in the synthesis of derivatives in the same way as are the alkyl halides. 3-Chlor propene or propenyl chloride, CH2 = CH—CH2CI, is known also as allyl chloride, the radical (CH2 = CH—CH2—) being known as allyl. 

We know that the more stable the radical, the faster it can be formed. This means that a hydrogen bonded to either a benzylic carbon or an allylic carbon will be preferentially substituted in a halogenation reaction. As we saw in Section 9.4, bromination is more highly regioselective than chlorination, so the percent of substitution at the benzylic or allylic carbon is greater for bromination. 

Radical addition of HBr to an alkene depends upon the bromine atom adding in the first step so that the more stable radical is formed. If we extend this principle to a conjugated diene, e.g. buta-1,3-diene, we can see that the preferred secondary radical will be produced if halogenation occurs on the terminal carbon atom. However, this new radical is also an allylic radical, and an alternative resonance form may be written. 

Halogen compounds in which the carbon-halogen bond is adjacent to a double bond, as in C=C—C—X are known as allylic halides. The simplest example is 3-chloropropene, CH2=CHCH2C1, which is made on a large scale by the radical chlorination of propene at 400° ... 

Allylic Systems. Allylic Halides. Allylic halides also undergo homolytic carbon-halogen cleavage by pentacyanocobaltate(II) to form equimolar quantities of halo- and allylcobalt complexes (21, 22, 23). It is assumed that this reaction involves generation of the allylic radical (Reaction 19), which then reacts with pentacyanocobaltate(II) (Reaction 20). 

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