Radical-chain addition, of hydrogen bromide

Some examples of radical-chain additions of hydrogen bromide to alkenes are included in Scheme 12.5. 

With light, peroxides, radical initiators, and in the absence of radical inhibitors a rapid radical-chain addition of hydrogen bromide occurs to yield 80% or more of 1-bromopropane ... 

Clearly, the formation of ROH and a bromine atom is energetically more favorable. The overall process of decomposition of peroxide and attack on hydrogen bromide, which results in the formation of a bromine atom, can initiate a radical-chain addition of hydrogen bromide to an alkene. 

The bromine atom adds to the less-substituted carbon of the double bond, generating the more stable radical intermediate. The regioselectivity of radical chain hydrobromi-nation is opposite to that of ionic addition. (See Section 5.3 for discussion of the ionic mechanism.) The early work on the radical mechanism of addition of HBr was undertaken to understand why Markovnikov 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. Some examples of radical chain additions of hydrogen bromide to alkenes are discussed in Section 11.4.5. 

Skell, P. S., Allen, R. G. (1963). cis-trans Isomerization in Vinyl Radicals. Gas Phase Radical-Chain Addition of Hydrogen Bromide to Propyne. Journal of the American Chemical Society, 86, 1559-1560. 

Since the bromine adds to the least-substituted carbon atom of the bond, thus generating the more-substituted radical intermediate, the regiospecificity of radical-chain hydrobromination of olefins is opposite that of ionic addition. The early work on the mechanism of the reaction was undertaken to understand why Markow-nikoff s rule was violated under certain circumstances. Anti-Markownikoff additions were eventually traced to reaction conditions under which peroxides or light were causing initiation of the radical-chain process. The radical chain addition of hydrogen bromide to olefins is a synthetically useful reaction, as illustrated by entries 1 and 2 in Scheme 12.4. 

Product mixtures from radical-chain addition of hydrogen chloride to alkenes are much more complicated than is the case for addition of hydrogen bromide. The problem is that the rate of abstraction of hydrogen from hydrogen chloride is not fast relative to the rate of addition of the alkyl radical to the alkene. This results in the formation of low-... 

We have just outlined the radical-induced addition of hydrogen bromide to an alkene. The steps of this chain reaction naturally divide into three kinds ... 

The anti-Markownikoff addition of hydrogen bromide to alkenes was one of the earliest free-radical reactions to be put on a firm mechanistic basis. In the presence of a suitable initiator, such as a peroxide, a radical-chain mechanism becomes competitive with the ionic mechanism for addition of hydrogen bromide ... 

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+ ... 

Conjugated dienes also undergo addition reactions by radical-chain mechanisms. Here, the addition product almost always is the 1,4 adduct. Thus radical addition of hydrogen bromide to 1,3-butadiene gives l-bromo-2-butene, presumably by the following mechanism ... 

The change in regiochemistry is a result of a change in the mechanism of the reaction, from an ionic mechanism in the Markovnikov reaction to a radical chain mechanism in the anti-Markovnikov reaction. The radical chain mechanism for the addition of hydrogen bromide to 1-butene is outlined in the following equations ... 

The regioselectivity of addition of hydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical chain reaction is readily initiated by peroxidic impurities or by light and leads to the anti Markovnikov addition product. The mechanism of this reaction is considered more fully in Chapter 11. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of a radical inhibitor. ... 

Gas-phase autoxidation of branched-chain hydrocarbons, e.g., isobutane, can be markedly accelerated by addition of hydrogen bromide (HBr-induced autoxidation).320 This variant of the normal autoxidation, where the added HBr provides the free radical, opened up the technical production of tert-butyl hydroperoxide and di-te/7-butyl peroxide. 

The mechanism for anti-Markovnikov addition of hydrogen bromide is a radical chain reaction initiated by peroxides. 

Free-Radical-lnitiated Hydrosilylation. In early reports, it was suggested that addition of hydrosilanes to multiple bonds could proceed as free-radical chain process because of a relatively low energy of the Si—H bond in comparison with that of the C—H bond. For this reason, many synthetic and mechanistic studies have been devoted to the hydrosilylation of carbon-carbon (C=C, CM])) bond initiated by free radicals generated in the reaction mixture (3,6,10,16). Free-radical addition of hydrosilanes resembles the addition of hydrogen bromide to alkenes and always occurs according to the anti-Markownikov rule (3). 

Like free-radical chlorination of methane (Section 4.16), the free-radical addition of hydrogen bromide to 1-butene outlined in Mechaiusm 6.8 is characterized by initiation and chain propagation stages. The irfitiation stage, however, involves two steps rather than one and it is Ais extra step that accounts for the role of peroxides. Peroxides are initiators they are not incorporated into the product but act as a source of radicals necessary to get the chain reaction started. 

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