Radicals, anti-Markovnikov propagation steps

A typical example of a nonpolymeric chain-propagating radical reaction is the anti-Markovnikov addition of hydrogen sulfide to a terminal olefin. The mechanism involves alternating abstraction and addition reactions in the propagating steps ... 

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+ ... 

Draw a reaction-energy diagram for the propagation steps of the free-radical addition of HBr to isobutylene. Draw curves representing the reactions leading to both the Markovnikov and the anti-Markovnikov products. Compare the values of AG° and Ea for the rate-limiting steps, and explain why only one of these products is observed. 

In the presence of a radical initiator, alkenes react with reactive molecules such as hydrogen bromide to give simple 1 1 adducts rather than a polymer. The initiator radical reacts rapidly with an HBr molecule to give a bromine atom (6.49), which starts the chain reaction. In the first propagation step, the bromine atom adds to the alkene 61 to give the adduct radical 62 (reaction 6.50). Since 62 abstracts a hydrogen atom from HBr by reaction (6.51) more rapidly than it would add to the alkene to form a polymer radical as in (6.43), the chain continues with reactions (6.50) and (6.51) as the propagating steps, and the product is the primary bromo compound 63. This anti-Markovniko addition is in the reverse direction to the polar addition discussed in Chapter 5. Since the radical chain reaction is faster than the polar reaction, the anti-Markovnikov product dominates if radicals are present. If the Markovnikov product is required, the reaction must be carried out in the dark, in the absence of free radical initiators, and preferably with a radical inhibitor present. 

Another common propagation step is addition of a radical to a double or triple bond as in the anti-Markovnikov addition of HBr to an alkene (Scheme 4.31). Carbon tetrachloride can be added to propylene in 80% yield (Scheme 4.32). 

The mechanism proposed for the peroxide effect involves a radical chain reaction. The initiation step (equation 9.31) produces a bromine atom, which then attaches to the less alkyl-substituted carbon atom of a carbon-carbon double bond (equation 9.32). The resulting alkyl radical then abstracts a hydrogen atom from HBr to produce the anti-Markovnikov product and regenerate a bromine atom in the second propagation step (equation 9.33). Termination steps, not shown, interrupt the chain reaction. 

Radiolysis of mixtures of thiols and olefins in the absence of oxygen leads to anti-Markovnikov addition across the double bond in a long chain reaction involving free radicals. The propagation steps for a terminal... 

The mechanism for the addition of hydrogen bromide to 1-phenylpropene in the presence of peroxides is a chain mechanism analogous to the one we discussed when we described anti-Markovnikov addition in Section 10.9. The step that determines the orientation of the reaction is the first chain-propagating step. Bromine attacks the second carbon atom of the chain because by doing so the reaction produces a more stable benzylic radical. Had the bromine atom attacked the double bond in the opposite way, a less stable secondary radical would have been formed. 

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