Double bonds radical reactions with

FIGURE 8.22 Reaction of a free radical with an unsaturated lipid and the subsequent rearrangement of double bonds and reaction with molecular oxygen. 

In the presence of peroxides, hydrogen bromide adds to the double bond of styrene with a regioselectivity opposite to Markovnikov s nrle. The reaction is a free-radical addition, and the regiochemistiy is governed by preferential fonnation of the more stable radical. 

Miller and Mayo studied the styrene oxidation and came to conclusion that this chain reaction occurs via addition of peroxyl radical to double bond of styrene with formation of polyperoxide as a product A.A. Miller and F.R. Mayo [54]... 

Radicals also exhibit high activity in addition reactions. For example, the peroxyl radical of oxidizing styrene adds to the double bond of styrene with the rate constant k = 68 Lmol-1 s-1, and dioxygen adds with k = 5.6x 10-10Lmol-1 s-1 (298 K). As in the case of abstraction reactions, the distinction results from the fact that the first reaction is... 

In addition to the reaction with the C—H bond, dioxygen attacks the double bond of olefin with free radical formation [9]. 

Another complication that can arise in these reactions is that a cycloaddition reaction of the radical intermediate formed in the tin hydride reduction, to an adjacent carbon-carbon double bond, can compete with the simple reduction reaction. This occurs when a five- or six-membered ring can be formed in an intramolecular cycloaddition reaction. For example, Beckwith and Lawrence96 found both five- and six-membered rings in the product when l-bromo-2,2,5-trimethylhex-l-ene was treated with tributyltin hydride (Scheme 15). 

Comparison of the results for catalytic isomerization of pent-l-ene to trans-pent-2-ene with the basic and one-electron donating properties of the catalysts led to the conclusion that two different reaction mechanisms operate in double bond isomerization reactions (a) an ionic mechanism which involves proton abstraction from the alkene molecule by the super base site (pAia = 37 for pentenes) and (b) a free radical mechanism which involves the abstraction of a hydrogen atom from the alkene by the one-electron donor center (Scheme 39). 

The main features of this hydroperoxidation reaction are that in any case a shift of the double bond is connected with this reaction, and that no free radicals are involved i.e., no hydrogen abstraction from the carbon atom a to the double bond prior to the C—O bond formation occurs as is the case in the well-studied autooxidation reactions. In the latter reactions two different hydroperoxides are formed as... 

Kinetics is used to investigate mechanisms of radical additions to alkenes. Outside the solvent cage, the initiator-derived radicals may undergo the desired bimolecular reaction with the substrate, or side reactions. When the substrate is an alkene, the exothermic intermolecular addition of the reactive radical (R ) to the double bond results in the formation of two new single carbon-carbon bonds in place of the double bond. This reaction represents conversion of an initiator into a propagating radical in radical polymerisations, and is becoming increasingly important in a number of synthetically useful intermolecular small molecule reactions. The addition of R to monosubstituted and 1,1-disubstituted alkenes is nearly always at the unsubstituted carbon atom (tail addition), and thus is normally not affected by the individual steric demand of the alkene substituents. Equation 10.4 is the expression for the rate of addition (R ) of R to an alkene where [M] is the monomeric alkene concentration ... 

Peroxy radicals(R02) react with organic compounds either by H-atom transfer or addition to a double bond. These reactions have rate constants ranging from <0.01 to 300 M 1 s 1 at 25°C (Howard, 1972 Hendry et al., 1974 Neta et al., 1990) and are rarely important under environmental conditions because of the low average concentration of R02 in surface waters (Table 15.5). However, H-atom transfers from phenol OH or aniline NH have large... 

Radical reactions with the n bond of vinyl monomers are not nearly as selective as ionic attack, and free-radical initiators cause the polymerization of nearly all vinyl and vinylidene monomers. (Some of these polymerizations are not elTicient because of side reactions. Propylene is acase in point as described in Section 6.8.5.) Resonance stabilization occurs to some extent with most vinyl monomers but it is important in radical polymerizations only when the monomers contain conjugated C—C double bonds as in styrene, 1,3-butadiene, and similar molecules ... 

---