1 Chapter 11. Free Radical Reactions A free radical is a species containing one or more unpaired electrons. Free radicals are electron-deficient species, but they are usually uncharged, so their chemistry is very different from the chemistry of even-electron electron-deficient species such as carbocations and carbenes. The alkyl radical (·CR3) is a seven-electron, electron-deficient species. The geometry of the alkyl radical is considered to be a shallow pyramid, somewhere between sp2 and sp3 hybridization, and the energy required to invert the pyramid is very small. In practice, one can usually think of alkyl radicals as if they were sp2-hybridized. Both alkyl radicals and carbocations are electron-deficient species, and structural features that stabilize carbocations also stabilize radicals. Alkyl radicals are stabilized by adjacent lone-pair-bearing heteroatoms and by π bonds, just as carbocations are, and the order of stability of alkyl radicals is 3° > 2° > 1°. However, there are two major differences between the energy trends in carbocations and alkyl radicals. • A C atom surrounded by seven electrons is not as electron-deficient as a C atom surrounded by six electrons, so alkyl radicals are generally not as high in energy as the corresponding carbocations. Thus, the very unstable aryl and 1° alkyl carbocations are almost never seen, whereas aryl and 1° alkyl radicals are reasonably common. • The amount of extra stabilization that adjacent lone pairs, π bonds, and σ bonds provide to radicals is not as great as that which they provide to carbocations. The reason is that the interaction of a filled AO or MO with an empty AO (as in carbocations) puts two electrons in an MO of reduced energy, whereas the interaction of a filled AO or MO with a half-filled AO (free radicals) puts two electrons in an MO of reduced energy and one electron in an MO of increased energy.2 Even though adjacent lone pairs, π bonds, and σ bonds do not stabilize radicals as much as they stabilize carbocations, the cumulative stabilizing effect of several such groups on a radical can be considerable. Benzylic radicals (those with the radical on a C atom next to a benzene ring, but not in a benzene ring) are particularly low in energy, as the radical center is stabilized by resonance with three π bonds. 11.1 Addition of HBr to Alkenes. The anti-Markovnikov addition of HBr to alkenes was probably the first free-radical addition reaction to be discovered. The discovery was inadvertent; around the turn of the twentieth century, scientists studying the regiochemistry of addition of HBr to alkenes found that the proportion of Markovnikov to anti-Markovnikov addition products varied inexplicably from run to run. Eventually it was discovered that impurities such as O2 and peroxides greatly increased the amount of anti-Markovnikov addition product (See Jones Figure 11.23). The results were later explained by a free-radical addition mechanism. Thus, 2-methylpropene will react with HBr in the presence of a peroxide such as t-BuO—O-t-Bu to regiospecifically yield the anti-Markvonikov addition product. H3CCH3CCH2HBrRO—ORH3C CCH3HCH2Brregiospecific,anti-Markovnikov addition The anti-Markovnikov regiochemistry derives from the addition of Br· radical to the less substituted C atom of the alkene (steric reasons) to give the more stable, more substituted radical (electronic reasons). In a polar, electrophilic addition reaction, the regioselectivity would be3 such that Br would add to the more substituted C atom of the alkene. RO—OR!or h!2 R—O•::H BrR O::.R OH+ Br::.:H3CCH3CCH2+ Br::.:H3C CCH3CH2Br.CH3CH3CCH2Br.H BrH3C CCH3HCH2Br+ Br::.:Mechanisminitiation stepspropagationsteps Note that the bromine radical left above can then go back into the reaction and react with another 2-methylpropene molecule. The reaction is ended when all of the chain-carrying radicals (those involved in the propagation steps) combine with other radicals in the termination step (for example, below): CH3CH3CCH2Br.H3C CCH3BrCH2Br Br::.: Br::.:Br2 Br::.: In contrast to HBr, the acids HCl and HI do not undergo free-radical addition to alkenes, even in the presence of peroxides or O2. Abstraction of H· from HCl is too endothermic, and addition of I· to an alkene is too endothermic. 11.2 Free-Radical Polymerization of Alkenes. The most important free-radical chain reaction conducted in industry is the free-radical polymerization of ethylene to give polyethylene. Industrial processes usually use (t-BuO)2 as the initiator. The t-BuO· radical adds to ethylene to give the beginning of a polymer chain. The propagation part has only one step: the addition of an alkyl radical at the end of a growing polymer to ethylene to give a new alkyl radical at4 the end of a longer polymer. The termination steps are the usual radical–radical combination and disproportionation reactions. 11.3 Free-Radical Substitution of Alkanes. Probably the best-known example of a free-radical reaction is the halogenation of alkanes with Br2 or NBS (N-Bromosuccinimide). This chain reaction is initiated by homolytic cleavage of Br2 induced by light or heat. The propagation step consists of two atom abstraction reactions.5 When more than one kind of H atom is present, the H atom that is removed is usually the one that will leave behind the lowest energy radical. H atoms are never removed from C(sp) or C(sp2), only from C(sp3). Among C(sp3)–H bonds, it is easiest to remove a H if a heteroatom such as N or O is attached to the C. If no such H atom is present, then the H atom that is removed is best allylic or benzylic (the C is attached to a C=C π bond or to a benzene ring). If there are no allylic or benzylic H atoms, then the order of reactivity of H atoms is 3° > 2° > 1°. Free-radical halogenation is most commonly applied to allylic or benzylic halogenation because the radicals formed at these positions are most stable. Note that alkenes can react with Br2
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