Radical Halogenation at an Allylic Carbon

CFCs are decomposed by sunlight to form chlorine radicals. 

Ozone is destroyed by a chain reaction with radical intermediates. 

Nitric oxide, NO-, is another radical also thought to cause ozone destruction by a similar mechanism. One source of NO- in the stratosphere is supersonic aircraft whose jet engines convert small amounts of Ng and Og to NO-. Write the propagation steps for the reaction of Og with NO. 

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. 

The bond dissociation energy for this process (87 kcal/mol) is even less than that for a 3° C-H bond (91 kcal/mol). Because the weaker the C-H bond, the more stable the resulting radical, an allyl radical is more stable than a 3° radical, and the following order of radical stability results  

The position of the atoms and the o bonds stays the same in drawing resonance structures. Resonance structures differ in the iocation of oniy rt bonds and nonbonded eiectrons. 

The allyl radical is more stable than other radicals because two resonance structures can be drawn for it. 

Problem 15.18 Draw a second resonance structure for each radical. Then draw the hybrid. 

---

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

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. 

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. 

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. 

---