Free-Radical Addition of Hydrogen Bromide to 1-Butene

Free-Radical Addition of Hydrogen Bromide to 1-Butene THE OVERALL REACTION  

Step 1 Homolytic Dissociation of a peroxide into two alkoxy radicals  

Step 4 Abstraction of a hydrogen atom from hydrogen bromide by the free radical formed in step 3  

The regioselectivity of electrophilic addition of HBr to alkenes is controlled by the tendency of a proton to add to the double bond to produce the more stable carbocation. Under free-radical conditions the regioselectivity is governed by addition of a bromine atom to give the more stable alkyl radical. 

Free-radical addition of hydrogen bromide to the double bond can also be initiated photochemically, either with or without added peroxides. 

Step 1 The weak O—O bond of the peroxide undergoes homolytic dissociation to give two alkoxy radicals. 

Step 2 An alkoxy radical abstracts a hydrogen atom from hydrogen bromide, generating a bromine atom and setting the stage for a chain reaction. 

Step 3 The regiochemistry of addition is set in this step. Bromine bonds to C-1 of 1-butene to give a secondary radical. If it had bonded to C-2, a less stable primary radical would have resulted. 

Step 4 The radical produced in step 3 abstracts a hydrogen atom from hydrogen bromide giving the product 1-bromobutane. This hydrogen abstraction also generates a bromine atom, which reacts with another molecule of alkene as in step 3. 

---

FIGURE 6 7 Initiation and propagation steps in the free radical addition of hydrogen bromide to 1 butene... 

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. 

Acid-Catalyzed Hydration of 2-Methylpropene 227 Hydroboration of 1-Methylcyclopentene 233 Oxidation of an Organoborane 235 Bromine Addition to Cyclopentene 237 Epoxidation of Bicyclo[2.2.1]-2-heptene 240 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243... 

The rate of addition depends on the concentration of both the butylene and the reagent HZ. The addition requires an acidic reagent and the orientation of the addition is regioselective (Markovnikov). The relative reactivities of the isomers are related to the relative stabiUty of the intermediate carbocation and are isobutylene 1 — butene > 2 — butenes. Addition to the 1-butene is less hindered than to the 2-butenes. For hydrogen bromide addition, the preferred orientation of the addition can be altered from Markovnikov to anti-Markovnikov by the presence of peroxides involving a free-radical mechanism. 

---