Addition Reactions to Vinyl Halides

Problem 7.6. Provide a justification for the observation that vinyl (C=C-X X=C1, Br) and allyl (C=C-C-X X = Cl,Br) halides undergo free radical addition reactions to the carbon-carbon double bond with about equal facility. 

Reaction with hydrogen bromide (HBr), which, unlike hydrogen chloride (HCl) can undergo both free radical and ionic addition reactions, produces both 1-bromo-2-chloroethane (BrCH2CH2Cl) and 1-bromo-l-chloroethane (CHsCHBrCl). Under free radical conditions (i.e., in the presence of peroxide catalyst), steric effects dictate that the former is the major product (Equation 7.68). 

In concert with the general observation that electron withdrawing groups in the dienophilic portion of the Diels-Alder electrocyclic Jt4s-Fjt2s addition reaction facilitate the process (raising both the HOMO and LUMO of the alkene), haloal-kenes can serve in this capacity. As shown in Equation 7.69,1-bromoethene (ethylene bromide, H2C=CHBr) serves as a dienophile in the reaction with cyclopentadiene to provide both endo-2-bromobicyclo[2.2.1]heptane and the corresponding exo-isomer. As expected on the basis of the Alder Endo rule (Chapter 4), the former predominates in the uncatalyzed reaction. 

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