Free radical addition of hydrogen bromide

FREE-RADICAL ADDITION OF HYDROGEN BROMIDE TO ALKENES... 

Free Radical Addition of Hydrogen Bromide to Alkenes... 

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

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity 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... 

Hydrogen bromide (but not hydrogen chloride or hydrogen iodide) adds to alkynes by a free radical mechanism when peroxides are present m the reaction mixture As m the free radical addition of hydrogen bromide to alkenes (Section 6 8) a regioselectiv ity opposite to Markovmkov s rule is observed... 

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

Griesbaum, K., A. A. Oswald, and E. N. Hall Allene Chemistry. II. Free-Radical Addition of Hydrogen Bromide to Allene. J. org. Chem. 29, 2404 (1964) and references cited therein. 

In the case of a free-radical addition of hydrogen bromide, a type of addition limited to hydrogen bromide only, the attacking species is not proton, but a bromine atom that will join the carbon in such a way as to generate a more stable free-radical species. It will add to the carbon carrying two fluorines. In the intermediate free radical, the single electron is better accommodated at the carbon holding chlorine, because chlorine can disperse the electrons in its d-orbitals. 

Is it reasonable that free-radical addition of hydrogen bromide should occur with orientation opposite to that of ionic addition Let us compare the two kinds of addition to propylene. 

In Sec. 6.17 a mechanism was presented for free-radical addition of hydrogen bromide. Equally consistent with the evidence given there is the following alternative mechanism ... 

---