Hydrogen bromide free-radical addition

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... 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

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. 

In the presence of peroxides, hydrogen bromide adds to the double bond of styrene with a regioselectivity opposite to Markovnikov s nrle. The reaction is a free-radical addition, and the regiochemistiy is governed by preferential fonnation of the more stable radical. 

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. 

This free radical, like the free radical initially generated from the peroxide, abstracts hydrogen from hydrogen bromide (step 4). Addition is now complete, and a new bromine atom has been generated to continue the chain. As in halogenation of alkanes, every so often a reactive particle combines with another one, or is captured by the wall of the reaction vessel, and a chain is terminated. 

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 ... 

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