Radicals, anti-Markovnikov rearrangement

Radicals react with alkenes to form a new radical that can react further to give addition reaction products. The initial reaction will generate the more stable radical, and rearrangement is not observed. In the presence of peroxides, alkenes react with HBr to give the alkyl bromide having Br on the less substituted carbon. This is called anti-Markovnikov addition. 

In the oxymercuration step, water and mercuric acetate add to the double bond in the demercuration step, sodium borohydride reduces the acetoxymercury group and replaces it with hydrogen. The net addition of H— and —OH takes place with Markovnikov regioselectivity and generally takes place without the complication of rearrangements, as sometimes occurs with acid-catalyzed hydration of aUcenes. The overall alkene hydration is not stereoselective because even though the oxymercuration step occurs with anti addition, the demercuration step is not stereoselective (radicals are thought to be involved), and hence a mixture of syn and anti products results. 

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