Halohydrin Formation Regiochemistry

Figure 7.3 Mechanism of the oxymercuration of an alkene to yield an alcohol. The reaction involves a mercurinium ion intermediate and proceeds by a mechanism similar to that of halohydrin formation. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry.

Figure 11.5 shows a mechanism that has been postulated for this reaction. First, an electrophilic mercury species adds to the double bond to form a cyclic mercurinium ion. Note how similar this mechanism is, including its stereochemistry and regiochemistry, to that shown in Figure 11.4 for the formation of a halohydrin. The initial product results from anti addition of Fig and OH to the double bond. In the second step, sodium borohydride replaces the mercury with a hydrogen with random stereochemistry. (The mechanism for this step is complex and not important to us at this time.) The overall result is the addition of H and OH with Markovnikov orientation. 

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