Question

Which is the wrong statement about oxymercuration- demercuration ?

• A. In the first step, oxymercuration occurs, i.e., water and $Hg(OAc{)}_2$ add to the double bond.
• B. In the second step, demercuration occurs, i.e., $NaB{H}_4$ reduces (-HgOAc) group to hydrogen
• C. The net reaction is addition of water according to Markovnikov's rule.
• D. Rearrangement takes place.

Answer 1

Answer: (D)

Rearrangement takes place.

Oxymercuration-Demercuration of Alkenes

Reaction type: Electrophilic Addition

Summary

• Overall transformation C=C to H-C-C-OH
• This is an alternative method for hydrating alkenes to give alcohols
• Typical reagents are mercury acetate, Hg(OAc)₂ in aqueous THF
• Unfortunately, mercury compounds are generally quite toxic
• Regioselectivity predicted by Markovnikov's rule (most highly substituted alcohol)
• The reaction is not stereoselective
• Reaction proceeds via the formation of a cyclic mercurinium ion (compare with bromination of alkenes)

• The mercurinium ion is opened by the attack of water to complete the oxymercuration.
• When the water attacks, it does so at the more highly substituted carbon.
• No rearrangement takes place
• Demercuration is effected by a reduction using sodium borohydride, NaBH₄
• If the reaction is carried out in the presence of an alcohol rather than water, then ethers are obtained.