Oxymercuration reaction

The oxymercuration reaction is an electrophilic addition organic reaction that transforms an alkene into a neutral alcohol. In oxymercuration, the alkene reacts with mercuric acetate (AcO-Hg-OAc) in aqueous solution to yield the addition of an acetoxymercuri (HgOAc) group and a hydroxy (OH) group across the double bond. Carbocations are not formed in this process and thus rearrangements are not observed. The reaction follows Markovnikov's rule (the hydroxy group will always be added to the more substituted carbon) and it is an anti addition (the two groups will be trans to each other).

Oxymercuration followed by demercuration is called an oxymercuration-reduction reaction. This reaction, which is almost always done in practice instead of oxymercuration, is treated at the conclusion of the article.

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Mechanism

Oxymercuration can be fully described in three steps(the whole process is sometimes called deoxymercuration), which is illustrated in stepwise fashion to the right. In the first step, the nucleophilic double bond attacks the mercury ion, ejecting an acetoxy group. The electron pair on the mercury ion in turn attacks a carbon on the double bond, forming a mercurinium ion in which the mercury atom bears a positive charge. The electrons in the highest occupied molecular orbital of the double bond are donated to mercury's empty dz² orbital and the electrons in mercury's dxz orbital are donated in the lowest unoccupied molecular orbital of the double bond.

In the second step, the nucleophilic water molecule attacks the more substituted carbon, liberating the electrons participating in its bond with mercury. The electrons collapse to the mercury ion and neutralizes it. The oxygen in the water molecule now bears a positive charge.

In the third step, the negatively charged acetoxy ion that was expelled in the first step attacks a hydrogen of the water group, forming the waste product HOAc. The two electrons participating in the bond between oxygen and the attacked hydrogen collapse into the oxygen, neutralizing its charge and creating the final alcohol product.

Regioselectivity and stereochemistry

Oxymercuration is very regioselective and is a textbook Markovnikov reaction; ruling out extreme cases, the water nucleophile will always preferentially attack the more substituted carbon, depositing the resultant hydroxy group there. This phenomenon is explained by examining the three resonance structures of the mercurinium ion formed at the end of the step one.

By inspection of these structures, it is seen that the positive charge of the mercury atom will sometimes reside on the more substituted carbon (approximately 4% of the time). This forms a temporary tertiary carbocation, which is a very reactive electrophile. The nucleophile will attack the mercurinium ion at this time. Therefore, the nucleophile attacks the more substituted carbon because it retains more positive character than the lesser substituted carbon.

Stereochemically, oxymercuration is an anti addition. As illustrated by the second step, the nucleophile cannot attack the carbon from the same face as the mercury ion because of steric hindrance. There is simply insufficient room on that face of the molecule to accommodate both a mercury ion and the attacking nucleophile. Therefore, when free rotation is impossible, the hydroxy and acetoxymercuri groups will always be trans to each other.

Oxymercuration-reduction

In practice, the mercury adduct product created by the oxymercuration reaction is almost always treated with sodium borohydride (NaBH₄) in aqueous base in a reaction called demercuration. In demercuration, the acetoxymercuri group is replaced with a hydrogen in a stereochemically insensitive reaction. The combination of oxymercuration followed immediately by demercuration is called an oxymercuration-reduction reaction.

Therefore, the oxymercuration-reduction reaction is the net addition of water across the double bond. Any stereochemistry set up by the oxymercuration step is scrambled by the demercuration step, so that the hydrogen and hydroxy group may be cis or trans from each other. Oxymercuration-reduction is a popular laboratory technique to create a "clean" alcohol (the acetoxymercuri group left over from the oxymercuration reaction is often undesirable).

See also

• Hydroboration
• Alcohol

References

1. ^ Bordwell, Frederick G.; Douglass, Miriam L. "Reduction of Alkylmercuric Hydroxides by Sodium Borohydride.", Journal of the American Chemical Society (1966), 88, pg 993-999. 

• Loudon, Marc G. (2002). "Addition Reactions of Alkenes.", Organic Chemistry, Fourth Edition, Oxford University Press, 165-168. 
• McGraw-Hill Higher Education (2000). Oxymercuration-Demercuration of Alkenes. 
• Schleifenbaum, Andreas (2001). "Oxymercuration.", Reaktionen, Reagenzien und Prinzipien.