Oxymercuration Demercuration for JEE

One of the most important reactions in the conversion of alkenes to alcohol is oxymercuration demercuration. It involves hydrating alkenes with mercuric acetate in a tetrahydrofuran/water mixture to form an organomercury adduct. This is followed by a demercuration step in which the adduct is reduced to alcohol using sodium borohydride. Hence, there is a formation of alcohol by oxymercuration demercuration of alkenes.

Electrophilic activation of an alkene by mercuric acetate, followed by water addition and reductive demercuration. It involves hydrating alkenes with mercuric acetate in a tetrahydrofuran/water mixture to form an organomercury adduct. The mechanism of oxymercuration - demercuration is based on Markovnikov's Regioselectivity, with the OH group attached to the most substituted carbon and the H group attached to the least substituted carbon.

Oxymercuration Demercuration Reaction

When alkenes react with mercuric acetate (CH3COO)2Hg or Hg(OAc)2, an oxymercuration product is formed, which is then reduced with NaBH4 in a basic medium to yield alcohols. The first step is the addition of OH and Hg to the double bond of alkenes, known as oxymercuration, and the second is the removal of Hg from carbene, known as Demercuration. Thus, 

$CH_3CH_2CH=CH_2\xrightarrow{(CH_3COO)_2Hg)}CH_3CH_2\underset{\underset{OH}{|}}{CH}-CH_2-HgOCOCH_3\xrightarrow{NaBH_4}CH_3CH_2\underset{\underset{OH}{|}}{CH}-CH_3$

This two-step procedure is known as oxymercuration-demercuration or oxymercuration-reduction. Because carbocations are not intermediates, the final product is alcohols, which correspond to Markovnikov's addition of water to alkenes without any rearrangement.

Oxymercuration–Demercuration Mechanism

The reaction mechanism is similar to that of electrophilic addition reactions.

• Step 1: The pi electrons bond to mercury, and the lone pair on the mercury bonds to the other vinyl carbon, forming a mercurial ion bridge. The mercurial ion forms as a result of the loss of an acetate ion.

Step1  Attack of Electrophile

• Step 2: The water molecules attack one of the carbons attached to Hg resulting in the cleavage of the C-Hg bond.

Step2 Cleavage of C-Hg Bond.

Step3 Deprotonation of Oxonium Ion

Step 4 Formation of Alcohol

Oxymercuration Demercuration Examples

The following are the examples of Oxymercuration–Demercuration reaction:-

Examples of Oxymercuration - Reduction Reaction

Oxymercuration Demercuration of Alkynes

As with alkenes, hydration (adding water) of alkynes requires a strong acid, usually sulfuric acid, and is aided by the mercuric ion (Hg2+). However, hydration of alkynes yields ketone products, whereas hydration of alkenes yields alcohol products. The addition of oxygen in both reactions follows the Markovnikov rule.

During the hydration of an alkyne, the initial product is an enol intermediate (a compound with a hydroxyl substituent attached to a double bond), which immediately rearranges to the more stable ketone via a process known as enol-keto tautomerization.

Oxymercuration of Alkynes

Hydroboration - Oxidation Reaction

Regiochemical and Stereochemical Outcome

A resonance hybrid of a carbocation and a bridged mercurinium ion is the bridged-mercurinium-ion intermediate. The partial positive charge is shared by the more substituted carbon atom and the mercury atom, reducing the possibility of a carbocation rearrangement. The carbon–mercury bond to the more substituted carbon is also longer and more easily broken.

The above-mentioned factors cause water to attack only the more substituted carbon, opening the three-membered ring.

The oxymercuration step is stereospecific because water's attack on the bridged mercurinium ion results in the anti-addiction of the hydroxyl group. A proton transfer completes the oxymercuration step, resulting in an organomercury compound.

Finally, the oxymercuration adduct is demercuration with sodium borohydride to yield alcohol with Markovnikov's orientation.

As hydrogen can replace the mercury species in either a syn or anti-fashion concerning the hydroxyl group during the demercuration step, the overall reaction produces a racemic mixture of two enantiomeric alcohols.

Oxymercuration is a textbook Markovnikov reaction; barring extreme cases, the water nucleophile will always attack the more substituted carbon, depositing the resulting hydroxyl group there. Examining the three resonance structures of the mercurinium ion formed at the end of step one explains this phenomenon.

An examination of these structures reveals that the positive charge of the mercury atom will occasionally reside on the more substituted carbon (approximately 4 percent of the time). This results in the formation of a temporary tertiary carbocation, which is a highly reactive electrophile. At this point, the nucleophile will attack the mercurinium ion. As a result, because the more substituted carbon retains more positive character than the less substituted carbon, the nucleophile attacks it.

Conclusion

As previously discussed, one of the major reactions in the conversion of alkenes to alcohol is oxymercuration–demercuration. The hydration of alkenes with mercuric acetate in a mixture of tetrahydrofuran and water results in the formation of an organomercury adduct. The reaction mechanism is similar to that of electrophilic addition reactions.