Oxymercuration Demercuration for JEE

VSAT 2022

Oxymercuration-Demercuration

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


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 the C-Hg bond.


Step2 Cleavage of C-Hg Bond.



  • Step 3: Oxonium ion deprotonation by the base acetate ion yields alcohol from the oxymercuration part.


Step3 deprotonation of oxonium ion


Step3 Deprotonation of Oxonium Ion


  • Step 4: The sodium borohydride treatment replaces the acetyl mercury atom with a hydrogen atom, forming a new C-H bond while breaking the C-Hg bond, resulting in the final alcohol product.


Step 4 Formation of alcohol


Step 4 Formation of Alcohol


Oxymercuration Demercuration Examples

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


Examples of oxymercuration-reduction 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


Oxymercuration of Alkynes


Hydroboration-Oxidation

Hydroboration-Oxidation is a two-step process for producing alcohol. In the alkene double bond, the hydrogen (from BH3 or BHR4) attaches to the more substituted carbon and the boron attaches to the least substituted carbon in an Anti-Markovnikov reaction. Furthermore, the borane acts as a LewisAnti-Markovnikov acid by accepting two electrons from an electron-rich alkene in its empty p orbital. This process gives boron an electron octet. This process is unique in that it does not require the use of a catalyst to be activated. The Anti-Markovnikov Hydroboration mechanism combines hydrogenation and electrophilic addition, and it is a stereospecific (syn addition), meaning that the hydroboration occurs on the same face of the double bond, resulting in cis stereochemistry. 


Hydroboration-oxidation reaction


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.

Competitive Exams after 12th Science

FAQs on Oxymercuration Demercuration for JEE

1. Does oxymercuration occur in alkene? How does oxymercuration differ from demercuration?

The oxymercuration reaction of an alkene is stereospecific, with a hydroxyl group anti-addition on the bridged-mercurinium intermediate. The oxymercuration adduct is treated with the sodium borohydride in the demercuration step to produce a mixture of alcohols with Markovnikov's regioselectivity. The main difference between oxymercuration reaction and demercuration reaction is that oxymercuration involves an electrophilic addition in which an alkene transforms into a neutral alcohol, whereas demercuration involves the conversion of an alkene into a Hg2+ salt and an organomercury intermediate.

2. What is Demercuration of Alkoxymercuration?

Alkoxymercuration-demercuration is a reaction that occurs when an alkene (a molecule with a carbon-carbon double bond) reacts with an alcohol in the presence of mercuric acetate to form an alkoxy mercury intermediate, which is then reduced with sodium borohydride to form an ether . In other words, we can say that 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.

Comment