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Alkoxymercuration Mechanism: Reaction, Example, Applications

Alkoxymercuration Mechanism: Reaction, Example, Applications

Team Careers360Updated on 02 Jul 2025, 05:26 PM IST

An alkene (a molecule with a carbon-carbon double bond) reacts with alcohol in the presence of mercury acetate to produce an intermediate known as alkoxy mercury, which is then reduced with sodium borohydride to produce an ether.

This Story also Contains

  1. Oxymercuration Demercuration Reaction
  2. Oxymercuration Demercuration Mechanism
  3. Alkoxymercuration Mechanism Reaction Example
  4. Applications

In order to make ether under the appropriate circumstances, an alkene must be treated with too much alcohol while being in the presence of an acid catalyst. For instance, when 2-methylpropene and methanol are run over an acid catalyst, 2-methoxy-2-methylpropane is created.

The Alkoxymercuration-Demercuration Reduction is a challenging mechanism to comprehend. Students commonly forget the alcohol reagent and the alkyl group in the outcome, not because it is more complicated than oxymercuration.

Oxymercuration Demercuration Reaction

The alkene is transformed into alcohol through an additional process known as the oxymercuration-demercuration reaction. In this reaction, the alkene first interacts with mercury (II) acetate (Hg(OAc)2) in an aqueous solution of THF, and then sodium borohydride reduces the alkene (NaBH4)

Below is an illustration of an oxymercuration-demercuration reaction.

Isopropanol is made from propane.

The response is shown below.

C_3H_6\:\xrightarrow[2.NaBH_4]{1. Hg(OAc)_2,THF,H_2O}\:C_3H_8O

1706980181532

In this reaction, propene is combined with mercury(II) acetate in the presence of tetrahydrofuran, and the result is reduced to isopropanol using sodium borohydride.

The hydroxyl group is linked to the most substituted carbon atom in this reaction, and the hydrogen atom is joined to the least substituted carbon atom, in accordance with Markonikov's rule of regioselectivity.

Commonly Asked Questions

Q: What is the role of the mercury(II) ion in the overall reaction?
A:
The mercury(II) ion acts as an electrophile, initiating the reaction by interacting with the π electrons of the alkene. It also serves as a leaving group in the subsequent demercuration step, being replaced by hydrogen.
Q: What are some limitations of alkoxymercuration?
A:
Limitations include the toxicity of mercury compounds, potential side reactions with certain functional groups, and the need for an additional demercuration step to obtain the final product. The reaction is also limited to alkenes and alkynes as substrates.
Q: Can alkoxymercuration be performed on alkynes?
A:
Yes, alkoxymercuration can be performed on alkynes. However, the reaction typically stops at the vinyl ether stage, not proceeding to a diether, due to the decreased reactivity of the vinyl mercury intermediate.
Q: How does alkoxymercuration compare to other methods of ether synthesis?
A:
Alkoxymercuration followed by demercuration is a useful method for synthesizing ethers from alkenes. Compared to traditional methods like the Williamson ether synthesis, it offers different regioselectivity and can be used with substrates that might not tolerate strong bases or nucleophiles.
Q: How does the structure of the alkene affect the product distribution in alkoxymercuration?
A:
The structure of the alkene affects the stability of the carbocation intermediate, which in turn influences the regioselectivity. More substituted carbons can better stabilize positive charge, leading to preferential formation of the Markovnikov product.

Oxymercuration Demercuration Mechanism

  • In the alkoxymercuration process, an alcohol and an alkene are combined in the presence of a mercury salt, such as mercuric acetate, and the result is a demercuration step that yields ethers.

  • The process is comparable to the oxymercuration reaction, however, alcohol is used in place of water.

  • The electrophilic addition process is followed in this reaction. The primary distinction is that a mercurial ion bridge stabilises the intermediate carbocation, preventing it from rearranging. Electropositive charges are present in metals. The mercury, which has a partial positive charge, is the electrophile in the acetate complex.

  • The Alkoxymercuration-Demercuration Reduction is a challenging mechanism to comprehend. Students commonly forget the alcohol reagent and the alkyl group in the outcome, not because it is more complicated than oxymercuration.

  • In the first step of this reaction, the pi electrons attach to mercury, and the lone pair on mercury forms an ion bridge with the other vinyl carbon. The carbocation is stabilised by the mercury ion, preventing it from rearranging. The mercurial ion is created as a result of the loss of an acetate ion.

  • The second phase of this reaction involves the combination of an alcohol molecule with the most substituted carbon, which opens the mercurial ion bridge.

  • In the third step of this procedure, the addition product is neutralised by a proton transfer to an alcohol-solvent molecule.

  • The fourth stage of the reaction pathway involves reducing the organomercury intermediate with sodium borohydride at basic conditions.

  • The OR group is joined to the most substituted carbon and the H group is attached to the least substituted carbon in the reaction mechanism, which is based on Markovnikov's regioselectivity. The absence of strong acids and the fact that no separate carbocation intermediate forms make the reaction advantageous since it prevents carbocation rearrangements.

Commonly Asked Questions

Q: What reagent is commonly used for demercuration?
A:
Sodium borohydride (NaBH4) is commonly used for demercuration. It reduces the carbon-mercury bond, replacing the mercury with hydrogen.
Q: What is the significance of the mercurinium ion in the mechanism?
A:
The mercurinium ion is a key intermediate in alkoxymercuration. It's a three-membered cyclic ion that forms when mercury(II) interacts with the π electrons of the alkene. Its formation explains the stereochemistry and regioselectivity of the reaction.
Q: How does the choice of mercury salt affect the reaction?
A:
Different mercury salts can affect the reaction rate and yield. Mercury(II) acetate is commonly used, but mercury(II) trifluoroacetate can increase reaction rates. The counterion can also influence the solubility of the mercury salt in the reaction medium.
Q: How does alkoxymercuration differ from oxymercuration?
A:
Alkoxymercuration uses an alcohol (ROH) as the nucleophile and solvent, resulting in an ether product. Oxymercuration uses water as the nucleophile and solvent, resulting in an alcohol product. Both follow the same general mechanism.
Q: What factors influence the rate of alkoxymercuration?
A:
The rate of alkoxymercuration is influenced by factors such as the structure of the alkene (more substituted alkenes react faster), the nature of the alcohol used (primary alcohols react faster than secondary or tertiary), and the presence of electron-donating or withdrawing groups on the alkene.

Alkoxymercuration Mechanism Reaction Example

After learning about the reaction, let's examine a concrete case to get a clearer visual. Consider the reaction of cyclohexene with ethanol and mercuric acetate as a model system. First, note that the reaction requires a carbon-carbon double bond, which is present in cyclohexene.

C_6H_{10}\:\xrightarrow{Hg(OAc)_2, CH_3CH_2OH, NaBH_4}\:(C_2H_5)_2O


1706980181406


Because we are using ethanol in this instance, we will eventually attach this molecule to our cyclohexene starting material and produce an ethyl ether. It's also important to remember that when the carbon-carbon double bond is broken, we also acquire a new carbon-hydrogen link in addition to the carbon-oxygen bond from the ether.

Commonly Asked Questions

Q: What is alkoxymercuration?
A:
Alkoxymercuration is an organic reaction that adds an alkoxy group (RO-) and a mercury(II) group to a carbon-carbon double bond. It's a type of electrophilic addition reaction that results in the formation of an organomercury compound.
Q: Why is alkoxymercuration considered regioselective?
A:
Alkoxymercuration is regioselective because it follows Markovnikov's rule. The alkoxy group attaches to the more substituted carbon of the double bond, while the mercury attaches to the less substituted carbon.
Q: What is the key difference between hydration and alkoxymercuration?
A:
The key difference is the nucleophile used. Hydration uses water (H2O) as the nucleophile, resulting in an alcohol product. Alkoxymercuration uses an alkoxide (RO-) as the nucleophile, resulting in an ether product.
Q: How does the mechanism of alkoxymercuration begin?
A:
The mechanism begins with the electrophilic mercury(II) ion attacking the electron-rich double bond, forming a mercurinium ion intermediate. This three-membered cyclic ion is similar to a bromonium ion in halogenation reactions.
Q: What role does the solvent play in alkoxymercuration?
A:
The solvent, typically methanol or ethanol, serves as both the reaction medium and the source of the alkoxy group (nucleophile) that will attack the mercurinium ion intermediate.

Applications

  • Oxymercuration is not just an alkene adding hydroxyl and mercury groups to water. Instead of waiting until a different reduction phase, the mercury in the carbon-mercury complex can spontaneously be replaced by hydrogen. In this way, mercury has the effect of acting as a Lewis acid catalyst. For instance, creating an enol by substituting an alkyne for alkene results in a ketone that has tautomerism. An ether results from replacing water with alcohol. Markovnikov's rule is followed in both scenarios.

  • When alcohol is present, the use of a vinyl ether enables the substitution of one alkoxy group (RO-) for another via an acetal intermediate. Under the right circumstances, an oxymercuration reaction between an allyl alcohol and a vinyl ether can produce 1706980181478, which is appropriate for a Claisen rearrangement.

Commonly Asked Questions

Q: How does alkoxymercuration-demercuration compare to hydroboration-oxidation in terms of regioselectivity?
A:
Alkoxymercuration-demercuration follows Markovnikov's rule, adding the alkoxy group to the more substituted carbon. In contrast, hydroboration-oxidation is anti-Markovnikov, adding the OH group to the less substituted carbon.
Q: Can alkoxymercuration be performed intramolecularly?
A:
Yes, intramolecular alkoxymercuration is possible if the starting material contains both an alkene and an alcohol group. This can lead to the formation of cyclic ethers.
Q: What is the environmental concern associated with alkoxymercuration?
A:
The main environmental concern is the use of mercury compounds, which are toxic and can accumulate in the environment. This has led to efforts to develop mercury-free alternatives for similar transformations.
Q: Why is alkoxymercuration often followed by demercuration?
A:
Demercuration is often performed after alkoxymercuration to remove the toxic mercury group and replace it with hydrogen, resulting in a more useful organic product.
Q: What type of alkenes work best for alkoxymercuration?
A:
Alkoxymercuration works well with a variety of alkenes, but it's particularly useful for terminal alkenes (those with the double bond at the end of the carbon chain) due to clear regioselectivity.

Frequently Asked Questions (FAQs)

Q: What are some recent developments or alternatives to traditional alkoxymercuration?
A:
Recent research has focused on developing mercury-free alternatives that achieve similar transformations. These include palladium-catalyzed alkoxylation of alkenes, gold-catalyzed additions, and other transition metal-catalyzed processes that can add alkoxy groups to unsaturated compounds with similar regio- and stereoselectivity.
Q: How does alkoxymercuration compare to oxymercuration-reduction in terms of overall yield and selectivity?
A:
Both reactions follow similar mechanisms and offer comparable yields and selectivities. The main difference is in the product: alkoxymercuration-demercuration produces ethers, while oxymercuration-reduction produces alcohols. The choice between them often depends on the desired product.
Q: Can alkoxymercuration be performed catalytically?
A:
While traditional alkoxymercuration uses stoichiometric amounts of mercury salts, efforts have been made to develop catalytic versions using lower amounts of mercury. However, these are less common and often less efficient than the stoichiometric reaction.
Q: How does the reaction temperature affect alkoxymercuration?
A:
Temperature can affect the rate and selectivity of alkoxymercuration. Higher temperatures generally increase the reaction rate but may decrease selectivity. Lower temperatures can improve selectivity but may require longer reaction times.
Q: What are some industrial applications of alkoxymercuration?
A:
Due to environmental concerns about mercury, industrial applications of alkoxymercuration are limited. However, it has been used in the production of some specialty chemicals and in research settings for the synthesis of complex molecules.
Q: How does alkoxymercuration behave with alkenes containing electron-donating groups?
A:
Electron-donating groups on the alkene generally accelerate alkoxymercuration by making the double bond more electron-rich. This increases its ability to interact with the electrophilic mercury(II) ion, potentially affecting regioselectivity as well.
Q: What is the significance of alkoxymercuration in the study of reaction mechanisms?
A:
Alkoxymercuration serves as a classic example of electrophilic addition mechanisms. Its study has contributed to our understanding of carbocation intermediates, stereochemistry in addition reactions, and the factors influencing regioselectivity.
Q: What role does solvent polarity play in alkoxymercuration?
A:
Solvent polarity can affect the reaction rate and yield. More polar solvents generally facilitate the reaction by stabilizing charged intermediates. However, the solvent is typically the alcohol being added, so its polarity is inherently linked to the reagent.
Q: Can alkoxymercuration be used in asymmetric synthesis?
A:
While traditional alkoxymercuration is not inherently asymmetric, chiral mercury catalysts or chiral alcohols have been explored for asymmetric variants of this reaction. However, these are less common than other asymmetric addition reactions.
Q: How does the presence of neighboring group participation affect alkoxymercuration?
A:
Neighboring group participation, such as from a nearby oxygen or nitrogen atom, can influence the stereochemistry and regioselectivity of alkoxymercuration. It may lead to the formation of bridged intermediates, affecting the final product structure.
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