Introduction of Alkoxymercuration Mechanism
In the presence of mercuric acetate, a carbon-carbon double-bonded molecule called an alkene reacts with alcohol to make an alkoxymercury intermediate. This intermediate is then broken down with sodium borohydride to make an ether.
In the acid-catalyzed addition of alcohols to alkenes, an alkene is treated with an excess of alcohol in the presence of an acid catalyst to produce an ether under the appropriate conditions. For instance, 2-methoxy-2-methylpropane can be produced by passing 2-methylpropene and methanol over an acid catalyst.
Understanding the Alkoxymercuration-Demercuration Reduction is difficult for organic chemistry students. Not because it is more complicated than oxymercuration, but rather because students frequently overlook the alcohol reagent and the alkyl group produced.
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What is Alkoxymercuration-Demercuration?
An alkene (a molecule containing a carbon-carbon double bond) reacts with an alcohol in the presence of mercuric acetate to form an alkoxymercury intermediate, which is then reduced with sodium borohydride to produce an ether.
First, an organic compound containing an alkene will undergo a reaction with alcohol and mercuric acetate, our mercury-containing reagent. This reaction leads to the formation of an alkoxymercury intermediate. This intermediate is then reacted with a reducing agent, sodium borohydride, to produce an ether as the final organic product.
Ethers are organic water derivatives in which both hydrogen atoms have been replaced by two carbon-based groups. Diethyl ether, for instance, is an excellent example of a generic ether.
Carbocation stability restricts ether synthesis from alkenes catalyzed by acid. Carbocation rearrangement can produce a more stable ion, as demonstrated in the following example.
The following illustration demonstrates that acid-catalyzed ether synthesis employing the alkoxymercuration-demercuration reaction pathway yields the Markovnikov product without carbocation rearrangement.
Alkoxymercuration-demercuration is a stereospecific two-step Markovnikov process for the production of ethers (anti addition). The stages of alkoxymerecuration and demercuration occur on opposite sides of the double bond, producing trans stereochemistry.
Alkoxymercuration-Demercuration Mechanism
Alkoxymercuration entails reacting an alcohol with an alkene in the presence of a mercury salt, such as mercuric acetate, and then demercuring with sodium borohydride to produce ethers.
Similar to the oxymercuration reaction, except alcohol is used instead of water.
This reaction follows the mechanism of electrophilic addition. A mercurium ion bridge stabilizes the carbocation intermediate, preventing it from rearrangement. The electropositive charge of metals. Mercury, which has a partial positive charge, functions as the electrophile in the acetate complex.
In the first step of this reaction, the pi electrons bond to mercury while the lone pair on mercury bonds to the other vinyl carbon, forming a mercurium ion bridge. The ion of mercury stabilizes the carbocation, preventing it from rearrangement. The formation of a mercurium ion is caused by the loss of an acetate ion.
In the second step of this reaction, an alcohol molecule combines with the most substituted carbon to open the mercurial ion bridge.
In the third step of this process, the addition product is neutralized by proton transfer to a solvent alcohol molecule.
Step 4 of the reaction pathway involves reducing the organomercury intermediate with sodium borohydride under basic conditions.
The reaction mechanism is based on Markovnikov’s regioselectivity, with the OR group attached to the carbon with the most substitutes and the H group attached to the carbon with the fewest substitutes. The reaction is advantageous because it does not require strong acids and prevents carbocation rearrangements due to the absence of a carbocation intermediate.
Example of an Alkoxymercuration Mechanism Reaction
Now that we understand the reaction, let’s examine a specific instance for a better visual reference. Consider the reaction of cyclohexene with ethanol and mercuric acetate as a model system. First, observe that cyclohexene contains the required carbon-carbon double bond for the reaction.
Because we are employing ethanol in this instance, we will eventually incorporate this molecule onto our cyclohexene starting material and generate an ethyl ether. It is also essential to remember that by breaking the carbon-carbon double bond, a carbon-hydrogen bond is formed in addition to the carbon-oxygen bond in ether.
Frequently Asked Questions on Alkoxymercuration Mechanism
Q.1 Is Alkoxymercuration anti Markovnikov?
Alkoxymercuration-demercuration is a Markovnikov-progressing stereospecific two-step method for producing ethers (anti addition).
Q.2 What exactly is the Markovnikov rule?
In addition to reactions with asymmetrical alkenes, the Markovnikov rule states that the electron-rich element of the reagent adds to the carbon atom with fewer hydrogen atoms bonded to it. The electron-deficient element bonds additional hydrogen atoms to the carbon atom. Vladimir Vasilyevich Markovnikov first proposed it in 1869.
Q.3 What is the result of this mechanism for alkoxymercuration?
An alkene combines with alcohol in the presence of mercuric acetate to form an alkoxymercury intermediate, which is then reduced with sodium borohydride to yield an ether.
Q.4 How does alkene react with mercuric acetate?
During oxymercuration, an alkene reacts in aqueous solution with mercuric acetate (AcO–Hg–OAc) to form an acetoxymercury (HgOAc) group and a hydroxy (OH) group across the double bond. No rearrangements are observed during this procedure because no carbocations are produced.
