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Alkoxymercuration Mechanism

Introduction

In the presence of mercuric acetate, an alkene (a molecule containing a carbon-carbon double bond) interacts with alcohol to make an alkoxymercury intermediate, which is subsequently reduced with sodium borohydride to provide an ether.

The acid-catalysed addition of alcohols to alkenes entails treating an alkene with an excess of alcohol in the presence of an acid catalyst to create an ether under the right conditions. For example, 2-methoxy-2-methylpropane is formed when 2-methylpropene and methanol are passed over an acid catalyst.

The Alkoxymercuration-Demercuration Reduction is a difficult mechanism for organic chemistry students to understand. It’s not because it’s more complex than oxymercuration; instead, students frequently overlook the alcohol reagent and the alkyl group in the result.

Table of Contents

What is Alkoxymercuration-Demercuration?

Alkoxymercuration-demercuration is a reaction in which an alkene (a molecule with 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, we will react to an organic compound containing an alkene with alcohol and mercuric acetate, which is our mercury-containing reagent. An alkoxymercury intermediate is formed as a result of this reaction. This intermediate is further reacted with sodium borohydride, a reducing agent, to provide an ether as the final organic product. Ethers are organic water derivatives in which two carbon-based groups have replaced both hydrogen atoms. Diethyl ether, for example, is a good representation of a generic ether.

Diethyl ether is an example of the general structure of an ether

Carbocation stability limits acid-catalyzed ether synthesis from alkenes. As seen in the case below, carbocation rearrangement can occur to generate a more stable ion.

The example below shows that acid-catalyzed ether synthesis utilising the alkoxymercuration-demercuration reaction pathway provides the Markovnikov product without carbocation rearrangement.

Alkoxymercuration-demercuration is a stereospecific two-step route for producing ethers that progresses in a Markovnikov fashion (anti addition). The alkoxymerecuration-demercuration stages occur on opposite sides of the double bond, resulting in trans stereochemistry.

Alkoxymercuration-Demercuration Mechanism

The Alkoxymercuration method involves reacting an alcohol with an alkene in the presence of a mercury salt, such as mercuric acetate, and then demercuring using sodium borohydride to produce ethers.

The reaction is similar to the oxymercuration reaction, but instead of water, it uses alcohol.

This reaction obeys the electrophilic addition mechanism. The main difference is that the carbocation intermediate is stabilised by a mercurium ion bridge, which prevents it from rearranging. Metals have an electropositive charge. The electrophile in the acetate complex is mercury, which has a partial positive charge.

Step 1: The pi electrons link to mercury in the first step of this reaction, while the lone pair on the mercury bonds to the other vinyl carbon, forming a mercurium ion bridge. The mercurium ion stabilises the carbocation, which prevents it from rearranging. The loss of an acetate ion leads to the formation of the mercurium ion.

Step 2: To open the mercurium ion bridge, an alcohol molecule combines with the most substituted carbon in the second step of this reaction.

Step 3: The addition product is neutralised by a proton transfer to a solvent alcohol molecule in the third step of this process.

Step 4: Reducing the organomercury intermediate with sodium borohydride at basic conditions is the fourth step of the reaction pathway.

The reaction mechanism is based on Markovnikov’s regioselectivity, with the OR group attached to the most substituted carbon and the H group attached to the least substituted carbon. The reaction is useful since it does not require strong acids and prevents carbocation rearrangements because no separate carbocation intermediate forms.

Example of an Alkoxymercuration Mechanism Reaction

Now that we’ve learned about the reaction, let’s look at a specific example to obtain a better visual reference. As a model system, consider the reaction of cyclohexene with ethanol and mercuric acetate. First, notice that cyclohexene has a carbon-carbon double bond required for the reaction.

Alkoxymercuration-demercuration of cyclohexene using ethanol

We will eventually incorporate this molecule onto our cyclohexene starting material and generate an ethyl ether because we are employing ethanol in this case. It’s also crucial to remember that by breaking the carbon-carbon double bond, we obtain a new carbon-hydrogen bond in addition to the ether’s carbon-oxygen bond.

Frequently Asked Questions on Alkoxymercuration Mechanism

Is Alkoxymercuration anti Markovnikov?

Alkoxymercuration-demercuration is a stereospecific two-step route for producing ethers that progresses in a Markovnikov manner (anti addition).

What do you mean by the Markovnikov rule?

The Markovnikov rule states that in addition to reactions to unsymmetrical alkenes, the electron-rich element of the reagent adds to the carbon atom with fewer hydrogen atoms bonded to it. The electron-deficient element adds to the carbon atom with more hydrogen atoms bonded. Vladimir Vasilyevich Markovnikov first proposed it in 1869.

What is the product of this Alkoxymercuration mechanism reaction?

In the presence of mercuric acetate, an alkene combines with alcohol to make an alkoxymercury intermediate, which is subsequently reduced with sodium borohydride to give an ether.

How does alkene react with mercuric acetate?

In oxymercuration, an alkene reacts in an aqueous solution with mercuric acetate (AcO–Hg–OAc) to produce an acetoxymercury (HgOAc) group and a hydroxy (OH) group across the double bond. Because no carbocations are produced during this procedure, no rearrangements are observed.

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