Alkoxymercuration-Demercuration of Ethers: Mechanism & Example

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson, you'll learn how ethers can be synthesized using alkoxymercuration-demercuration. You'll study the reaction mechanism and see an example.

The Magic of Mercury

When you hear the word mercury, what's the first thing that comes to your mind? Perhaps you think about an old automobile company that used to make cars. Or maybe you think about the shiny, silvery liquid that's sometimes used inside the thermometers in a chemistry lab. Regardless of your view of mercury, you probably don't think of it in the context of organic chemistry.

It turns out though that mercury can play an important role in a specific kind of reaction in organic chemistry called alkoxymercuration-demercuration. Today we are going to be learning about this transformation by carefully studying the reaction mechanism and looking at a specific example of the reaction. Let's see what mercury can do!

What is Alkoxymercuration-Demercuration?

Let's start by talking about the reaction in general terms first. Alkoxymercuration-demercuration is a reaction in which an alkene (a compound containing a carbon-carbon double bond) is reacted with an alcohol in the presence of mercuric acetate that initially yields what's called an alkoxymercury intermediate, which produces an ether after reduction with sodium borohydride. That definitely is a mouthful and seems technical so let's break it down piece-by-piece.

Essentially what we're talking about doing is first reacting an organic compound that contains an alkene with an alcohol and mercuric acetate, which is our mercury-containing reagent. This reaction produces an intermediate called an alkoxymercury intermediate. This intermediate is then further reacted with a reducing agent called sodium borohydride to ultimately produce an ether as the final organic product. Ethers are organic derivatives of water in which both hydrogen atoms have been replaced by two carbon-based groups. For example, diethyl ether is a good example of what a general ether looks like.

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

A Specific Example of an Alkoxymercuration-Demercuration Reaction

Now that we're familiar with the reaction itself, let's take a look at a specific example to get a better visual aid of what it is we're talking about. Let's use the reaction of cyclohexene with ethanol and mercuric acetate as our model system. Notice first that cyclohexene contains a carbon-carbon double bond which is needed for the reaction.

Alkoxymercuration-demercuration of cyclohexene using ethanol

Since ethanol is the alcohol we are using in this case, we will ultimately incorporate this molecule onto our cyclohexene starting material and form an ethyl ether as the product. In losing the carbon-carbon double bond, it's also important to realize that we gain a new carbon-hydrogen bond in addition to the carbon-oxygen bond of the ether.

Mechanism of Alkoxymercuration-Demercuration

Let's now discuss how the alkoxymercuration-demercuration reaction happens by taking a look at the detailed reaction mechanism using our previous model system as an example.

Step 1

In the first step of the mechanism, a reaction takes place between the alkene and mercuric acetate, forming a three-membered ring system with a positive charge on the mercury atom.

The first mechanistic step in an alkoxymercuration reaction

Step 2

With the three-membered mercury intermediate formed, here is where our alcohol acts as a nucleophile (electron pair donor) and bonds with one of the carbon atoms of the three-membered ring. When this happens, the ring opens to form our alkoxymercury intermediate we mentioned previously.

The second step in an alkoxymercuration reaction mechanism

Step 3

In the final step of the reaction, a hydride ion (supplied by sodium borohydride) attacks the carbon atom bonded to the mercury atom and simultaneously kicks off the mercury as a leaving group. This is where the new carbon-hydrogen bond comes from and also represents why sodium borohydride is needed as part of the reaction.

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