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8.6 Alkylation of Enolates
In this section, we will look at reactions between enolates and electrophiles.
elimination will be favored over substitution.
When we try to make the enolate by treating a ketone with hydroxide, we run into a major obstacle.
There is a small amount of enolate, but there is a lot of ketone and hydroxide present.
If we introduce some alkyl halide into the reaction flask, we run into a big problem.
There are competing side reactions that create a mixture of products when excess hydroxide reacts with alkyl halide.
In order to avoid this problem, we will need to form the enolate under certain conditions.
We won't have to worry about the base reacting with alkyl halide if we are able to do this.
We will need a base that is stronger than the ones we have been using to do this.
The name of the compound is LDA.
The negative charge on a nitrogen atom is less stable than a negative charge on an oxygen atom, which makes LDA a very strong base.
LDA is used as a strong, sterically base, which hindered what we need in our situation.
LDA can be used to convert the ketone into the enolate.
We will have mostly enolates in our reaction flask.
The risk of competing side reactions is greatly reduced when we introduce some alkyl halide into our reaction flask.
To form an enolate, we use LDA to deprotonate the ketone.
Don't get confused when you see THF in the reagents.
In step 2, we use an alkyl halide to install the alkyl group, where R is a primary alkyl group, and X is a halogen.
The starting ketone was symmetrical.
The more-substituted enolate is more stable because it has a more substituted pi bond.
The less-substituted enolate is expected to form more rapidly.
The sterically hindered base is expected to make it easier to remove one of the protons.
This is a classic example of how things work.
Stability and energy levels are the most important aspects of thermodynamics.
The argument says that we should form the more stable enolate.
A mixture of products is observed.
There are many ways to achieve this.
The thermodynamic enolate is what you need to form.
Some textbooks teach one or two ways to do this, while others don't.
If you are responsible for knowing how to alkylate the more-substituted side, you should look through your textbook and lecture notes.
A alkylation reaction is what this is.
LDA is used to form an enolate.
In step 2, we are using an alkyl halide to alkylate.
An ethyl group will be installed on an alpha carbon because the alkyl halide is ethyl chloride.
The less-substituted enolate is mostly given by the use of LDA as our base.
There is an extra group introduced at an alpha position if we look at the difference between the starting material and the product.
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