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Aldol Addition: Mechanism & Reaction

Instructor: Korry Barnes

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

The goal of this lesson will be to gain an understanding of a reaction in organic chemistry known as the aldol addition by carefully studying the general reaction form and the reaction mechanism.

Clicking Molecules Together Like Railroad Tracks

If you look back in history before cars were invented, one of the primary methods of traveling long distances was by means of a steam-powered train. This was especially true in the early years of the United States as people started moving into the west and settling new territories. Someone had to build the railroad tracks though, and it took countless man-hours to construct enough tracks to stretch from one region of the country to the next.

Individual pieces of railroad tracks probably aren't that useful, but when lots of them are connected together you can build large and complex networks of tracks useful for transportation. Did you know that we can use the same type of logic for some organic compounds? You see, sometimes simple organic compounds can be 'joined' together just like railroad tracks to give larger, more complex structures that are more useful than the individual building blocks. In our lesson, we are going to be learning about a very powerful reaction in organic chemistry that has the ability to form carbon-carbon bonds, called the aldol addition. Let's get started!

What Is an Aldol Addition?

What exactly is an aldol addition? Might be a good place to start right? An aldol addition is a reaction that occurs between the enolate of an aldehyde or ketone and the alpha-carbon of another aldehyde or ketone to form a beta-hydroxy aldehyde or ketone. Clear as mud right? Let's take the technical and wordy definition and break it down into simpler parts and terms. First off what's an enolate? An enolate is an intermediate in organic chemistry that contains a formal negative one charge on either a carbon atom or an oxygen atom.

If the negative charge is on the carbon atom there's a double bond between carbon and oxygen, but if it's on the oxygen atom there's a double bond between two carbon atoms. This type of scenario is called resonance when the negative charge can be placed on more than one atom. Consider as an example the enolate of butanone.


An enolate is an intermediate in organic chemistry derived from an aldehyde or ketone
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Enolates form when a base reacts with an aldehyde or ketone and pull off one of the hydrogen atoms directly adjacent to the carbon-oxygen double bond (carbonyl group).


An enolate results when an aldehyde or ketone reacts with a base
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The aldol addition in a nutshell is when the newly formed enolate reacts with another aldehyde or ketone to form a new carbon-carbon bond and thus a larger organic compound. Kind of like building railroad tracks! The general form of the aldol addition is enolate + aldehyde or ketone = aldol product.


The general form of an aldol addition reaction
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Mechanism of the Aldol Addition

Now that we're familiar with what a general aldol addition reaction looks like, let's break the reaction down into individual steps by studying the reaction mechanism. Fortunately, there are only a few steps we need to consider. Let's use the reaction of acetone with benzaldehyde as our model reaction to learning the mechanistic steps.

Step 1: Formation of the Enolate

Step 1 of the reaction involves acetone reacting with a base to actually form the intermediate enolate needed for the reaction. The base takes one of the hydrogens of acetone and then acetone keeps the pair of electrons associated with what used to be the carbon-hydrogen bond.


Step 1 of the aldol addition mechanism in which the enolate is formed
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Step 2: The Carbon-Carbon Bond Forming Reaction

In the second step of the reaction, the enolate reacts with the carbon-oxygen double bond of benzaldehyde. This is where the new carbon-carbon bond forms and the two individual compounds are linked together. When the enolate reacts with benzaldehyde, we have to break the existing carbon-oxygen double bond and this places an extra pair of electrons (shown as a negative charge) on oxygen.


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