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Aldol Condensation: Mechanism & Reaction

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  • 0:04 Aldol Condensation Background
  • 2:59 Aldol Condensation Mechanism
  • 4:27 Aldol Condensation Reaction
  • 04:58 Lesson Summary
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Lesson Transcript
Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Use this lesson to learn about aldol condensation, a very interesting organic chemistry reaction. Read about the purpose of an aldol condensation and understand its mechanism and reaction.

Aldol Condensation Background

Did you know that our bodies utilize the aldol condensation reaction to break down glucose sugar? They certainly do! In fact, our bodies also use this reaction to synthesize, or create, glucose sugar molecules. Aldol condensation is a reaction that results in the formation of a carbon-carbon bond when an enol (or enolate) molecule reacts with an aldehyde (or ketone).

A great way to remember the basics of an aldol condensation is to look more closely at each term, aldol and condensation. Aldol is a molecule that consists of an aldehyde, represented by the prefix 'al-', and an alcohol group, represented by the suffix '-ol.' You've been looking at an example below. As for our friend condensation, think of combining (or condensing) two molecules and removing a small molecule in the process.

Example of an Aldol Product
example

I know you may be wondering what an enol or enolate molecule is. An example of the enol and enolate form of acetone is shown below. Essentially, an enol is a compound that has an alcohol (OH) group substituted into an alkene (C=C) molecule. And an enolate ion is a compound of the anions, or negatively charged molecules, of an enol. Think of enolate as the cousin of enol; in other words, an enolate ion is an enol that carries a negative charge.

Example of the Enol and Enolate Forms of Acetone
enol

There are few items to keep in mind with the relatives enol and enolate. Enols absolutely love donating electrons, which is why they are called nucleophiles. It just so happens that enolate ions are also nucleophiles. You may be thinking, if enols and enolates enjoy donating electrons, what compounds will accept those electrons? Great question! In the aldehyde or ketone molecule used in this reaction, a carbonyl group is present. This carbon atom, in the carbonyl group, loves accepting electrons from the nucleophiles enol or enolate. You can call this carbon, or any other atom that functions this way, an electrophile.

Generally, aldol condensations are recognized in chemistry as very useful reactions. In fact they are one of a limited amount of reactions that can form new carbon-carbon bonds. Now, why would this be useful? Another great question! Whether it is a research chemical laboratory or pharmaceutical industry lab, sometimes forming a carbon-carbon bond may be required when synthesizing, or creating, new organic compounds. Having the handy aldol condensation reaction available makes this part of the synthesis possible.

As we saw in the definition and will see shortly with the mechanism, you only need two molecules for an aldol condensation reaction: (1) a molecule that has a carbon-hydrogen bond next to a carbonyl (CO) group, and (2) a molecule that has a carbonyl group. The first molecule is called an a-hydrogen. Now that we know the nuts and bolts for an aldol condensation reaction, let's look at the mechanism required to run this reaction.

Aldol Condensation Mechanism

Step 1: Before you start the aldol condensation reaction, you have to perform an acid-base reaction. Now, why is this important? Well, the acid-base reaction ensures the enolate ion is formed. The hydroxide ion (OH) is your base and the a-hydrogen atom is your acid. The hydroxide ion reacts with the a-hydrogen to form a reactive, nucleophile called the enolate ion.

Step 1 of the Aldol Condensation Reaction
aldol step 1

Step 2: Once you form an enolate ion, the next step is to make an intermediate. In order to do so, the enolate must attack the carbon atom in the aldehyde compound. Keep in mind that the enolate ion is a nucleophile and goes after the electrophile, the carbon atom in our pal aldehyde.

Step 2 of the Aldol Condensation Reaction
step 2

Step 3: After the intermediate is formed, another acid-base reaction is performed. In this case, the end result is a deprotonation of water to form an hydroxide. I know, I know, deproto-what? Deprotonation is the process of removing a proton (H) from a molecule. In this case, a proton is removed from the water molecule, formula H2O, to make a hydroxide, formula OH. When this happens, the aldol product is formed.

Step 3 of the Aldol Condensation Reaction
step 3

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