Danielle has taught middle school science and has a doctorate degree in Environmental Health
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.
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.
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 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 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.
Keep in mind that it is helpful to distinguish between the terms 'mechanism' and 'reaction.' A mechanism is simply your blueprint or instruction manual needed to run a reaction. Given that we have reviewed the instruction manual for an aldol condensation reaction, let's look at different examples of aldol condensation reactions.
Aldol Condensation Reaction
You can say that aldol condensation reactions come in many flavors, or types. They range from an Wieland-Miescher ketone reaction to the Robinson annulation reaction. Regardless of the reaction, the common denominator is that the same mechanism, or instruction manual, is used. Shown below is a different aldol condensation reaction. As you view the reaction, think about the importance of the mechanism, what small molecule was removed, and how each reactant condensed to form the aldol product.
Let's review what we've learned!
Adol is a molecule that consists of an aldehyde and an alcohol group and condensation is combining or condensing two molecules and removing a small molecule in the process. So, 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 keytone.
This happens when an enol, or a compound that has an alcohol group substituted onto an alkene molecule or _enolate, a compound of the anions of an enol, reacts with a carbonyl group and water is removed. An alpha hydrogen and carbonyl group are needed to run this reaction.
There are three simple steps in the mechanism of an aldol condensation: (1) get rid of the alpha hydrogen, (2) form a new carbon and carbonyl bond, and (3) convert that carbonyl group (CO) into a COH using deprotonation, which is the process of removing a proton from a molecule.
Aldol condensation reactions come in different types, such as the Wieland-Miescher reaction and the Robinson annulation reaction.
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