Condensation reactions are reactions in which two molecules combine to form a larger molecule, producing a small molecule such as H2O as a byproduct. They typically form a new carbon-carbon bond and result from a nucleophilic enolate and an electrophilic carbonyl.
P.S. Here is the 2-hour super-comprehensive version if you are not in a time crunch.
There are a few varieties of condensation reactions, and all require those two pieces. Sometimes metal-complex catalysts are added, but luckily most undergrad Orgo classes focus on base-catalyzed mechanisms! You'll likely see these reactions again later in carbohydrate chemistry or in Biology class.
So, how do we actually form an enolate? We have to deprotonate the alpha-carbon of a carbonyl. Remember that alpha-carbons are much more acidic than standard sp3-hybridized carbons! Let’s quickly go over three condensation reactions here:
A base is used to deprotonate the alpha-hydrogen. The type of condensation reaction the resulting enolate undergoes depends on what kind of electrophile is present.
The aldol condensation reaction occurs between an enolate and an aldehyde to yield a beta-hydroxy ketone.
The Claisen condensation reaction involves an ester (or diester) and an enolate. It yields a beta-keto ester or a beta-diketone by displacing the acyl group through nucleophilic acyl subsitutition. The above reaction resulted in a beta-diketone.
The Dieckmann condensation reaction is an intramolecular reaction of a diester. The enolate forms on the alpha carbon of one carbonyl and then directly attacks the other. This forms a ring with a beta-keto ester.
Two things to note: 1) the products can undergo dehydration to form an enone (an alpha-beta unsaturated ketone) through an E1cB mechanism and 2) these reactions are reversible through hydrolysis! Let's see what each of our products looks like after dehydration:
There are plenty of different types of condensation reactions:
P.S. Congratulations! You just learned a form of polymerization! Carbonyl compounds can combine to form polymers like nylon through this mechanism!