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. 

Condensation Overview

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:

Mechanism

enolate-formation

Enolate Formation

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. 

Aldol Condensation

aldol-condensation

Aldol Condensation

The aldol condensation reaction occurs between an enolate and an aldehyde to yield a beta-hydroxy ketone.

Claisen Condensation

claisen-condensation

Claisen Condensation

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.

Dieckmann Condensation

dieckmann-condensation

Dieckmann Condensation

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:

dehydrated-condensation-products

Dehydrated Condensation Products

Detailed Videos

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!

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