Huisgen Cycloaddition: Mechanism & Overview

Instructor: Korry Barnes

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

The primary focal point of this lesson will be on an organic chemistry reaction known as the Huisgen cycloaddition. Our points of discussion will be the general reaction form and how the reaction occurs in terms of the mechanism.

A Unique Kind of Reaction

Do you have a favorite type of reaction from your organic chemistry class? Today in Ashley's organic chemistry lecture course, her instructor is going over a type of reaction that seems to have really caught her attention. They are learning about reactions called cycloaddition reactions. A cycloaddition reaction is when two organic molecules combine with one another, producing a ring structure as the product. This is an easy term to remember, as the first part of the word, cyclo, comes from Greek, and means circle. (Just think cyclo=cycle, as in bicycle.) Thus, a cycloaddition adds two organic molecules together to create a circle or ring.

The specific cycloaddition that Ashley's class is learning about is called the Huisgen cycloaddition reaction. This type of cycloaddition reaction is very important to organic chemists, and has a lot of utility in the industrial and pharmaceutical sub disciplines of chemistry. Let's listen in along with Ashley's class and learn about the Huisgen cycloaddition reaction together!

Reaction Overview

The Huisgen cycloaddition reaction occurs between two organic building blocks: an alkyne and an azide. An alkyne is an organic compound that contains a carbon-carbon triple bond, and an azide is a compound that contains three nitrogen atoms connected successively to one another.

Alkynes are organic compounds that contain a carbon-carbon triple bond

Azides contain three nitrogen atoms connected successively to one another

When the alkyne and the azide react with one another, the product that forms is a five-membered ring in which three of the atoms are nitrogen and two of the atoms are carbon. This type of ring system in organic chemistry is called a triazole, and is an important building block for pharmaceuticals and agricultural compounds.

Reaction Mechanism

Now that we know what's involved in the Huisgen cycloaddition and we know what kind of product it produces, let's take a look at the mechanism to see how the reaction happens. The good news for Ashley and her class is that the reaction mechanism is rather simple. The Huisgen cycloaddition reaction is classified as a pericyclic reaction, which means that the reaction proceeds by a concerted mechanism.

Have you ever heard that term concerted before? In chemistry terms, it simply means that the reaction happens all in one step. In a concerted reaction, all of the bond-making and bond-breaking events happen simultaneously. One of the alkyne carbons donates a pair of electrons to the nitrogen atom that bears a positive charge while simultaneously, the nitrogen that has a negative charge donates a pair of electrons to the other alkyne carbon atom.

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