Azide-Alkyne Huisgen Cycloaddition Reactions

Instructor: Korry Barnes

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

The focal point of this lesson will be on a specific type of reaction in organic chemistry called a cycloaddition reaction. Our discussions will center around the azide-alkyne Huisgen cycloaddition.

Cyclic Compounds and Monopoly

Cycles occur quite frequently in our lives don't they? A cycle could refer to a cycle on a washing machine or even a cycle of exercises at the gym. Have you ever taken a piece of string or rope and tied the two ends together to make a loop? That would be another example of creating a cycle because when the two ends are joined together, it creates a continuous loop of string.

Did you know that some organic compounds can be described as cycles too? An organic compound that contains a ring system of some sort is referred to as cyclic. We know that a cycle in Monopoly means going all the way around the board, but where do cyclic organic compounds come from? How are they made? That's actually the subject of our lesson today where we'll be talking about a specific class of reaction called an azide-alkyne Huisgen cycloaddition, which is very important for the construction of cyclic compounds.

What's Needed for the Reaction?

In order for us to have a Huisgen cycloaddition reaction we need two basic reactants, an organic azide and an alkyne. An organic azide is a compound that contains a carbon-based group bonded to three nitrogen atoms, all in a row. Sometimes an azide is represented by the symbol R-N=N=N where the 'R' group can be virtually any carbon-based group. An alkyne is any organic compound that contains a carbon-carbon triple bond.

When the azide and alkyne react with one another they produce a five-membered ring that contains the three nitrogen atoms from the azide, called a triazole.

The Huisgen cycloaddition involves an azide reacting with an alkyne to produce a triazole as the product

Variations of the Huisgen Cycloaddition

Since the discovery of the Huisgen cycloaddition reaction, many different variations have been reported in the scientific literature that take advantage of the methodology.

Reactions that Incorporate Different Atoms and Groups

It's possible to take an alkyne that is functionalized with an oxygen atom, called an ether functional group, and react it with an azide bonded to an aromatic ring. This type of reaction forms a triazole that incorporates both groups from the reactants onto the product. For an organic chemist needing to make a triazole containing an ether and an aromatic ring it can be very useful.

A particular variation of the Huisgen cycloaddition that incorporates an oxygen atom and aromatic ring into the product

Double Cycloaddition Reactions

It's also possible to perform a double Huisgen-type cycloaddition on the same molecule! By strategically placing two azide groups on one of the reactants, the cycloaddition reaction can occur twice and give a product that contains two triazole rings. However since the cycloaddition is occurring twice, we need to add extra of the alkyne reactant.

If two azide groups are incorporated on the same molecule the cycloaddition can occur twice and give a product with two triazole rings

Cycloadditions that Form Polymers

Polymers are very important organic compounds used in things ranging from rubber for our tires all the way to the Tupperware that's in our kitchen cabinets. Some polymers can actually be made from a Huisgen-type cycloaddition reaction, in what's called 'click chemistry.' Click chemistry is a type of chemical reaction that is usually high-yielding and involves coupling or 'clicking' different molecules together.

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