Chemical and Biomolecular Engineering

Official blog of the Lehigh University Chemical Engineers

Diels-Alder Reactions

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What’s the Diel with Alder reactions? (I’ll stick to my day job) In organic chemistry, we study the reactions and properties that govern organic life. For the majority, we examine molecules and reactions that contain carbon, hydrogen, oxygen, and nitrogen. These elements are the basis for all organic material, with carbon being the most important. (the common expression being carbon-based lifeform). Organic chemistry is an extremely broad topic, but when boiled down, organic chemists aim to figure out how to produce different material. This is referred to as synthesizing a material. An organic chemist has to figure out how to get from point A to point B, it’s kind of like solving a maze. One of the major components of class is learning and employing mechanisms for reactions. Mechanisms describe how a reaction occurs and in what order. One such mechanism/reaction is the Diels-Alder reaction, which I first learned about in lecture and then actually carried out in lab.

Diels-Alder Reaction

Diels-Alder Reaction

The Diels-Alder reaction is very significant because it forms carbon-carbon bonds. The chemists whom the reaction is named after received the Nobel Prize in Chemistry for the novel discovery. The image above explains the mechanism for the reaction. A diene (2 carbon-carbon double bonds) reacts with a dienophile (C=C) to produce a six carbon ring with a single double bond. This ring is called cyclohexene. The red arrows indicate the movement of electrons which exist in the pi pairs. Pi pair bonds form double bonds. The reaction if fairly simple, it always occurs the same way with the same groups reacting. The only difference is that there can be other substituents (molecules) on the diene or dienophile, or the reactants can be rings. The Diels-Alder reaction solely refers to the above reaction, but by altering the diene and dienophile, the final product can be altered to something the chemist desires.

After learning about the reaction, I was able to complete a Diels-Alder Reaction in organic lab. We started with dicyclopentadiene which had to be “cracked” into cyclopentadiene. Cyclopentadiene acts as the diene for the reaction. To “crack” the dicyclopentadiene, we used fractional distillation, which involves boiling the mixture to remove a certain compound by recondensing it. Cyclopentadiene has a much lower boiling point than the original substance, so we were able to remove it and use for the reaction. Next, we dissolved maleic anhydride in several solvents and then slowly added the cyclopentadiene. Maleic anhydride is a while solid and acts as the dienophile in the reaction. After setting the reaction on ice, we observed the Diels-Alder reaction, with the product being cis-Norbornene-5,6-endo-dicarboxylic Anhydride (I’ll call it the “product”). This is a perfect example of how a complex substance can be produced based on the simple Diels-Alder mechanism. The product is an interesting white crystalline structure that looks like snow. To recover and purify the product, we used a recrystallizing technique by heating, cooling, and then filtering. This lab was a great way to get tangible reinforcement from what was taught in lecture. Check out the pictures below that highlight the product from the lab.

Reaction

Reaction

Crystal Product

Crystal Product

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Author: Ben Dunmire

I am a sophomore Chemical Engineering major at Lehigh. On campus, I am the president of Club Baseball and a member of AIChE and NSCS. Outside of school, I enjoy the outdoors, fishing, and any type of athletic activity. You can reach me at bcd216@lehigh.edu.

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