Atropisomers in Organic Chemistry: Definition, Examples & Chirality

Instructor: Korry Barnes

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

What are atropisomers? In this lesson, we will be learning about these specific isomers by exploring the definition, discussing specific examples, and briefly talking about their chirality.

Do the Twist

Have you ever seen one of those old dancing movies like Grease? A lot of times we tend to think of some of those old movies to be rather cheesy and silly, but in their time they were big hits in the box offices all over the world. One of the most common dance moves in those types of movies was the twist, where the dancers would simply rotate their hips back and forth as they moved on the dance floor.

Did you know that sometimes organic compounds can twist too? Because of the bond rotation that's possible with single bonds a molecule can freely rotate in space. Sometimes, however, even though a molecule may have a single bond that you would think could freely rotate or twist, it won't be able to do so. That's what we are going to be talking about in this lesson, molecules that can't twist or rotate about single bonds that are known as atropisomers. Let's explore this unique case!

Definition of Atropisomers

Let's get rolling by first talking about how to define atropisomers. In organic chemistry atropisomers are stereoisomers that result from a hindrance of bond rotation about single bonds due to steric, making the isolation of the individual isomers possible. That's a technical sentence so let's break down the definition piece-by-piece.

When compounds are said to be stereoisomers of one another it means that they have the same chemical formula, the same atom connectivity, but are different from one another in the way they occupy three-dimensional space i.e. their shape. Steric strain is a type of strain energy that is experienced by a molecule when the atoms of the molecule are brought into close proximity of one another or forced to occupy the same physical space.

It's kind of like someone being in your personal space bubble when that happens your natural reaction is to back away right? It's the same thing here with steric strain. The atoms want to maximize space between each other to minimize any repulsive interactions. Now, normally we would expect single bonds to be able to rotate freely.

However, in case of atropisomers, it turns out that the steric strain is so much that the molecules are 'stuck' or locked into one specific conformation and they don't actually rotate at all.

Examples of Atropisomers

Now that we're familiar with the definition of atropisomers, let's take a look at a couple of example cases to get a better visual idea of the concept. Consider a set of compounds composed of two benzene rings bonded together, with each of the benzene rings also containing nitro and carboxylic acid groups. In both cases, the bold black bonds indicate bonds that are coming out of the page at us.

At first glance the two compounds may not look that different from one another but upon closer inspection, we hopefully should see that the benzene ring, on the molecule on the left, is twisted in such a way that the carboxylic acid group is coming out of the page at us along with the nitro group on the bottom benzene ring.

These molecules are examples of atropisomers of one another

But if we examine the molecule on the right it's the complete opposite of its partner! The reason these two compounds exist is because, in order for the molecule on the left to become like the one on the right, the bond would have to rotate and cause the carboxylic acid and nitro groups to clash or run into each other. This steric strain is enough such that the rotation doesn't happen and both compounds are 'locked' into their respective conformations.

Another good example of atropisomers would be Felodipine, which is a medicine used to treat high blood pressure. In this case, notice the dashed and wedge bonds of each isomer. The dashed bond is going away from us into the page and the wedged bond is again coming out at us.

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