Test 2: Stereocenter Test

The gold standard chirality test for almost all molecules involves finding stereocenters.

Defining Stereocenters

Concept: What is a stereocenter?

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Video Transcript

This brings us to test two. We're going to need some kind of reliable way to tell if molecules that aren't rings are chiral or not. We know that the internal line of symmetry test or what I call test one is crap for chains and branches and stuff like that, so we're going to need some kind of test. It turns out I have just the thing for you guys, what we're going to talk about now is called stereocenters.
It turns out there's another test that we can use and this test is actually going to be the gold standard for most molecules. It might be a little bit harder to use on some kinds of molecules, but it's going to work every time.
How does it work? Well, we have to define what is a stereocenter. A stereocenter is any atom that creates a stereoisomer after swapping groups That is called, like I said, a stereocenter.
What is that definition? Basically, what I'm trying to say here is that if you swap atoms, any two atoms on the same carbon, if you swap their position and if you wind up getting a different molecule or a molecule that has a different shape afterwards, that is called a stereocenter.
Here, I've given you two different examples. In this first example, I went ahead and I had a bromine in the front then I swapped it with an H in the back. And what we'll notice is that now the H is in the front and the bromine is in the back. This is a type of stereocenter because after I inverted these two groups, I didn't get the same exact molecule. In one of them, the bromine is in the front, in the other one, the bromine is in the back. Since there's no plane of symmetry here, those are actually different molecules.
Another example would be this ring, how this ring here has a methyl on the front and an H in the back. And after I swap those two groups, the methyl goes to the back and the H goes to the front, those are going to be also different molecules because once again, we have a ring and rings can't rotate easily. That means if it's on the front, it's going to stay on the front. If it's on the back, it's going to stay on the back. This would also be an example of what we call a stereocenter. The stereocenter itself is the atom or the group of atoms that it's attached to. 

Any atom that creates a stereoisomer after swapping two groups is called a stereocenter.

Types of Stereocenters

Concept: The difference between chiral and trigonal centers.

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Video Transcript

Let's define what stereocenters are because it turns out there's actually two different types of stereocenters. You may hear your professor refer to these as stereogenic centers, but that's just the same thing as stereocenter. Like I said, the definition is any group that creates stereoisomers after swapping two atoms or any atom that creates stereoisomers after swapping groups. Here are the two different ones.
The first and most common one that we're going to talk about is called the chiral center. I just want to make this point. A chiral center is a type of stereocenter, but they're not exactly the same thing because like I said, there's other types of stereocenters as well. You have to be careful with that because a lot of times I hear students refer to these interchangeably, like a stereocenter is always a chiral center. No, that's not true. There are some stereocenters that are not chiral centers.
But what is a chiral center? A chiral center is going to be any atom that has four different substituents. That's it. Super easy definition. If I have an atom in the middle and it's attached to molecule atom A, atom B, atom C, atom D, that is what we call a chiral center and it would be denoted with a star. I would use a little star to say that that is a chiral center.
Why is it so special that it has four different groups? I'll explain that in a little bit, but that's just the only thing you need to know. If has four different groups, it's a chiral center. And as you guys might have guessed, a chiral center is a predictor of chirality. If I have a chiral center, that means that that molecule is going to be chiral.
Let's go on to the next step. The one over here is the trigonal center. A trigonal center is another type of stereocenter, but it's a little bit different. What this is, it's a double bond, so automatically that's kind of different. A double bond that's capable of making E or Z isomers. Do you guys remember what E or Z is? That's just a fancy way of saying cis or trans. E or Z is the more technically correct term because a lot of these will have more than two substituents, but it's the same concept where you have a double bond that can form into two different positions, two different shapes.
When you have that kind of double bond, that is a stereocenter, but it's not chiral by itself. These are actually achiral. It's possible to have a stereocenter and for the molecule to be achiral if that stereocenter happens to be a trigonal center.

Example: Does the following molecule contain a stereogenic center? Is it chiral?

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Example: Does the following molecule contain a stereogenic center? Is it chiral?

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Example: Does the following molecule contain a stereogenic center? Is it chiral?

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Example: Does the following molecule contain a stereogenic center? Is it chiral?

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Example: Does the following molecule contain a stereogenic center? Is it chiral?

 

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Example: Does the following molecule contain a stereogenic center? Is it chiral?

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