5. Chirality
Test 2:Stereocenter Test
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Let's talk about let's define what stereo centers are because it turns out that there's actually 2 different types of stereo centers. Okay? So you may hear your professor refer to these as stereogenic centers, but that's just the same thing as stereocenter. Okay? And like I said, the definition is any group that creates stereoisomers after swapping. Okay, after swapping 2 atoms. Okay? Or any atom that creates stereoisomers after swapping groups. So here are the 2 different ones. The first and most common one that we're going to talk about is called the chiral center. Okay? Now I just want to make this point. A chiral center is a type of stereo center, but they're not exactly the same thing. Because like I said, there 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 chiral center is always a chiral center. No, that's not true. There are some stereo centers that are not chiral centers. Okay? So what is a chiral center? A chiral center is going to be any atom that has 4 different substituents. Okay? That's it. It's a super easy definition. So if I have an atom in the middle and it's attached to molecule atom a, atom b, atom atom that is what we call a chiral center and it would be denoted with a star. Okay, I would use a little star to say that that is a chiral center. Why is it so special that it has 4 different groups? I'll explain that in a little bit. But that's just the only thing you need to know. If it has 4 different groups, it's a chiral center and as you guys might have guessed, a chiral center is a predictor of chirality. So if I have a chiral center, that means that that molecule is going to be chiral. Okay? So now let's go on to the next step. The one over here is the trigonal center. A trigonal center is another type of a 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. Or is the more technically correct term because a lot of these will have more than 2 substituents, but it's the same concept where you have a double bond that can form into 2 different positions or 2 different shapes. When you have that kind of double bond, that is a stereo center. Okay? But it's not chiral by itself. So these are actually achiral. So it's possible to have a stereo center and for the molecule to be achiral if that stereo center happens to be a trigonal center.
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