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- Many times in chemistry we'll see different molecules that have the same constituent atoms. For example, these two molecules here, they both have four carbons. One, two, three, four. One, two, three, four. So if I were to write their chemical formula, it would be C4 and then they both have, one, two, three, four, five, six, seven, eight, nine, ten. One, two, three, four, five, six, seven, eight, nine, ten hydrogens. So both of them, both of them have the chemical formula C4H10. C4H10, but they're still fundamentally different molecules and you can see that because they have different bonding. For example, over here we have a carbon that is bonded to three other carbons and a hydrogen. Over here I can't find any carbon that's bonded to three other carbons. I can find ones that are bonded to two other carbons, but not one that's bonded to three other carbons. So, how we've put the atoms together, is actually different. They're bonded to different things. And so when we have the situation where you have the same constituent atoms, where you have the same chemical formula, but you're still dealing with different molecules because either how their bonds are made or what their shape is, we call those isomers. So an isomer, isomer, you have the same chemical formula, same chemical formula. But you could have different bonding but different, different bonding, bonding or shape, bonding, shape or orientation. Orientation. So over here you have just different bonding and this type of isomer is called a structural isomer. So these characters are structural isomers, same constituent atoms, but different bonding. Structural isomers. So that's structural isomers right over there. Now when you look at this pair or this pair, you'll say those don't look like structural isomers. Not only do they have the same constituents, both of these for example have four carbons, four carbons and they both have one, two, three, four, five, six, one, two, three, four, five, six, seven, eight, and they both have eight hydrogens. So these are both C4H8, it's looks like they're bonded similarly. For example, I mean the left hand side here, these look identical and one the right hand side, you have a carbon bonded to another carbon that's bonded to three hydrogens, carbon bonded to another carbon that's bonded to three hydrogens. Carbon bonded to a hydrogen, carbon bonded to a hydrogen, so it looks like the structure of the bonding, everything's bonded to the same things, but you might notice a difference. Over here, on the right hand side, this CH3 is on the bottom right, while over here it's on the top right and you might say okay well we know, what's the big deal there, these, you know, all these molecules, they're all moving around, maybe they're rotating with respect to each other and these things could, this thing could have rotated down to become what we have up here. If this was a single bond. A single bond would allow for that type of rotation, it would allow for these things to rotate around each other. For the molecule to rotate around that bond, but a double bond does not allow that rotation. So this fixes these two things, this fixes these two things in place. And because of that, these are actually two different molecules. Over here on the top, you have the CH3 groups, they're both, they're both, I guess you could say, facing down or their both on the same side of the double bond, while over here they're on different sides of the double bond and so this type of isomerism, where you have the same constituents and you even have the same bonding, this is called stereoisomerism. So over here we're caring much more about how things sit in three dimensions. We don't just care about what's bonded to what or the constituents and actually this one is, as we'll see, is also a stereoisomer because this carbon is bonded to the same things in either case. So these are both, these are both situations, there are both stereoisomers, stereoisomers, and this particular variation of stereoisomer is called a cis trans isomer. Cis is when you have the two groups on the same side, cis, and trans is when you have the two groups on the opposite sides of the double bond. Cis trans isomers. Cis trans isomers. Isomers, and these are often called geometric isomers. Geometric, geometric isomers. So that's a subset, so when I'm talking about cis trans or geometric, I'm talking about these two characters over here. They are a subset of the stereoisomers. Now what's going on over here? I have no double bond, I'm not talking about cis and trans. The carbon, as I've just said, is bonded to fluorine, chlorine, bromine, and a hydrogen, fluorine, chlorine, bromine, and a hydrogen. How are these two things different? And the way that they're different is if you were to actually try to superimpose them on each other. You will see that it is impossible. There are mirror images of each other and because there's four different constituents here, you can actually not superimpose this molecule onto this molecule over here and actually because of that, they actually have different chemical properties, and so this over here, these two characters, which is a subset of stereoisomers. Stereoisomers are concerned with how things are positioned in three dimensions, not just how their bonding is different, but this subset where you have these mirror images that cannot be superimposed, we call these enantiomers. So these two characters, these are enantiomers. Enantiomers, and enantio comes from Greek, the Greek word or the Greek root opposite. So these are opposites of each other, they cannot be superimposed, they're mirror, they're mirror images. So all of these are different variations of isomers and once again, you might say, okay theses are clearly two different molecules that have different bonding, but even cis trans isomer will have different chemical properties. These two in particular, they aren't that different but they do have different chemical properties, but sometimes they're so different that one might be able to exist in a biological system while the other is not. One might be okay for your health, and the other might not be okay for your health. Same thing for enantiomers. One might be biologically active in a certain way and the other one might not be biologically active in that same way.