Now isomers are molecules with same molecular formula, but different connectivity or spatial orientation. Here the 2 types of isomers we can delve into are Structural or Constitutional isomers, these have the same molecular formula but different connectivity. And then Stereo Isomers, These have the same molecular formula and connectivity, but different spatial orientation. So what exactly do we mean by this? Well, if we take a look at this middle compound, we have as its formula, c 4h8. Now, when we mean structural or constitutional isomers, that means we have the same 4 carbons, but they're connected differently to one another. In this example, we have the 4 of them forming a chain. But what I could do here is have 3 of them forming a chain, and then that 4th one branching off of that middle one. We also need to include the double bond, so I decide to make a double bond between these two carbons. This will represent one possible structural isomer of this middle compound. They have the same molecular formula of C4H8, but their connections or connectivities are definitely different. Now, stereo isomer, what do we mean by that? Well, they have the same molecular formula and connectivity, so we'd still have our 4 carbons in a chain, but they have different spatial orientation. These two carbons in the middle would still be double bonded, but now I have one of these carbons pointing up, and one of them pointing down. This would represent a different spatial orientation because in our original compound both of these carbons are pointing up. By making them opposites of each other now, this represents its stereo isomer. Right? So just remember when it comes to isomers, we have 2 big categories of Structural AKA constitutional isomers, and then stereo isomers.
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Stereoisomers
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Stereoisomers themselves are further divided into 2 types. We have our geometric stereo isomers and our optical stereo isomers. With geometric stereo isomers, these are molecules with different spatial arrangements around a double bond. Optical isomers on the other hand, these are molecules that are non super superimposable mirror images of each other. Now when we say non superimposable, these mirror images cannot be placed one over the other. So just imagine you have a dog, and this dog is looking into a mirror. This mirror image of itself would mean that these two dogs are optical stereo isomers. If we were to take this dog and take it out of the mirror, and try to slide it over this dog here, we would see that they don't perfectly line up. Because if you slid it all the way over to the left, we have this spot here. So when you when you slide it over, it would appear over here, but that doesn't match up with this dog where the spot is on this side. Now this and this would more or less match up with this and this, but again, it's this portion here when you slide it over the other dog that would not match up. That makes these 2 optical stereo isomers of each other. And remember, the fastest way to look at it is to look at 2 molecules, imagine there's a mirror between them. The molecule on the left, when it looks into the mirror, does it see the molecule on the right? If it does, that means they're optical stereo isomers of each other. That's the easiest way to understand this type of stereo isomer. Now that we've talked about these 2 additional types of stereo isomers, click on the next video and let's take a look at an overall chart.
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Types of Isomers
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So here we're taking an overall look at the different types of isomers that exist. So remember, structural isomers have the same molecular formula, but different connectivity. In both of these image we have C 4 H 10, 4 carbons, 10 hydrogens. The one on the left has them all oriented in a chain, But the one on the right has 3 of them in a chain, and one branching group. So they're connected differently, but they still have the same molecular formula of H10, C4H10. On the other side we're looking at stereo isomers, which we just learned can be further divided into geometric and optical isomers. Here in the geometric one, we're able to tell that they're geometric isomers by the inclusion of a double bond. Now if we're looking at the double bond we see we have these 2 CH3 groups on the same side with each other. And then here we have them on opposite sides of each other. Later on we'll learn that when they're on the same side, these two groups, they are cis. And then when we have 2 groups opposite each other, they're called trans. Optical isomers, the way we are able to tell we have optical isomers is just imagine you're looking into a mirror. Looking into a mirror we'd see the inversion of ourselves. But another way we can show that we have an optical isomer, is we look at the bonds that show spatial orientation. Instead of looking into a mirror, if we looked into a mirror what we would see would be the same molecule. This is looking into a mirror, so o h would be over here, and then this h would be back here, and this c h three would be here. That would be our mirror image. Sometimes it's hard to depict that because you're moving things and orienting them a different way. An easier approach would just be to look at the bonds that have spatial orientation, and invert them. So here, this is a dashed, wedged bond, we change it to a solid wedged bond. This is a solid wedged bond, we change it to a dashed wedged bond. Doing it this way by inverting the bonds that show actual spatial orientation, and holding everything else in the same position, this actually becomes its optical isomer. This is its mirror image. K? So again, you can look into a mirror and actually draw this, which is a bit trickier, or you can just invert the bonds that are solid or dashed. Right? So these are the different types of isomers that exist. Remember isomers here have the same molecular formula, but they may have the same connectivity or spatial orientation, or different connectivity and spatial orientation. Depending on what happens, you can fit under structural isomers or stereo isomers.
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example
Isomers Example
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Based on the pair of molecules, identify a structural, optical, geometric isomers, or identical. So if we take a look here at the first one, we have bonds that have spatial orientation. Usually this is a key to give away that we're dealing with optical isomers. Now if we imagine that this is looking into a mirror, it would see its reflection back. The h would be looking this way, which it is. The CH three would be looking this way, which it is. We'd have our BR here. And looking in the mirror, the Oh would be here in the back. So this is the mirror image of this original one here on the left. So they are optical isomers. For the next one we have the presence of a double bond, which usually indicates that we have a geometric isomer but we have to check. In this one both CL's are on the same side, and on this one both CL's are still on the same side. Now, they're not geometric isomers. For them to be geometric isomers, we'd have to have one where the CL's are on different sides. Both are saying the same thing, so they represent identical molecules. For the next one, what do we have here? So we have this structure here and we have this structure here. If we look, what do we see? We see that we have what? We have this chain here, and branching off of it is this Oh, I mean, this c h three and this o h. And then what else do we have? We have this chain here which has 4 carbons as well, and coming off of it is this CH2, and then here, Oh. It's better to draw it this way to show the actual connection to the O. So if you look, they both would have the same number of Carbons, Hydrogens and Oxygen, but they look like they're connected differently. So here we'd say that these are structural isomers. Same molecular formula but different connectivity. So this is how we classify each one of these three options.
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Problem
Problem
Examine the two molecules and determine if they are identical or isomers.