Isomers: Study with Video Lessons, Practice Problems & Examples

Isomers are molecules with the same molecular formula but different structures or spatial orientations. They are categorized into structural (constitutional) isomers, which differ in connectivity, and stereoisomers, which maintain connectivity but vary in spatial arrangement. Stereoisomers further divide into geometric isomers, characterized by different arrangements around a double bond, and optical isomers, which are non-superimposable mirror images. Understanding these distinctions is crucial for grasping concepts in organic chemistry, particularly in reactions involving alkenes and chirality.

Isomers Concept 1

Now isomers are molecules with the same molecular formula, but different connectivity or spatial orientation. Here the two types of isomers we can delve into are Structural or Constitutional isomers, which have the same molecular formula but different connectivity. And then Stereo Isomers, which 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, C₄H₈. Now, when we mention structural or constitutional isomers, it means we have the same 4 carbons, but they're connected differently to one another. In this example, we have the four of them forming a chain. But what I could do here is have three of them forming a chain, and then that fourth 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, a 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 two major categories: Structural AKA constitutional isomers, and then stereo isomers.

Stereoisomers Concept 2

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-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 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.

Types of Isomers Concept 3

So here we're taking an overall look at the different types of isomers that exist. Remember, structural isomers have the same molecular formula but different connectivity. In both of these images, we have C₄H₁₀, 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 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 OH would be over here, and then this H would be back here, and this C H₃ would be here. That would be our mirror image. Sometimes it's hard to depict that because you're moving things and orienting them in 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. Okay? 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.

Isomers Example 1

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 giving 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 CH₃ 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 CH₃ and this OH. 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, O. 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.

Draw a constitutional isomer of butane.

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Draw all structural isomers for C₃H₈O.

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Provide a cis isomer for the following compound.

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Here’s what students ask on this topic:

What are the different types of isomers in organic chemistry?

Isomers in organic chemistry are molecules with the same molecular formula but different structures or spatial orientations. They are categorized into two main types: structural (or constitutional) isomers and stereoisomers. Structural isomers have the same molecular formula but differ in the connectivity of their atoms. Stereoisomers, on the other hand, have the same molecular formula and connectivity but differ in the spatial arrangement of their atoms. Stereoisomers are further divided into geometric isomers, which differ in the arrangement around a double bond, and optical isomers, which are non-superimposable mirror images of each other.

How do structural isomers differ from stereoisomers?

Structural isomers, also known as constitutional isomers, have the same molecular formula but differ in the connectivity of their atoms. This means that the atoms are connected in different ways, resulting in different structures. Stereoisomers, however, have the same molecular formula and connectivity but differ in the spatial arrangement of their atoms. Stereoisomers are further divided into geometric isomers, which have different arrangements around a double bond, and optical isomers, which are non-superimposable mirror images of each other.

What are geometric isomers and how do they differ from optical isomers?

Geometric isomers are a type of stereoisomer where the spatial arrangement of atoms differs around a double bond. They are categorized as cis (same side) or trans (opposite sides) based on the positions of substituent groups. Optical isomers, another type of stereoisomer, are non-superimposable mirror images of each other. These isomers are also known as enantiomers. The key difference is that geometric isomers involve different spatial arrangements around a double bond, while optical isomers involve mirror images that cannot be superimposed on each other.

How can you identify optical isomers?

Optical isomers, or enantiomers, can be identified by their non-superimposable mirror images. To determine if two molecules are optical isomers, imagine placing a mirror between them. If one molecule looks like the mirror image of the other but cannot be superimposed on it, they are optical isomers. Another method is to look at the bonds that show spatial orientation. By inverting the bonds that are solid or dashed while keeping everything else in the same position, you can identify the optical isomer.

What is the significance of isomers in organic chemistry?

Isomers are significant in organic chemistry because they have the same molecular formula but different properties due to their different structures or spatial arrangements. This can lead to different chemical and physical properties, which are crucial in various chemical reactions and biological processes. For example, structural isomers can have different boiling points and reactivity, while stereoisomers can have different biological activities. Understanding isomers is essential for grasping concepts in organic chemistry, particularly in reactions involving alkenes and chirality.