Isomers - Online Tutor, Practice Problems & Exam Prep

Isomers are compounds with the same molecular formula but differ in their connectivity or spatial orientation. Structural isomers, also known as constitutional isomers, share the same molecular formula but have different connectivities. For instance, butene (C₄H₈) can have its four carbon atoms arranged in a straight chain or with a branching structure, altering how the atoms are connected. Stereoisomers, on the other hand, maintain both the molecular formula and the connectivity of atoms but differ in the arrangement of atoms in space. Geometric isomers, a subtype of stereoisomers, differ in their spatial arrangement around a double bond, classified as cis or trans based on the relative position of substituent groups. Optical isomers are non-superimposable mirror images of each other, like a left and right hand, which cannot be perfectly overlaid. Understanding isomers is crucial in chemistry as they have the same molecular formula but can have vastly different properties due to their structural or spatial differences.

Isomers

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. 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 C4H8. Now only 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 four of them forming a chain. What I could do here is have three 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 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.

So just remember when it comes to isomers, we have two big categories of structural AKA constitutional isomers and stereo isomers.

Stereoisomers

Stereoisomers themselves are further divided into two types. We have our geometric stereoisomers and our optical stereoisomers. With geometric stereoisomers, these are molecules with different spatial arrangements around a double bond. Optical isomers, on the other hand, are molecules that are non-superimposable mirror images of each other.

Now when we say nonsuperimposable, 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 stereoisomers. 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 slide 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 two optical stereoisomers of each other.

And remember, the fastest way to look at it is to look at two 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 stereoisomers of each other. That's the easiest way to understand this type of stereoisomer. We've talked about these two additional types of stereoisomers. Click on the next video and let's take a look at an overall chart.

Types of Isomers

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 images, we have C₄H₁₀, four carbons, 10 hydrogens. The one on the left has them all oriented in a chain, but the one on the right has three of them in a chain and one branching group. So they're connected differently, but they still have the same molecular formula of H₁₀C₄H₁₀.

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 two 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 we have two 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 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 this same molecule. But this is looking into a mirror, so OH will be over here. And then this H would be back here and this CH₃ 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 wedge bond which hated to a dashed wedged bond. Doing it this way by inverting the bonds with that show actual spatial orientation and holding everything else in the same position, this actually becomes its optical isomer. This is its mirror image.

OK, so again, you can look into a mirror and actually draw this, which is a bit trickier. Or we 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

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 Chapter 3 will be looking this way, which it is. We'd have our BR here, and looking at the mirror, the OH would be here in the back. So this is the mirror image of this original 1 here on the left O. 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 CLS are on the same side, and on this one both CLS 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 CLS 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 Chapter 3 and this. Oh, and then what else do we have? We have this chain here which has four carbons as well, and coming off of it is this Chapter 2 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. OK, formula, but different connectivity.

So this is how we classify each one of these three options.

Examine the two molecules and determine if they are identical or isomers.

• A. Identical
• B. Isomers
• C. Neither identical nor isomers
• D. Both identical and isomers

Draw a constitutional isomer of butane.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What are constitutional isomers in organic chemistry?

Constitutional isomers, also known as structural isomers, are compounds that have the same molecular formula but different physical and chemical properties because their atoms are connected in different orders. This means that while they contain the same number of each type of atom, the atoms are arranged differently in space, leading to different structures. These differences can affect the shape of the molecule, the functional groups present, and the overall reactivity of the compound.

For example, consider the molecular formula $C_4{H}_{10}$. This formula can represent two different structures: butane, which has a straight-chain of four carbon atoms, and isobutane, which has a branched-chain with a central carbon atom connected to three other carbons. Despite having the same number of carbons and hydrogens, butane and isobutane have different boiling points and reactivities due to their distinct structures. Identifying constitutional isomers is an important skill in organic chemistry as it helps predict and explain the properties and reactivities of various organic compounds.

What are isomers in organic chemistry?

Isomers in organic chemistry are molecules that have the same molecular formula but differ in the arrangement or connectivity of their atoms, resulting in distinct properties. There are two main types of isomers: structural isomers and stereoisomers.

Structural isomers, also known as constitutional isomers, have the same molecular formula but their atoms are connected in different ways. This can lead to variations in the complexity of their structures, such as chain isomerism where the carbon skeleton differs in branching, positional isomerism where functional groups are attached to different carbons, and functional isomerism where the functional groups themselves differ.

Stereoisomers have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This category includes enantiomers, which are mirror images of each other and cannot be superimposed, and diastereomers, which are not mirror images and have different physical and chemical properties.

Understanding isomers is crucial in organic chemistry because they can have vastly different chemical behaviors and biological activities, despite having the same numbers and types of atoms.

How do you draw isomers in organic chemistry?

Drawing isomers in organic chemistry involves illustrating the different structures that molecules with the same molecular formula can have. Here's a step-by-step guide:

1. Identify the Molecular Formula: Start with the molecular formula of the compound you're dealing with. For example, C₄H₁₀ represents butane.
2. Draw the Carbon Skeleton: Sketch all possible carbon chain arrangements. For C₄H₁₀, you can have a straight chain of four carbons or a branched chain with three carbons in the main chain and a methyl group attached.
3. Add Hydrogens: Fill in the hydrogen atoms so that each carbon has four bonds (following the tetravalency of carbon). In the straight chain, each internal carbon will have two hydrogens, while terminal carbons will have three. In the branched isomer, the secondary carbon will have one hydrogen, and the primary carbons will have three.
4. Consider Functional Groups and Double Bonds (if applicable): If the compound includes double bonds or functional groups, you'll need to draw all the variations where these features can be placed.
5. Check for Stereoisomers: If there are chiral centers, draw the enantiomers (mirror images that are not superimposable) and diastereomers (isomers that are not mirror images).
6. Label

What are structural isomers?

Structural isomers, also known as constitutional isomers, are molecules that have the same molecular formula but differ in the way their atoms are connected or bonded together. This means that while they contain the same number of each type of atom, the arrangement of these atoms in space is different, leading to distinct structures. These differences can result in variations in physical and chemical properties despite the isomers having the same molecular formula.

For example, consider the molecular formula C₄H₁₀. It can represent two different structural isomers: butane and isobutane (or methylpropane). In butane, the carbon atoms are arranged in a straight chain, whereas in isobutane, there is a branched chain with a central carbon atom connected to three other carbon atoms. These structural variations can affect boiling points, melting points, and reactivity, which is why structural isomerism is an important concept in chemistry, particularly in the study of organic compounds.

What are stereoisomers?

Stereoisomers are molecules that have the same molecular formula and sequence of bonded atoms (constitution), but they differ in the three-dimensional orientations of their atoms in space. This distinct spatial arrangement arises because of the presence of chiral centers, double bonds, or rings in the molecules, which can lead to two main types of stereoisomerism: enantiomerism and diastereomerism.

Enantiomers are stereoisomers that are non-superimposable mirror images of each other, much like your left and right hands. They have identical physical properties except for the direction in which they rotate plane-polarized light and their reactions in a chiral environment.

Diastereomers, on the other hand, are not mirror images of each other and have different physical and chemical properties. They can have multiple chiral centers and their spatial arrangement leads to more than just two possible isomers.

Understanding the concept of stereoisomerism is crucial in fields such as organic chemistry, pharmaceuticals, and biochemistry, as the different arrangements can lead to vastly different properties and biological activities.