Skip to Main Content

Isomers Part I


Hi guys, today we're going to take a look at the various kinds of isomers you might encounter on the MCAT. In part one, we're gonna talk about all the types of isomers that aren't optical isomers, and this is a 5B topic. So a good place to start is what is an isomer anyway? An isomer is a molecule that has the same molecular formula but a different arrangement of atoms compared to some other molecule.

Any two molecules with the same numbers of atoms of each element will be isomer if they don't have the exact same structure. There are many ways in which molecules can have a different structure. And this chart lists the relationship between several of these terms. We're gonna go through each of these in turn. Some kinds of isomers will have different chemical and physical properties while others will be more similar.

We'll go over which ones do what. So we'll start out with what I called constitutional or structural isomers. This maybe the easiest type if isomer to understand. In a constitution isomer, the molecules have the same number of each element, but the elements do not have all the same bond to each other. I have a couple examples here.

This one on the top left is butanol. It has the formula C4H10O. Now all of these other molecules here also have the formula C4 H10O, but the atoms are bound to each other in different ways. So we can see here that is isobutanol, diethyl ether, and t-butanol are all constitutional isomers of regular butanol and of each other.

There are a couple more constitutional isomers that I could've drawn, but I think you get the idea. One good thing to note that because these structure is so different for each of these isomers, they create and react to, different intermolecular forces. And therefore, the chemical and physical properties can vary significantly between them.

They will often react differently and boil or melt at different temperatures. The ether forms for example, will tend to be significantly less polar than the alcohol forms. The next type of ice summer we're going to go over is called a stereoisomer and there are many subtypes of this isomer. Two molecules will be stereoisomers of each other if they have the same elements bound to each other, but arranged differently in space.

I have an example drawn here. We can see that for all three molecules drawn here, each element stays bound to the same elements. The methyl groups all on the left side stay the same and this right hand carbon is always bound to an H an Or an amine. What's different is where the The amine, and the H are located in space.

We can see that in these two isomers on the right, the functional groups have traded places with each other, although they are all still bounced that central carbon. These two isomers here are actually two different types of stereoisomer and we'll talk about that next. So the first subtype of stereoisomers we'll talk about is what's called a conformational isomer, or a conformer.

Conformational isomers differ only in the conformation, or their 3D rotation and folding. These three molecules here aren't really three different molecules at all, they're just different conformations of the same molecule obtained by rotating this carbon here around that central bond. Two different rotations of the same molecule are often called rotamers by the way.

You might also remember the chair and the boat conformation of cyclohexane. These are also conformational isomers. They differ only in how the CC bonds have been rotated which causes the overall molecule to fold into a different shape. Since conformational isomers are the same molecule, they will usually have identical chemical and physical properties.

They'll react the same way with the same things and boil and melt at the same temperature. This is because most formers can move between alternate confirmations easily. So the reactivity and properties are the average of all the potential states. Some confirmations are more or less stable than others. However, for example the chair conformation of cyclohexane has less intramolecular electron repulsion than the boat conformation.

So when given a choice, cyclohexane prefers to be in the chair conformation, as it is the most stable. In order to convert it to the boat confirmation, you need to add in extra energy. The chemical and physical properties of cyclohexane will also be more like that of this most stable conformation, the chair then it'll be a of the boat confirmation.

So if conformers can't enter convert easily, they may have different properties. The other wrinkle regarding confirmation isomers is that some enzymes for example will only work if the substrate is in a particular conformation that fits the enzymatic site. Since most conformers can convert back and forth between shapes pretty easily, this usually doesn't matter much for the reactivity but it could potentially matter if you were dealing with a problem where a molecule is stuck in one confirmation.

Conformers will tend to have the same reactivity and physical properties, but only if they can freely interconvert with one another. So the second type of stereoisomer we're going to talk about are what are called configurational isomers. On like a conformational isomer, in a configurational isomer the atoms have the same element-element bonds but they have a different 3D configuration relative to each other.

This last bit is important. We don't have different bonds in a configurational isomer, but the bonds are arranged in a different location or a different order. Here's two examples, on the right we have ethene-1,2-diol to dial. These guys have two carbons with a double bond and those two carbons are bound to an H and an But one of the H and an Groups are on different sides in each molecule.

These are the same bonds in terms of the identities of the elements but they're in different locations relative to the carbon and the other constituents. The molecules on the left here are similarly arranged differently in space. If we went clockwise around these three substituents, starting at the amine, we will run into an H and then an Before getting back to the amine. If we do that for the bottom molecule, we run into the Then the H, and then we get back to the amine.

The central carbons and their substitutions differ in terms of what's called chirality which means basically handedness. There'll be more on this in the next lesson. Now, so one subtype of stereoisomer is the geometric isomers. In geometric isomer, each element is bound to the same element, but their spatial configuration is different in regards to a rigid structure, like a double bond or ring.

So we can see here that both carbons are bound to an H and an But in one, the OHs are both up, and in the other they are across from each other. We call the isomer where the functional groups are across from each other the transform and we call the form where they are on the same side the CIS form. The names for these compounds will then be, trans-ethene-1,2-diol and cis-ethene-1,2-diol.

Trans is Latin for across. So that's one way we can remember it, these guys are across from each other on this double bond. Now, geometric isomers have a rigid configurational difference and they may have different intermolecular forces and so their chemical and physical properties can be different.

They can have different boiling points or densities and some reactions may work with only one isolate or the other. For example, a reduction that required unhindered access to the double bond. Now, what happens if we have something more complicated than ethene-1,2-diol? We've got all functional groups on these guys now and no H's. What do we consider the main group for the purposes of cis and trans?

Is it the longer carbon chain or is it the? To deal with cases like this, we use a different but related system called the E/Z system. So the first step to naming E/Z isomers is to assign a priority to the atoms on either side of the double bond carbons using the Cahn-Ingold-Prelog rules. There's more information on this in the lesson on stereochemistry, but here is a copy of the rules again, for reference.

We start out looking for the bound atom with the highest atomic number, and give it the highest priority, 1. If there's a tie, we look at the atom's substituents, and give the original atom a higher priority if it has higher atomic weight substituents than the other. So oxygen is heavier than carbon, so we give a 1 to the oxygen an An 2 to this left hand side equal carbon.

The other double bond carbon is bond to two carbons. So we look down the chain to see which has the heavier substituents. The ethyl group's first carbon is bound to another carbon, while the methyl group's is not, so we give them priorities of one and two respectively. Now on the left here, we see that the And the ethyl carbon are both on the up side of the molecule.

When the two highest priority groups are on the same side of the double bond, we call this the Z isomer from the German word zusammen, which means together. This word also sounds a lot like the same. So that's how I remember this. Now, when the high priority functional groups are on opposite sides, like the And ethnic group on the other molecule, we call it the E isomer, from the German word entgegen which means against or opposed to.

So when writing the names of these compounds they would be called (Z)-4-ethylhex-3-en-3-ol and (E)-4-ethylhex-3-en-3-ol. One other configuration to note, if the substituents are both on the same carbon, it is not really cis, or trans, or E, or Z, but it is called the geminal configuration. This comes from the Latin word gemini, or twin, just like how this carbon has twin Groups.

This is not a stereo isomer of ethene-1, 2-iol since O H is actually bound to a different carbon, rather, this is considered a structural isomer. So in summary we can go back to our original isomer chart. Constitutional or structural isolates have the same elements, but different element to element bonds, while still have the same element element bonds but in a different arrangement.

For conformational isomers, they simply have their bond rotated in 3D space, while in configurational isomers they actually have different relative positions for the bonds while still being attached to the same elements. Geometric isomers have different positions relative to a rigid element, while optical isomers, as we'll discuss in the next lesson, have different chirality or handedness.

So in the second part of this lesson, we are going to talk about this last remaining group, the optical isomers and several of their own subtypes.

Read full transcript