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Tips on Recognizing Structural Isomers - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Tips on recognizing the different Isomers. There are 3 main different types of Isomers. What is an Isomer? An Isomer is something that has the same chemical compound or chemical formula but different ways of arranging the atoms.

There are three main types of Isomers. There is Structural Isomer, which are the same atoms but different way of bonding them. They've completely different chemical properties from one another.

There are Geometric Isomers, which are the same atoms but different arrangements of those atoms around the double bond. So here the key phrase is double bond. Geometric is double bond. They have some different properties, some similar properties. If they have the same branching on the same side, it's called Cis, this helps me remember. Cis means same. Trans is different, we'll go over that in a second or recognise those in a second.

Lastly you have Optical Isomers and this is where Chirality comes in and we're not going to go into it now. But different arrangements of 4 different groups around the central Carbon atom, they have all the same properties for the most part.

Here below we have 4 different types of Isomers. We're going to practice identifying different types, because a lot of times students have some issues identifying the types of Isomers. All of these are definitely Isomers because they have the same chemical formula, which is C3H3F2Cl. So here's our basic one, this one.

We're going compare it to this one over here, to 2. We have the same things on this Carbon, they're the exact same, great. Except for the only difference, are these guys. These guys are switched, interesting, around the double bond. Now double bonds cannot move. As you know, you've probably done this in class. Double bonds are inflexible, meaning they cannot twist and turn like single bonds can. They are stable, they are stationery in place. I have to actually change, where the F anD H are located in order for them to be isomers of each other or to be similar to each other.

So if you notice, there the Hydrogen is on the bottom and top. This is what we call a trans-Isomer. Here we have the Hydrogens both in the bottom. This is meaning the same, these are called the Cis. Now it can be completely flipped over. If I flip these around, I make both the Hydrogens on top, totally fine, the same thing. It's a Cis Isomer. If I completely switch this and made the left Hydrogen on top and the right Hydrogen on the bottom, that's okay too. It's still trans. So these are Geometric Isomers. They are just different around that central Carbon atom.

Let's look at this one down here, number 3. If you notice the difference are up here, the way this Fluorine and Hydrogen are swapped. This Fluorine and Hydrogen are completely one has picked up and moved, and so no matter how you look at it, you can't make it look the same as the one on top. They're both trans, they both have their H's in separate places but they have different arrangements around the central Carbon atom. Okay, about 4 different groups around the central Carbon atom.

We have central Carbon atom, different arrangement for different groups this is also different groups. HCL, this long chain and Carbon. So these guys are Optical Isomers of each other. This Carbon atom is just arranged differently.

If you look at number 4, this one, this Hydrogen and this Fluorine just swapped places. They're attached now to completely different atoms. The Fluorine was attached to this atom; that Carbon and now it's attached to that Carbon. It completely moved different way of bonding everything. So these guys are Structural Isomers.

So this is an example of all 3 and hopefully you can clearly identify the difference between all 3. Because that a lot of times, students get them mixed up; the Geometric, Structural and Optical. Structural has completely different, Geometrical think double bond, Optical, think 4 different groups are around the central Carbon atom, that are different. Don't forget Cis and trans and this is where Chirality comes in.

Hopefully that helped you understand the difference between all 3 and hopefully you can really recognize, how all 3 are similar yet different.