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Isomers- Stereoisomers - 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.

Isomers are compounds of the same molecular formula but different molecular structures. There are four important types of isomers. Structural isomers atoms are bound in different orders while stereoisomers are bound in the same order but oriented differently. Geometric isomers involve different arrangement about a double bond. Optical isomers involve four groups in varying arrangements around a central atom.

Alright. So let's talk about isomers. Isomers are two or more compounds that have the same molecular formula but different molecular structures. Alright. So let's look at these guys. These guys are all C5H12. And I want you to understand the difference between all these. Okay. If I write this in the board, they're all and I didn't write the hydrogens because I didn't want to take up so much space, but they are there. But like if these guys are actually not isomers of each other, and I want to make sure you understand that. These are just written differently on the board. So this, if you compare this one to this one, this is actually just a flipped image of it. Same exact thing to the flipped image on this, on the board. This guy is just rotated and this guy is just written vertically. But these guys are all the same. These are not isomers of each other. Isomers are actually different molecules of the same molecular formula. So let's actually talk about the three main isomers that you're going to probably see.

The first one you're going to definitely see are structural isomers. And these are the same chemical formula that atoms are bonded in different orders and this one is probably one you're going to see in class most often. Sop we have C5H12, there are different ways you can write C5H12. We can write it as a straight chain, a straight carbon chain. That's pentene. We can write it as this carbon chain with a branched methyl group and that's two methyl butane. I mean there are several ways wee can write it. We can write it as a block kind of structure looking like this with two meth- dimethyl propane, and there are several more that we can actually draw. There are thousands upon thousands upon thousands of isomers you can actually draw structurally [IB] they're actually vastly structurally different. And so they have different chemical properties and different physical properties from each other but the same molecular formula.

Another type of isomer are stereoisomers. Stereoisomers is when all atoms are bonded in the same order but arranged differently in space. This is actually a bit more difficult to comprehend and understand. There are two main stereoisomers I want to make sure you know. That's geometric isomers and that's different arrangement of groups around a double bond. So if you're talking about single bonded carbons, we have this, they can actually rotate around a single bond. That carbon can actually turn itself freely and have not an issue at all. But if it's double bonded, this double bond locks it into place. So you can't actually turn and twist this carbon. So that hydrogen that's up there will always be up there. The hydrogen that's down there will always be down there because it doesn't have the freedom that single bonds do. Same with triple bonds. Triple bonds are the same way. They are locked into place.

So what happens if you have two different isomers, one with two different functional groups or whatever's attached to this carbons and bonding are up. We're going to call that a cis, they're the same. They're both going in the same direction. They're both upwards in the double bond from each carbon. So we're going to call this a cis two butene meaning that they're both, the carbons are both upwards. But there is a possibility that one is up and the other one is down. That's called a trans two butene. So if they're going in the same direction, we're going to call that cis and they're going in opposite direction and we're going to call that trans. And that actually makes a vast huge difference. Notice the melting point of this guy is -39 degrees celsius, sorry. -139 degrees celsius while this guy's melting point is -106 which is a huge difference. And so not only are their physical properties very different, their chemical properties are very different too. So these guys are what we call geometric isomers of each other. Okay.

And last type and the most difficult to understand are what we call optical isomers. This has a different arrangement of a [IB] of four different groups around the central carbon atom. So, let's say we have this structure, we have this carbon and it's bonded to these different things around it, and it's the same different, the same things around it however, they're arranged just differently around the carbon atom. These guys are the same thing. However they can't ever be the same exact molecule. Let's make it actually look a little bit more clear. Our hands are stereoisomers, oh sorry, optical isomers of each other, they're the same things. they've all thumbs, pinkies, middle, forefingers, but they are mere images of each other. There's no way that we can actually like put one on top of the other and have them be the same. They're mere images of each other. So these are what we call optical isomers. And optical isomers are actually they have the same exact chemical properties, the same exact physical properties, it's just they might behave differently, they might react differently with different compounds or molecules that are also optical isomers of each other. So they might react differently in a reaction however the arrangement around the central, around itself is different.

So [IB] let's talk about four [IB] that the isomers in this practice problem down here. So we have this compound here and we want to figure out these guys are all isomers, some type of isomer of this particular compound. So let's look at this guy compared to this guy. Okay. So the main difference between the structure of this and this is this flourine is down but this flourine up here is up. Okay. So this is actually different and because they can't freely switch around, this guy is what we call geometric isomer of this. What about this one? This one actually has, let's see a different arrangement of this carbon it's chlorine, hydrogen, flourine and this carbon has chlorine, flourine, hydrogen. This is an optical isomer of this particular one because around here they are completely different. They're arranged differently. Down here, if you look at the difference between this guy and this guy. Well, flourine's down here where flourine's up here. It's completely, attached to a completely different carbon. So we're going to call this a structural isomer. So this is an example of the three different isomers compared to this particular guy. [IB] all isomers f this one.

So the three main isomers, we have optical, geometric and structural and basically they're all the same idea. They're all the same molecular structure but different, form differently in space. So that's what an isomer is.