Ring Strain

Now I want to talk about an analytical technique that's used to measure the amount of energy in a molecule. Alright, and that technique is called heated combustion. All right, so the heat of combustion is like I said, basically a machine that blows up molecules to see how energetic they are. And basically the theory behind it is this. The higher the heat of combustion are basically the more heat that's released by the explosion, the higher the energy of the molecule. And if the energy is very high, that means we would expect it to not be very stable. Okay, vice versa. Same thing would be if you have a low heat of combustion, then you have low energy in the molecule, which means it must be a very stable molecule. Alright, so that's really all we need to know for right now. But I just want you guys to understand the relationship here because your professor could use any word he wants. He could say, pick the most stable molecule or pick the most energetic molecule, or pick the one with lowest heat of combustion. You need to understand what all three of those things mean and how they relate to each other. All right. So just think that heat of combustion releases energy. So those should be the same direction. And stability should be the opposite of whatever those are.

So now there's actually two different ways that Al Canes can become unstable. The first one, and probably the most difficult to explain is the shape. Okay, there's just this rule in organic chemistry that a straight chain chain is gonna be less stable than a branch chain. Alright, where does this come from? Actually, this is a topic of hot debate, and there's actually research going on right now to figure out why straight chains are less stable than branch chains. It's not something that's talked about in your textbook very much, and all I would say is like it has to do with very, very complicated things has to do with stuff that's well beyond the scope of this course. So what I would say is, instead of trying to understand it, just memorize it. That a straight chain. If I had a six carbon straight chain and if I had a six carbon branch chain, OK, the actual we're not talking about inter molecular forces, inter molecular forces. If you think of Vander Waals forces, that's something completely different. Okay, that has to do with what state of matter it's in. I'm talking about actually how energetic it is like how much energy it releases when I burn it. That is gonna be higher for the straight chain. And that's gonna be lower for the branch change. Why? Because the branch chain is gonna be more stable. The straight chain is gonna be less stable. All right, so it's just something you should know. The second thing is strain. There's actually a lot of different types of strain. I'm going to talk about two right now, but there's actually even more than that.

so. And a lot of these were found in cycle all canes. So the first one is angle strain. Okay. Angle strain exists when Tetra Hydro bonds are forced out of their ideal bond angle of 109.5 degrees. Okay, remember that Tetra he drools always wanna have 19.5. And the smaller your rings get, the more difficult it is for those carbon carbon bonds to maintain that kind of bonding. So let me just give you an example of a triangle. I'm not sure if you guys remember from geometry, but a triangle the like. If you have an equal, literal, equal lateral triangle, each of these bonds is gonna be for each of these corners is gonna be 60 degrees. Okay, is 60 degrees close to 109.5. Not even close. Remember that both all of these carbons have two hydrogen. So what that means is that they are tetra. He drills and they want to have 19.5 bond angles. But they don't because of that stupid triangle. So what that means is that 60 is far less than 19.5. So this is going to be highly strained. And if it's highly strained, guess what that means. That means it's gonna have high energy. And if it has high energy, then you can guess the rest. That means as low stability. All right, so let's keep going. The square is a little bit better, actually, a lot better. It's at 90 degrees, but that's still pretty far off from 109.5. All right, so in this case, this one's a little bit better. But still, it still has angle strain. Now a five member ring is getting a lot closer. What we want, Ah five member bring will have bond angles of 108 and 18 is almost the same as one of 9.5 like there's almost no difference that. So, in terms of angle strain of five member during cyclo, Penton has very little angle strain. Are you guys quota? It's almost It's almost perfect. Okay, then let's look at finally a six member ring. A six member ring would actually have bond angles of 120 degrees if it was drawn. If it was the way, if it actually existed in this plane or form. Okay, well, 120 degrees is way more than 109.5. So I would expect that a cyclo hexane would actually be one Ah, less stable than cyclo painting. But actually, it turns out to not be true. Cyclo hexane actually turns out to be the most stable ring out of all the rings. Okay, out. If you could make a many carbons, you could make tons of carbons. But cyclo hexane is the most stable ring. Why is that that if the bond angles so off? Oops. I didn't mean to do that. I meant to highlight it. If the bond angles so off, if it's 1 20 then why would it be the most stable? Okay, and we're gonna learn that in a little bit.

then we have torch. It'll strain torch. It'll strain is a different type of strain that exists when carbons that have hydrogen overlapping space basically that hydrogen is will be eclipsed in space. So let me give you another example of a cyclo butane here. This is just the square. But now we're looking at it in a three D version. Later on, I'm gonna teach us what the name of that is. But don't worry about it right now. And what this is is all these ages are facing the same exact direction. They're all looking at each other exactly in the same way. So that means that they're all eclipsed over each other. For example, if I had an eyeball looking here, I would see that all of these are eclipse. And then all the red ones are eclipse. That is called torch inal strain torch. It'll strain is that strain that comes from having eclipsed bonds. Okay, now you can see that cyclo plantain is a little bit better, but it still has some eclipsed bonds here and here it has some eclipse bonds, usually cycle. Penton will kind of move out of the plane like it will bend a little bit so that it won't have so maney eclipse bonds. Okay, But it turns out that if you were if your professor were to ask you what is the main source of instability for cycle A painting, by the way, this is just a a three D version of that. Okay, is the main cause of the strain. I'm sorry. The main cause of instability is it Ring strain, which is right over there. Or is it torch inal strain, which is those hydrogen is there and the answer is that it's actually torch inal strain torch inal strain is the reason that cyclo Penton isn't very stable or isn't as stable because the fact that we still no matter how much it folds, it's always gonna have some overlapping H is here and some overlapping h is here, and that means that they're gonna be within eclipse. They're gonna have an eclipse confirmation and they're going to be kind of running into each other and bumping into each other. And that's not very good

Now, what I want to do is I want to show you 23 d versions of cyclo hexane on. I want you guys to tell me which one do you think is gonna have the lowest heat of combustion? Alright, I haven't even taught you about cyclo hexane yet, but I just want us to predict which would have the lowest heat of combustion. So go ahead and just pause the video. And then when you're done thinking also think, What does lowest heat of combustion mean? So once you're done thinking on positive video, All right. So I'm pretty sure that you guys got that lowest heat of combustion meant lowest energy and lowest energy met most stable. Okay, so we're looking for the one that has the least amount of strain. Okay. And what we found is that basically this one over here would actually have quite a bit of torch. It'll strain. Why? Because check it out. I have these hydrogen. Is that air poking at each other? They're basically, like running into each other's space. Okay, so that would be one source of strain. Another source of strain would be that you have these hydrogen is down here better like pretty much in each other's way. Okay, They're also eclipse. Okay, so this would not be the best confirmation for cycle. Heck, sane. Now, over here on this one, this one is way better because we actually don't have any direct Horschel strain. Now you might be wondering. Well, Johnny, I see that we have these. H is here. They're facing the same way, but they have a carbon in between. That means that actually pretty far apart from each other. Okay, there's actually very little torch it'll strain on this kind of confirmation. Okay? And I wanted to tell you guys that it turns out that this is what cyclo hexane is actually gonna look like in real life in real life. Instead of having 120 degree bond angles and having everything be eclipsed, all the hydrogen is be eclipsed instead. What it does is it forms a puckered confirmation and turns into what we call a chair. Alright? And later on, when I talk about cyclo hexane, I'm gonna be talking all about chairs. Why? Because chairs have bond angles of 109. awesome, and they have almost no torch inal strain, so they're like the ideal. They're like the ideal cycle cycle, all canes, because they have pretty much the perfect bond angle and because they're all twisted in that chair. Confirmation. There is no torch. It'll strange, very little torch it'll stream.