Now we’re going to discuss the most important intermediate of all organic chemistry, the carbocation.
Determining Carbocation Stability
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Let's talk about intermediates. So remember that intermediates are high energy compounds that can be isolated, but our exist at a higher energy state than the products or the react. It's OK, That's the definition of an intermediate. And it turns out the most popular the most widely used type of intermediate in this in this course is gonna be the Carvel Catalan. So what that means is I wanna go more into depth on exactly what are the properties of carbon Catalans because those are the most popular intermediates. Alright, so remember that Carvell Catalans are just carbons that have three bonds instead of four. So they're always gonna have an empty orbital. Okay, that empty orbital, we give it a positive charge because it's missing a bond, alright, and carve a cat. I end, it turns out, are going to be stabilized by something. There is something that can make that positive charge a little bit better on that's something called hyper conjugation. Okay, What hyper conjugation says is that the electron density from neighboring Sigma bonds Okay, so adjacent sigma bonds can share electron density with the carbon cattle with that empty orbital. So if I have the more signal bonds I have close to it that are able to donate, the more stable. My positive is going to be okay because of the fact that my positive is missing a ton of electrons. If I can share a little bit of these electrons into that positive charge, it's gonna make it a lot more stable. Okay, So basically, hyper conjugation is defined by the de localization of charge of the interaction of a P orbital empty, pure, it'll within adjacent eclipsed Sigma Bond. Okay, it turns out that there's only one group that's really good at having Eclipse Sigma Bonds to pee orbital's. And that's our groups because our groups remember they always have hydrogen is coming off of them. So those hydrogen zehr gonna be able to eclipse with the P orbital. Okay, so what that means is that the more substituted my carpal Catalan or the more our groups I have attached to it, the more stable. Okay, so I want to show you guys a chart of carbon kata instability and show you guys the differences between what's a really bad karma cat and what's a good one? So a bad one would be all the way down here, like at primary. Okay, this would be called a primary Carvel Catalan. Because I only have one. Our group that's gonna have h is that can donate electrons because remember, in our group just means it has It's a carbon with some ages on it. Okay, that would be an example off a really, really bad um carro cada. And because it only has one side donating electrons Now, on the opposite end of the spectrum, a tertiary is awesome because the tertiary is gonna have hydrogen is here, here and here. And they're all gonna be able to donate electron density to that positive charge. Stabilizing it. So what that means is tertiary carbo cannons are always gonna be your most stable, no matter what. Okay, then. In between, we have other ones, like secondary. And then these guys here, what are those? These are actually going to be once that can resonate. This is called a Lilic. And this is Ben Zelic. Okay. All these words mean, Is that a little? It just means next to a double bond. So that's what this is. Okay, Ben's Ilic means is next to a benzene. Remember when we talk about resident structures way back that a positive charge next to a dull bond could resonate. And it would form another resident structure that looked like this. Okay, that is a form of de localization. Where now my positive charge isn't just stuck in one place. It could be distributed over several atoms, so that's gonna be more stable. So a Lilic, Carvel Catalans are more stable than secondary. Okay, because secondary, it's good, it's It's better than primary because it has the ability to donate with hyper conjugation. But it doesn't have the resonance capability, so it's not as good as a Lilic. Same with Ben's Ilic. Ben's Ilic is also better than secondary. Okay, but the Onley thing is that tertiary is actually always gonna be the best, even even if it can't resonate. That's fine, because the tertiary one has so much hyper conjugation that that was actually gonna be more stable overall than even the Ben Zoellick or the L Ilic ones that can resonate. OK, so might just wanna put here that these can resonate. Okay, but then this one just has The tertiary is just the most stable overall All right, So now I want to do this example. And I want you guys to think about this for a second and say which of the following alcohol Hey, leads would generate the most stable Carvel. Katyn. Now, I am kind of asked you guys to think about an extra step, so I'm just gonna tell you what that extra step is. Alright? Just you guys know maybe you've learned this already or not. Okay? Or maybe you remember it or you don't, But that's fine. Alcohol highlights. What, you're gonna talk about more in a little bit. Have the ability toe leave, Okay? And if alcohol it's have the ability to leave the the molecule completely, that means that they're gonna leave a Carvel cat and behind. Okay, So what you need to do is think after this f leaves and becomes an f negative, and I get a positive charge here. What kind of car market and would that be? Do the same thing with all four of thes. Okay. And then figure out which of them is going to be the most stable Carvel Catalan. All right, So go ahead and try to solve this question, and then I'll answer it
Carbocations are stabilized by a phenomenon called hyperconjugation.
Hyperconjugation is the delocalization of charge by the interaction of an empty p-orbital with an adjacent, eclipsed σ-bond. Basically it’s like resonance for single bonds (more complicated than that but don’t worry about it).
Since this is only possible with -R groups, the more substituted the carbocation, the more stable.
Which of the following leaving groups would generate the most stable carbocation?
Predicting the most stable carbocation
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All right. So this one would have had a carbo carrion in the primary position, because that f is going to go away. So just gonna be attached toe one carbon. Is that stable? Not really. This one would have a carbon copy in the secondary position, because that O t s is going to go away, by the way. That's a sulfa knee. Esther, That's not really an alcohol. Hey, lie. But it does the same thing. Okay? Just we'll talk about that more in a little bit. Okay? Um, then we have this iodine that would make a car broke out in there. That would be tertiary. We already know that's gonna be the winner, because I said nothing's better than tertiary. Okay? And then what about this last one? It would have a carbo cat in here, which would actually be primary. Okay, I know that's confusing, but think about it. This carbon right here is the one that has a positive charge. How many other carbons is it attached to? Just one. Okay. So that one's primary. Because even though it looks like it's attached to mawr, it's actually only attached toe one carbon, whereas this Carvel Cat in here is like legit. Attached to three other carbons is attached to a one, two and three. So that's why that one's tertiary, whereas the red one over here for number four is primary. Alright, so hopeful. That made sense. The most stable one would be three. All right, so let's go ahead and move on.