Carbocation Stability - Online Tutor, Practice Problems & Exam Prep

Intermediates, particularly carbocations, are high-energy compounds with three bonds and a positive charge due to an empty orbital. Their stability is enhanced by hyperconjugation, where electron density from adjacent sigma bonds stabilizes the positive charge. Carbocation stability increases with substitution: tertiary carbocations are the most stable, followed by allylic and benzylic, which can resonate. Primary carbocations are the least stable. Understanding these concepts is crucial for predicting reaction mechanisms and outcomes in organic synthesis.

Determining Carbocation Stability

Let's talk about intermediates. So remember that intermediates are high energy compounds that can be isolated but exist at a higher energy state than the products or the reactants. Okay? That's the definition of an intermediate. And it turns out the most popular, the most widely used type of intermediate in this course is going to be the carbocation. So what that means is I want to go more into depth on exactly what are the properties of carbocations because those are the most popular intermediates. Alright? So remember that carbocations are just carbons that have 3 bonds instead of 4, so they're always going to have an empty orbital. Okay? That empty orbital, we give it a positive charge because it's missing a bond. Carbocations, it turns out, are going to be stabilized by something. There is something that can make that positive charge a little bit better and that's something called hyperconjugation.

What hyperconjugation says is that the electron density from neighboring sigma bonds, okay, so adjacent sigma bonds can share electron density with the carbocation, with that empty orbital. Okay? So if I have the more sigma bonds I have close to it that are able to donate, the more stable my positive is going to be. 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 going to make it a lot more stable. So basically, hyperconjugation is defined by the delocalization of charge of the interaction of a p orbital, empty p orbital with an adjacent eclipsed sigma bond. It turns out that there's only one group that's really good at having eclipsed sigma bonds to p orbitals and that's r groups because r groups, remember they always have hydrogens coming off of them, so those hydrogens are going to be able to eclipse with the p orbital. So what that means is that the more substituted my carbocation or the more R groups I have attached to it, the more stable.

So now I want to show you guys a chart of carbocation stability and show you guys the differences between what's a really bad carbocation and what's a good one. So a bad one would be all the way down here like at primary. This would be called a primary carbocation because I only have one r group that's going to have H's that can donate electrons because remember an r group just means it's a carbon with some H's on it. That would be an example of a really, really bad carbocation because it only has one side donating electrons. Now on the opposite end of the spectrum, a tertiary is awesome because a tertiary is going to have hydrogens here, here and here. And they're all going to be able to donate electron density to that positive charge stabilizing it. So what that means is tertiary carbocations are always going to 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 gonna be ones that can resonate. This is called allylic and this is benzylic. All these words mean is that allylic just means next to a double bond, so that's what this is. Benzylic means it's next to a benzene. Remember when we talked about resonance structures way back that a positive charge next to a double bond could resonate and it would form another resonance structure that looked like this. Okay? That is a form of delocalization where now my positive charge isn't just stuck in one place. It can be distributed over several atoms, so that's going to be more stable. So allylic carbocations are more stable than secondary. Okay? Because secondary, it's good, it's better than primary because it has the ability to donate with hyperconjugation, but it doesn't have the resonance capability, so it's not as good as allylic. Same with benzylic. Benzylic is also better than secondary. Okay? But the only thing is that tertiary is actually always going to be the best. Even if it can't resonate, that's fine because the tertiary one has so much hyperconjugation that that one's actually going to be more stable overall than even the benzylic or the allylic ones that can resonate. So I might just want to put here that these can resonate. But then this one just has the tertiary is just the most stable overall.

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 alkyl halides would generate the most stable carbocation? Now I am kind of asked you guys to think about an extra step, so I'm just going to tell you what that extra step is. Just so you guys know, maybe you've learned this already or not, or maybe you remember it or you don't, but that's fine. Alkyl halides, which we're going to talk about more in a little bit, have the ability to leave. And if alkyl halides have the ability to leave the molecule completely, that means that they're going to leave a carbocation behind. Okay? So what you need to do is think after this F leaves and becomes an F- and I get a positive charge here, what kind of carbocation would that be? Do the same thing with all 4 of these and then figure out which of them is going to be the most stable carbocation. Alright. So go ahead and try to solve this question and then I'll answer it.

Predicting the most stable carbocation

All right. So this one would have had a carbocation in the primary position because that F is going to go away, so it's just going to be attached to one carbon. Is that stable? Not really. This one would have a carbocation in the secondary position because that OTS is going to go away. By the way, that's a sulfonate ester; it's not really an alkyl halide, but it does the same thing. Okay? But we'll talk about that more in a little bit. Then we have this iodine that would make a carbocation there. That would be tertiary. We already know that's going to be the winner because I said nothing's better than tertiary. And then what about this last one? It would have a carbocation here, which would actually be primary. 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. So that one's primary because even though it looks like it's attached to more, it's only attached to one carbon, whereas this carbocation here is legitimately attached to three other carbons. It's attached to 1, 2, and 3. So that's why that one's tertiary, whereas the red one over here for number 4 is primary. Hopefully, that made sense. The most stable one would be number 3. All right? So let's go ahead and move on.

Here’s what students ask on this topic:

What is a carbocation and how is it stabilized?

A carbocation is an intermediate in organic chemistry characterized by a carbon atom with three bonds and an empty orbital, giving it a positive charge. Carbocations are stabilized through hyperconjugation, where electron density from adjacent sigma bonds (C-H or C-C bonds) is shared with the empty p orbital of the carbocation. This delocalization of electron density helps to stabilize the positive charge. Additionally, carbocations can be stabilized by resonance, where the positive charge is delocalized over multiple atoms, as seen in allylic and benzylic carbocations.

Why are tertiary carbocations more stable than primary carbocations?

Tertiary carbocations are more stable than primary carbocations due to hyperconjugation and inductive effects. In a tertiary carbocation, the positively charged carbon is bonded to three alkyl groups, which can donate electron density through their sigma bonds, stabilizing the positive charge. In contrast, a primary carbocation has only one alkyl group to provide this stabilization. The greater the number of alkyl groups, the more electron density can be shared, making tertiary carbocations the most stable.

What is hyperconjugation and how does it affect carbocation stability?

Hyperconjugation is the delocalization of electron density from adjacent sigma bonds (C-H or C-C bonds) into the empty p orbital of a carbocation. This interaction helps to stabilize the positive charge on the carbocation. The more adjacent sigma bonds available for hyperconjugation, the more stable the carbocation becomes. This is why tertiary carbocations, which have three alkyl groups providing sigma bonds, are more stable than secondary or primary carbocations.

How does resonance stabilization work in carbocations?

Resonance stabilization in carbocations occurs when the positive charge can be delocalized over multiple atoms through π bonds. For example, in allylic carbocations, the positive charge is adjacent to a double bond, allowing the charge to be shared between the carbon atoms involved in the π system. Similarly, in benzylic carbocations, the positive charge is adjacent to a benzene ring, allowing the charge to be delocalized over the aromatic system. This delocalization reduces the energy of the carbocation, making it more stable.

What is the order of stability for different types of carbocations?

The order of stability for different types of carbocations is as follows: tertiary > allylic ≈ benzylic > secondary > primary. Tertiary carbocations are the most stable due to extensive hyperconjugation. Allylic and benzylic carbocations are also highly stable due to resonance stabilization. Secondary carbocations have moderate stability with some hyperconjugation, while primary carbocations are the least stable due to minimal hyperconjugation and no resonance stabilization.