Radical Stability - Online Tutor, Practice Problems & Exam Prep

Radicals are high-energy, short-lived species that are electron deficient, making them unstable. Stability increases with more alkyl (R) groups due to hyperconjugation. Tertiary radicals are stable, but allylic and benzylic radicals are even more stable due to resonance, which delocalizes electron deficiency. Understanding these stability trends is crucial for predicting reaction outcomes, particularly in reactions involving radicals. Recognizing resonance structures is essential for determining the most stable radical in a given scenario.

The radical stability trend.

Radicals are very high energy and very short-lived, so anything that we can do to stabilize them will have a significant impact on their likelihood to be formed. So, what that means is that we have to figure out the trend of stability for radicals. And I just want to show you guys right now. Basically, this is the trend, and what you're going to notice is that I'm going to compare this trend to the trend for carbocations. Now, if you don't know the trend for carbocations yet, that's okay. I'm just going to point out the major differences here.

First of all, radicals are electron deficient. What I mean by that is that there's an orbital. Right? And usually, each orbital has space for how many electrons? Two, okay. That's the Pauli Exclusion Principle. But in this case, we have a radical with just one electron. So that would be what we call a partially filled orbital. That's not very stable. Okay? So any way that we can push electrons into that orbital that will make it more stable. There is an effect that does that, and that effect is called hyperconjugation. What hyperconjugation says is that the more R groups you have around an empty or partially filled orbital, the more stable it will be.

So in this case, what I want to do is I want to say, okay, the more R groups are on my radical, the more stable it's going to be. Easy. And notice that that trend does hold true. This actually holds true for both carbocations, which are completely empty orbitals, and radicals. They're really the same thing, so notice that for my increasing stability, I have here that I have a tertiary here. A tertiary carbocation is very stable, and I also have a tertiary radical at the top here. Okay? But notice there's a slight difference here. It turns out that tertiary is the best type of carbocation that I can form. But it's not the best type of radical. I actually have a different type of radical here that's more stable, and that's because it turns out that unlike carbocations, allylic and benzylic radicals are actually going to be the most stable. Now allylic and benzylic are just words to mean that you're next to a double bond or you're next to a benzene ring. So what that's saying is that if you can resonate, that's going to make the radical more stable than anything. And here I have drawn the allylic and the benzylic. This would be allylic. This would be benzylic. Notice that both of these are directly next to a double bond, so a double bond and the radical could switch places through a resonance structure. And what that would do is that would delocalize that electron deficiency over several atoms, stabilizing it.

So what I want you guys to be mindful of is that this is actually going to be important for reactions. These sites here are very, very crucial for reactions that we're going to learn later because they're very stable. Okay? So what I want you guys to do here is determine which of the following radicals would be the most stable by looking at this trend. Just basically looking, forget the carbocation one because we're not talking about those. We're just talking about radicals. Figure out which of these would be the most stable and why. Don't forget to look at resonance structures to make sure that you're looking at both of the ways that the radical could be represented because remember that in a resonance structure, they're constantly in hybrid of each other. Okay? So you can't determine stability just based on one of the resonance structures.

Determine which of the following radicals is the most stable.

Alright. So according to my trend, the first thing that you really want to identify is which of these can resonate or which of them is allylic because if it's not allylic, it's not going to be very stable. So what we notice is that the allylic position is just any position next to a double bond, so this would be allylic. This would be allylic. This would be allylic. All those 3 would be allylic. How about d? Is that allylic? No. Because there's no double bond directly next to it. A double bond would have to be there or here in order for that to be allylic. So I automatically know that is not going to be my most stable because it can't resonate. Okay? Now what I'm looking at is these radicals that can resonate and I'm saying well, which of those would be the most stable? What I notice is that this one is primary. This one is secondary and this one is secondary. They look pretty the same. In fact, maybe there's one that kind of seems like a loser and that would be because A can resonate but it's only primary. The other 2, it's like how would I tell the difference between those? They're both secondary. Well, remember that I said in order to really determine stability for radicals, you have to resonate them. So what we want to do is we want to draw the resonance structures for all three of these and see what they look like afterwards.

So I'm just going to show you what one of the resonance structures would look like. You would use the fishhook arrows to move one electron at a time. So I would say this radical moves into this bond to try to make a double bond, but it can't make a double bond by itself. It needs one more electron. Well, where can it pull an extra electron from? The pi bond. So it's going to take an electron from that pi bond and now we're going to make a new double bond there. The problem is that that pi bond was made out of 2 electrons. Now I just removed one of them, so now I just have this one random electron hanging out. That electron is going to jump onto the carbon next to it and that's going to become a radical. So that means that the resonance structure for this guy would look like this. Now if I go ahead and count up what type of radical that is, not only is it allylic, but it is secondary. Oh, see how that changed? Before it was primary and now it's secondary.

Now what we have to do is draw the same thing for the others and what we would find out is that the resonance structure for B is now going to have a radical there. The resonance structure for C is going to wind up having a radical here. Now if we analyze how substituted each radical is, this one is now tertiary and this one is secondary. So do we have a winner? The winner has to be B because not only is it allylic, but it can also resonate to a tertiary location. And remember that I said the more groups that you have around a radical, the more stable it is. So notice that my trend says that primary is less stable than secondary, which is less stable than tertiary. So if I have the ability to make it allylic and tertiary, that's the best thing. And that's exactly what this one is down here, so this one's going to win. And the corresponding structure will win. So it's B. Alright?

Here’s what students ask on this topic:

What factors influence the stability of a radical in organic chemistry?

Radical stability in organic chemistry is influenced by several factors. The primary factors include hyperconjugation and resonance. Hyperconjugation occurs when alkyl (R) groups donate electron density to the electron-deficient radical, stabilizing it. Therefore, more R groups increase stability, making tertiary radicals more stable than secondary or primary radicals. Resonance is another crucial factor; radicals adjacent to double bonds (allylic) or benzene rings (benzylic) can delocalize the unpaired electron over multiple atoms, significantly enhancing stability. Understanding these factors is essential for predicting reaction outcomes involving radicals.

Why are allylic and benzylic radicals more stable than tertiary radicals?

Allylic and benzylic radicals are more stable than tertiary radicals due to resonance stabilization. In allylic and benzylic radicals, the unpaired electron can delocalize over a conjugated system, such as a double bond or a benzene ring. This delocalization spreads the electron deficiency over multiple atoms, reducing the overall energy of the radical. In contrast, tertiary radicals are stabilized primarily through hyperconjugation, where adjacent alkyl groups donate electron density. While hyperconjugation is effective, resonance provides a more significant stabilization effect, making allylic and benzylic radicals the most stable.

How does hyperconjugation stabilize radicals?

Hyperconjugation stabilizes radicals by allowing adjacent alkyl (R) groups to donate electron density to the electron-deficient radical center. This occurs through the overlap of σ-bonds (C-H or C-C) with the partially filled p-orbital of the radical. The electron donation from these σ-bonds helps to reduce the electron deficiency, thereby lowering the energy of the radical and increasing its stability. The more alkyl groups present, the greater the hyperconjugation effect, making tertiary radicals more stable than secondary or primary radicals.

What is the trend of radical stability in organic chemistry?

The trend of radical stability in organic chemistry is as follows: allylic and benzylic radicals are the most stable due to resonance stabilization. Tertiary radicals are next in stability, followed by secondary and primary radicals. Methyl radicals are the least stable. This trend is primarily influenced by hyperconjugation and resonance effects. Hyperconjugation stabilizes radicals by electron donation from adjacent alkyl groups, while resonance allows the delocalization of the unpaired electron over a conjugated system, significantly enhancing stability.

Why is it important to understand radical stability in organic reactions?

Understanding radical stability is crucial in organic reactions because it helps predict the likelihood of radical formation and the outcome of radical-based reactions. Stable radicals are more likely to form and persist long enough to participate in subsequent reaction steps. This knowledge is essential for designing efficient synthetic pathways, particularly in reactions like radical polymerizations, halogenations, and certain types of rearrangements. Recognizing the factors that stabilize radicals, such as hyperconjugation and resonance, allows chemists to manipulate reaction conditions to favor the formation of desired products.