Concept: Concept: Stability of Conjugated Intermediates5m
Now let's focus on the specific intermediates that conjugation helps to stabilize. Regardless of the type of intermediate you are, whether you're a carbocation or a carbanion, the ability to resonate is always going to make you more stable. But as we learned from prior lessons in organic chemistry one, resonance doesn't effect all intermediates equally. Some intermediates benefit more from resonance and some intermediates benefit less. We can see this with the trends of carbocations versus radicals.
What we see is let's start with the first one. Carbocations. We see that obviously, primary carbocations are terrible and as we add R groups, this would be primary, secondary, tertiary. As we add our R groups they become more stable. That should make sense with everything else you've learned about carbocations thus far.
But notice that we have these carbocations in the middle that are both primary. So I included these primary carbocations, but what's the difference? Now you know that these are conjugated. These are conjugated primary carbocations. As you can see, they have at least three atoms that can resonate in a row. So this would be called primary allylic and this would be called primary benzylic. Either way, both of these are going to be more stable than a typical primary because of their ability to resonate.
Now if we look at the same trend for radicals, we notice a similar pattern, which is that as you add R groups they become more stable. And as you add conjugation, they become more stable. However, there's a big difference, which is that for carbocations we tend to say that hyperconjugation or the addition of R groups is actually going to be the best thing for a carbocation. If you have a tertiary carbocation, that is the most stable.
Whereas for radicals, radicals are actually the most stabilized by resonance meaning that allylic and benzylic positions are truly ideal for radicals to exist because that's the most stable type of radical that you can get. Regardless of the type of intermediate, it's always going to make it more stable, but as we compare and contrast carbocations versus radicals, we see that radicals benefit even more from conjugation than a typical carbocation would.
That being said the stability of the allylic position is going to open up reactions to happen at that location that are radical and carbocation intermediated. We're going to see that as a general pattern that these intermediates love to form in that allylic position due to the conjugation that they can achieve.
So I'm just going to show you guys two really brief examples before we get into the specific types of reactions. But as you can see, imagine that you have a radical in the allylic position and a radical reagent, you could see a termination step occurring. A termination step that would then take these two radicals, form a new sigma bond and we would get a product that looks something like this.
Another possibility would be what if you had a carbocation form in that allylic position and you reacted it with something negatively charged or let's just say with an available lone pair, well, then you could get just basically a nucleophilic attack on that reaction site. So then we would form, actually pretty much the same exact compound, in this case, due to two different mechanisms.
So these are not reactions for you to memorize. I'm really just trying to introduce the idea that we're always going to be watching out, now that we understand it, we're always going to be watching out for this allylic position because if we can form a reactive intermediate at that position, it's highly likely to react with another reagent and I'm going to wind up getting reactions at the allylic position where the double bond remains, but now I have a new atom attached to that allylic position.
That's all for this topic. Let's move on to the next one.
Four compounds are shown. These compounds react more quickly, or they react with more favorable equilibrium constants, than similar compounds with less conjugated systems. In each case, explain the enhanced reactivity.
Draw the important resonance contributors for the following cations, anions, and radicals.