In Organic Chemistry 1, we learned about a position word called allylic or allyl. What allylic simply meant was it was the position that's next to a double bond. Now that position is actually a little bit more important than you might think. The reason is because now we just stated that conjugation depends on 3 atoms in a row that can resonate. Double bonds typically have 2 of these atoms, but they're always missing the third one. Notice this double bond. I'm going to erase this in a second. But notice this one I'm circling, right? It's missing it has 1, 2 atoms that can resonate but it's missing the third. Many times, these double bonds are looking for some kind of orbital, reactive orbital to be placed on the third atom so that they can participate in resonance. What that means is that typically, carbocations, carbanions, and radicals are usually unstable. Usually, we say these are reactive intermediates. They don't like to form. But when they're paired with double bonds on the allylic positions, they become unusually stable due to conjugation. Meaning that whereas most carbocations are not very stable, the one next to the double bond will be unusually stable. It will be better than normal. Now what I want to do is refresh ourselves kind of on the resonance structures of these reactive intermediates because we'll be drawing a lot of resonance in this section. Let's go ahead and start off with the simplest situation which is cations. Do you guys remember how cations move? I told you guys that cations always move with one arrow. I always talk about how if you have a cation in that allylic position, you can draw it like a door opening on a door hinge. So I just say you draw it like the door opens and now you replace the other side with a cation. Your cation and your double bond switch places and that's your resonance structure. Then if we wanted to show the complete structure, you would just show that this one also goes back. Okay? So, the cation resonance structure is the easiest one to draw.
How about basically a lone pair? Now that lone pair, if it's on a carbon, a lone pair is going to be a negative charge. Now it's not always going to be a negative charge. It just depends on what atom it is. Remember I was saying this has to do with formal charges. So in this case, since it's a carbon, it's going to be what we call carbanion. Do you guys remember how many arrows lone pairs move with? They always move with 2. We would actually start from the region of highest electron density just like any mechanism we've ever drawn. You would start off with these electrons moving towards the closest bond. Okay? Now if we make that bond, we have to break a bond because we're violating the octet of this carbon right here. It already had 4 bonds. We're about to make the 5th one. We have to break the bond. So what we're going to get is 2 arrows. Make a bond, break a bond. And we're going to wind up getting something that looks like this. So, negative charge here and now the double bond is on the other side. So that would be applied to lone pairs but also anything that's an anion because really they're the same exact concept.
Last one is radicals. So, what if we have just one electron next to its O bond? Now remember that radicals actually move with 3 arrows. They move with half-headed arrows, so it's a little weird. We would start off by making part of a double bond with 1, but now the double bond next to the radical breaks off into its own radicals. Then we'd get one radical joining us here and the final radical being dropped off at the end where it's going to become its own standalone radical. So now what we have is 2 electrons joining to make a new pi bond, and that leftover radical on the side, that would be for radicals. So as you can see, maybe this is like a nice pattern, but we've got one arrow, 2 arrows, and 3 arrows. These are just ways to think about kind of categorize these resonance structures. Resonant structures are something you're still going to have to do for the rest of organic chemistry. You have to stay on your toes about that. That's it for this topic. Let's move on.