 Okay, let's go over the resonance rules now. So there are essentially five rules when you're dealing with resonance, and I have them all written up here on the board right now. So I'd like you to look over them, write them in your notes, and I'll lecture and then afterwards if you have any questions, feel free to ask them. So the first rule when you're dealing with resonance is that you got to remember that individual resonance forms are imaginary, not real. So what I have written here are the two individual resonance forms, or we call those contributors of the molecule benzene. And what you'll see here is that they have their pi bonds, the ring with the three pi bonds in it. If we look at this structure, we can see this structure over here is a little bit different where the pi bonds or the electrons in those pi bonds have moved to in between the other carbons. So the carbons that they weren't in between before. In actuality, neither one of these structures is the correct structure of benzene. What we find is that it's a combination of those two contributors, those resonance contributors, and we call that a hybrid. And if we were to draw that, we would draw something like this where instead of having up here alternating double and single bonds, we actually have a ring of electrons that indicates here that we have a bond order of one and a half in between each of these carbon atoms instead of one, two, one, two, one, two, we have one and a half, one and a half, one and a half, one and a half, one and a half, and so on. So this is actually what benzene looks like, not either one of these structures. The other thing we want to remember about these resonance structures is that these aren't transient forms of the molecule benzene. Benzene doesn't look like either one of these two things, these two contributors, that always looks like this resonance hybrid. The reason that we don't draw this normally is, and you'll see in a second because we like to move electrons around in organic chemistry, the reason we don't draw this one exclusively is because it's very difficult for us to use that structure to show the motion of electrons, which is very important like I was saying in organic chemistry. Okay, so let's move on to the second rule. The second rule states that resonance structures differ from only the placement of lone pair and pi electrons. So if you're in advanced organic chemistry right now and you're watching this video, you might think, well what about non-bonding resonance forms? We're dealing with undergraduate organic chemistry, so we won't think about that right now. But hopefully you can see here, right, we have the two resonance forms of the acetate anion. The acetate anion is actually a conjugate base of the acetic acid. And you'll see that form one has the three lone pair electrons on oxygen, the top oxygen, and form two has only two lone pairs. But we only have a single bond here and we have a double bond here and the first one and a double bond and a single bond here. So what I'm going to show you is how we actually show the motion of these electrons to show the two different resonance contributors and how those electrons have been placed in different portions of the molecules to actually indicate which contributor you are referring to. So I'm going to show the motion with the red arrows here. Okay, so hopefully you can see that. These electrons go down here and make that pi bond there. The pi bond electrons go from here and give us that third lone pair. So what we find is the structure is different only the placement of the lone pair and the pi electrons. And in fact, remember, neither one of these resonance contributors of the acetate anion is actually the true molecule or particle, what we would find is the acetate anion looks more like something like this with a negative one-half and a negative one-half charge on each of those oxygens. So hopefully you look at that kind of crooked right cross-eyed and be like, I've never seen that before. Okay, that's because we don't usually deal with this type of structure in organic chemistry or in chemistry at all. We really just deal with these full negative charges, not these partial ones. So hopefully you are beginning to understand how it's kind of difficult for us to use these resonance hybrids to show the motion of electrons. So let's move on to the third role for resonance. Individual resonance forms do not have to be equivalent. So when we see the conjugate base of acetone, or we call this the acetone enolate, this is acetone that's been deprotonated. So one of its hydrogens from this carbon has been removed. What we see here is that we have a carb anion on this carbon, okay, so a carbon with five electrons around it giving it a negative charge. What we can do is show the other resonance form of the acetone enolate by drawing our curly arrows. So hopefully you can see, right, there's a double bond there, so we have to take those electrons and move them in between those two carbons, and there was a double bond here and now there's another lone pair, so we move those electrons up to the oxygen. And what we find is that these structures are not equivalent, right, because this structure, this resonance form, has the lone pair and the negative charge on carbon, and this one has the lone pair and negative charge on oxygen. And what you find is that in those cases, unlike the two cases that we've shown before, benzene and the acetate ion, cases where the resonance forms are not equivalent, what you'll find is that the hybrid looks more like one of the forms than the other one. In this case, the resonance hybrid looks more like the second form here. Why? Because we think usually of a charge preferring to be on the heteroatom than a carbon, okay, and in this case, right, we have the charge on the oxygen relative to the carbon in the other case, therefore the second of those two resonance forms we would think would contribute more to the actual hybrid form, the structure of the hybrid form. Okay, so hopefully that makes sense. Let's move on to the fourth rule of resonance, and this should be hopefully obvious to you that resonance structures have to obey equivalency rules. Okay, so we can't just take these electrons and move them to there and not do anything with these electrons, because if we do that, well, what do we find? We have two, four, six, eight, ten electrons around carbon, and of course, carbon cannot have ten electrons, it can only have eight electrons around it at the most, therefore this is organic chemist nightmare is to see a carbon atom that has more than eight electrons around it, therefore this is not a valid resonance form, okay? So in order to create a valid resonance form, you would have to do exactly what we did over here, which is to move that second set of electrons from the pi bond and move them over to the host, okay? So hopefully everybody understands this. Okay, and then the last rule, and we've been alluding to this the entire time, we've been talking about resonance forms, but the resonance hybrid has a lower potential energy than any individual resonance form. So remember in chemistry, what does that mean, or in anything, right? A lower potential energy means that it's more stable, so therefore the hybrid form of these resonance forms is more stable than any one of the resonance contributors itself. So in other words, right, when we were looking at these two resonance hybrids here of benzene and deacetate anion, right, what we find is these ones are of a lower potential energy or more stable. So I can see y'all are still writing, I'll give you a couple minutes for questions. Is there any questions right now? Okay, think about it, we could talk about this for a little bit longer.