Main content
Current time:0:00Total duration:7:45

Common mistakes when drawing resonance structures

Video transcript

- [Voiceover] I see a lot of mistakes when students draw resonance structures, and so I wanted to make a video on some of the more common mistakes that I've seen. So let's say we wanted to draw a resonance structure for this carbocation. Some students would take these electrons and move them down to here and say, all right, so on the right, now, I would have this, and this is my resonance structure. Let me highlight those electrons in blue here, so these electrons here move down to here. But this is incorrect, so let me write "no" here. So the resonance structure on the right, this is an incorrect resonance structure, why is this resonance structure not possible? Well, let's draw in the hydrogens on the carbons, and it will be much more obvious. So this carbon right here has one hydrogen on it, same with this carbon, and this carbon right here has two hydrogens on it, and the carbon with a plus one formal charge must have one hydrogen. So let's put in those hydrogens for the resonance structure on the right, and it should be obvious why this resonance structure is incorrect. Let's focus in on this carbon right here, the one I marked in red. How many bonds are there to that carbon? Well here's one bond, two, three, four, and five, that's five bonds to a carbon, that does not happen, you can't show carbon with five bonds, because that would be 10 electrons around this carbon, and carbon can never exceed an octet of electrons. Because of carbon's position on the periodic table, in the second period, there's four orbitals, and each orbital can hold a maximum of two electrons, which gives us four times two, which is eight. So carbon can never exceed an octet. There's another reason why this is wrong. If we go to this top carbon here, there's only three bonds around that carbon, so that carbon would have a plus one formal charge. So we added another formal charge, and we have carbon with five bonds, so this is incorrect, this is not a correct resonance structure. So what is the proper resonance structure to draw? Well, let's show that down here. You take your electrons, and you move them in the direction of the positive charge, of the positive one formal charge, and so let's show that. The electrons in, let me make them blue again, the electrons in blue move over to here, like that. And that moves the positive formal charge over to this carbon. If we draw in our hydrogens, it'll be clear why this is correct. So we put in a hydrogen here, we put in a hydrogen here, and we put in a hydrogen here. So let me draw in those three hydrogens on the right. Okay, now it's very obvious, let me point this out in red. It's obvious that this carbon here in red has a plus one formal charge, it has three bonds around it. So one, two, and three. And this carbon, this carbon over here on the right that had the plus one formal charge, now its formal charge is zero, because there are four bonds around it. So one, two, three and four, so now the formal charge is zero. So this is the correct resonance structure. Now, it looks a little bit confusing when I have those hydrogens drawn in there, which is why we leave them off. Let me go ahead and draw it again on the right just for clarity. That's why we leave off those hydrogens when we\re drawing our resonance structures, because they get in the way, and once you understand what's going on it's not necessary to draw in those hydrogens. Let's do another example, and again, I'll start with the wrong way to do it, and then we'll talk about the correct way. So, a student might say, all right, I have a negative one formal charge on this nitrogen, so I could take this lone pair of electrons and move into here, which would push these electrons over to here, so let me go ahead and draw what some students might think is a correct resonance structure. So let me put in my lone pair of electrons, and let's follow some electrons along. So electrons in light blue on this nitrogen move into here, and electrons in, let's say dark blue, move down to here. And then finally, electrons in magenta remain behind on the nitrogen. So on the right, why is this not a correct resonance structure? So again, this is the wrong way to do it. Well, think about your hydrogens. So we'll start with this carbon right here, this carbon has one hydrogen, this carbon has one hydrogen, and this carbon down here has two. So if we put in those hydrogens over here on the right, hopefully it's obvious why this is incorrect. Let's look at this carbon down at the bottom of the ring, so this carbon right here I just marked in red. How many bonds do we have? Well, here's one, two, three, four, and five, so there are five bonds to that carbon, and we know carbon can never have five bonds. Carbon can never exceed an octet of electrons. So immediately we know that this is not a correct resonance structure. All right, let's talk about the right way to do it. So you take these electrons, and you move them into here, and then these (mumbles) electrons have to go somewhere, and they move out onto this carbon, so now, let's draw the correct other resonance structure here. So we'll put in our double bond, we'll put our electrons on this carbon, that gives this carbon a negative one formal charge, and then we have some electrons, a lone pair of electrons left on the nitrogen. I'll use the same colors as before, so these electrons right here in light blue move in to form our double bond. The electrons in dark blue move off onto this carbon, so the electrons in dark blue are on this carbon that I just marked in dark blue which gives that carbon a negative one formal charge. And the electrons in magenta remain behind on the nitrogen. All right, the reason why the carbon -- now I'll go ahead and mark it in a different color. This carbon I just marked in magenta has a negative one formal charge, is because, remember, there's one hydrogen on that carbon, so let me draw in that hydrogen over here. Let me see if I can squeeze it in over here like that. This carbon right here has three bonds to it and a lone pair of electrons, which gives that carbon in magenta a negative one formal charge. Now this nitrogen over here would have a formal charge equal to zero. And so this on the right, would be the correct resonance structure, and again, drawing in hydrogens is a waste of time, it gets in the way. Let me go ahead and draw the resonance structure again, I'll take out that hydrogen so it'll look cleaner, and it also takes less time when you're not drawing in all of your hydrogens. So you just put a lone pair of electrons and write a negative one formal charge, and you have to know that there's still a hydrogen on this carbon that I marked in magenta. So if you're having trouble drawing resonance structures, usually the problem is not thinking about your hydrogens, forgetting about putting in your hydrogens. And once you put those in, it's a lot easier to see if your dot structure, if your resonance structure, I should say, is correct, so be careful about that. And resonance structures are just practice. The more you draw, the better you're going to get and if you make a mistake, it's not a big deal. You learn from your mistake, and you keep on practicing.