Hey guys, in this video, I'm going to show you how to draw molecular orbital diagrams for 3-atom conjugated systems. Let's get started. So guys, because a 3-atom conjugated system is an odd number of atoms, what we're usually going to end up dealing with is an allylic position. Remember that the allyl position or an allylic position means it's the position next to a double bond, and that's usually the case when you have an odd number of atoms because many times there's a double bond and then there's something next to the double bond. That's like the 3rd atom. Okay? Now what we know about allylic positions from previous lessons is that allylic positions are able to resonate. Remember that the allylic position can resonate from one side of the double bond to the other. Okay? So it turns out that the whole idea of resonating an allylic position can actually be explained through molecular orbitals. You already know how to explain it using resonance. You should be able to draw resonance arrows and be able to show how the allylic position can move from one side to the next. But it actually turns out that the fundamental explanation of an allylic position reaction can actually be explained through molecular orbital theory. So that's what we're going to do right here. I'm going to show you guys how molecular orbitals explain the reactive positions of an allylic ion. So here we go. It says simplified LCAO model of a propanyl ion and here what I've drawn is a general propanyl ion. Basically we've got our 3-atom conjugated system. So we have 3 atomic orbitals, like let's just say a, b, and c, right. And what we see is that we have a double bond that we know is going to contribute 1 pi electron for each atom, so we have 12. And then we have this unknown ion or this unknown non-bonding orbital in position C and that I've labeled with a question mark. And the reason is because what I'm about to explain to you applies to any ion, no matter what the identity of that question mark is. Whether it's empty, whether it's an empty orbital or whether it's a radical or whether it's a lone pair. It doesn't matter. What I'm able to teach you applies in all those situations and what the molecular orbital theory would say about this molecule is that how would you structure the molecular orbitals? Well, it says that you would have, first of all, a molecular orbital with 0 nodes, 1 node and 2 nodes, and that we would put those pi electrons in order of Aufbau principle. So we would put the 2 electrons from that first double bond into psi 1, right, which is the most stable the bonding orbital. And then what we would do is we would put whatever is left over, whatever is here, whether it's 0 electrons, 1 electron, or 2 electrons, they would go into psi 2. Does that make sense so far? No matter what the question mark is, we know it's going into psi 2. Now what is unique about psi 2? Look at psi 2. Notice that psi 2 actually has a node at atom B. If this is atom A, atom B and atom C like we had before, we had a, b, and c, right? Notice that there's a node at atom B. What does that mean? What that means is that regardless of the identity of that ion, it cannot react at position B. It can only react at either position A or at position C, but because no electrons ever pass through atom B, you will never find the allylic position reacting there. So what this does is it explains the theory behind the resonance structures that we've learned how to draw for so long. Remember that an allylic position ion can resonate to the other side of the bond, but it can never resonate to the middle. You've never seen that, for example, this, let's say this is a positive charge, you can't move it to the middle. You can only switch places with the double bond, but you can't move it to the middle. And the reason is that the molecular orbital theory states that there's no way that that ion can react at the middle carbon because that orbital doesn't even have any electrons. It can only react at the orbitals with electrons, which would either be orbital C in this case or orbital A in this case, but never B. Isn't that so cool? So what we're going to do next is an example.