Below is the written transcript of my YouTube tutorial video – What is Resonance.
Leah here from leah4sci.com and in this video we'll break down the concept of resonance as it'll show up on your Organic Chemistry course. You can find this entire video series along with my practice quiz and study guide by visiting my website leah4sci.com/Resonance.
In the Orgo series link below, we looked at how to draw the lewis structure for molecule and ions. So let's take a look at the NO3- Lewis Structure. Starting with the skeleton we have Nitrogen single bound to 3 Oxygen atoms. Looking at the check list we have atoms, let's look at the octet electrons and formal charge quickly, again you can see this step by step in the Lewis Structure video. What this comes to is the Nitrogen double bound to one Oxygen and single bound to the two other Oxygens. Nitrogen has a formal charge of plus 1, the two single bound to Oxygens each have a negative formal charge and that gives us a net of NO3 minus 1. But what if I chose to put the pi bond on the lower right Oxygen or the pi bond on the lower left Oxygen? Would that be incorrect? And the answer is NO.
If I have the pi bond on the lower left Oxygen, that Oxygen is neutral, Nitrogen still has a positive charge and the two other Oxygen atoms each has a negative charge. If I put the pi bond on the lower right Oxygen, same thing. That Oxygen is neutral,, Nitrogen still has a formal charge of plus 1 and the two other Nitrogen atoms each have three lone pairs and a formal charge of negative 1. So how is this possible? And on your exams, how do you know which structure to show if there are three possible structures that all appear to be correct? And the answer is Resonance.
So what exactly is Resonance? According to the dictionary, resonance is the sound or vibration produced by one object caused by the sound or vibration caused by another object. That to me is a very tedious and potentially confusing definition. I like to think of resonance as something vibrating back and forth. What specifically vibrating back and forth? The electrons that are moving from one atom to another on the same molecule. When we look back at these three molecules, it's not just about the fact that they're all correct but more importantly how to interconvert between one and the other to show that resonance, to show those electrons moving back and forth.
To do that we have to use curve arrows which you may also hear as pushing arrows. The idea behind an arrow in resonance and in mechanism is the movement of electrons. The arrow tail always goes where the electrons are on your molecule meaning where they're starting out. And the arrow head shows exactly where those electrons will wind up. It's not one electron that's moving, it's two! And I like to think of it as one electron carried on each point of the arrow. This is to differentiate between a radical arrow, the radical arrow looks like a fish hook. It only has a half arrow head coz it's only one electron. With these arrows, you want to ask yourself which electron are capable of moving and where can they move to?
Let's start with the structure all the way on the left. To get from the left structure to the middle structure, it looks like we want the pi bond to move up, but that's not the case. In fact if we do that, we're going to violate the octet on both Oxygen atoms. If we take these electrons and move them up, that means Oxygen which already has a full octet now has a total of ten electrons and the Oxygen here loses that pi bond for a total of six electrons. Instead you want to start your resonance at the most negative electrons so we'll take these two electrons that has a negative charge on Oxygen and bring them down towards the positive charge Nitrogen because opposites attract and that's how we form the pi bond which will now show here in blue. These are the two electrons. But Nitrogen which already has the full octet would now have a total of 10 electrons which is not allowed. So Nitrogen has to let go of two electrons and we'll show these two red electrons as a pi bond collapsing on to the Oxygen atom.
Notice that this arrow tail is on the lone pair and the head winds up in between two atoms. It winds up in between because we're forming a bond but here the arrow tail starts in between because we're showing a bond moving ends up on an atom because the electrons are landing on that atom. And then we'll show a double headed arrow to show the back and forth resonance and here's what we have. The blue electrons form the pi bond, the red electrons that moved, are shown here as the two new electrons on that atom. We still have the two green lone pairs. And the third lone pair is the former pi bond that collapsed.
Now how do we go from the middle structure to this structure on the right? Once again it looks like the pi bonds has to collapse downward from the upper Oxygen to the lower right oxygen, but in fact that's not the case. If the Oxygen on the right is going to lose its negative charge, that's where the electrons have to start moving. So we'll start the arrow at the two electrons that we're showing in pink and move in between the Nitrogen and Oxygen because we're about to form a pi bond. One more time Nitrogen has more than 8 electrons, it has a more than full octet so we have to kick out two electrons. We start the arrow at the blue bond to show that these electrons are moving. We end the arrow at the Oxygen because that's where the electrons will collapse. And then we'll show a double headed arrow.
After the double headed arrow, always show what changed. Meaning what do we have as a result? As a result of the pink electrons moving, we now have a pi bond that will show in pink where the two electrons are the former lone pair that used to be on Oxygen. The blue electrons that collapsed onto the Oxygen atom will now show as the third lone pair sitting on Oxygen and we already have the formal charges in place where Oxygen when it has three lone pairs has a negative charge, an Oxygen that only has two lone pairs and a pi bond is neutral. One more thing you have to show with resonance is a bracket around all the structures. And what this tells you is that everything in the brackets is constantly resonating back and forth, back and forth.
So it's not a question of which structure exist in nature. Do we have this one for twenty five percent of the time, this one for thirty five percent of the time, NO! In fact, none of these represent the real structure of the Nitrate Ion, they simply show you different extremes of what that intermediate looks like. Let's look back at the initial structure. If we have electrons on Oxygen moving down, moving on this direction, moving that direction, moving on that direction, the electrons are constantly moving back and forth, back and forth, so quickly that it's impossible to show it on a two-dimensional paper. That's why we have to ask, where are these electrons moving from and where are they moving to? Well, they move in this direction. Okay so we'll show one structure that has this negative, it moves in this direction. We'll show another structure that has that negative, it moves on that direction. We'll show another negative. But this is just a way for us to represent it on paper. In reality, the electrons are moving so fast between all three structures that what we have is this resonance hybrid intermediate.
An interesting point to think of this came from one of my students. And this is what he said. A dinosaur is an imaginary creature, some people will argue that. I've never seen a dinosaur so I have to imagine what it looks like. A unicorn is another imaginary creature. Again I've never seen one so I have to imagine what it looks like. But if a dinosaur and a unicorn were to mate, their offspring would be a hybrid, an in between which is a rhinoceros. Looks a little bit like a dinosaur, a little bit like a unicorn but it didn't get it's unicorn parent's looks and it's something in the middle. It's not that it resonates back and forth where half the time it looks like a dinosaur and half the time looks like a unicorn, but instead it has qualities of both parents and exist as something in the middle. If you want to try to explain its parentage then you can describe the dinosaur as one extreme and describe the unicorn as the other extreme. And that's the idea behind showing resonance structures where the electrons are on one atom at one extreme and the other atom at the other extreme. But the resonance hybrid, that rhinoceros is the actual structure and that is how the molecule exists.
Depending on the molecule, it can vary from two to nearly infinite numbers of resonance structures. Let's take a look at the Enolate ion as an example. If we have an enolate, Enolate comes from Alkene, Ol is an alcohol, eight is a deprotonated negative alcohol, with three lone pairs on Oxygen and a negative charge. These negative electrons can resonate towards the sp2 hybridized carbon to form a pi bond between carbon and oxygen. But carbon which already has a complete octet cannot accept that extra bond unless it kicks something out. So we'll show the electrons forming the pi bond getting kicked onto the carbon on the left. The reason we don't kick it on to carbon on the right is because it'll have too many electrons. But pushing it to carbon on the left now both carbons still have a complete octet.
Show the double headed resonance arrows in-between to show that it's going back and forth and then start with your skeleton. Since you sigma bonds don't break in resonance structures it's easiest to start with the skeleton and ask yourself what stayed the same and what changed. Oxygen has three lone pairs but now has two blue lone pairs that hadn't moved. The red electrons form a pi bond between carbon and Oxygen so we'll show it right here. The purple electrons that used to be a pi bond are now sitting on the carbon at the end as a lone pair. And of course don't forget to enclose your resonance structures in brackets. The next step is checking formal charge specifically to see that you have a conservation of charge. Remember, electrons can move, charges can go from one atom to another. But the net charge of the total molecule will always stay the same before and after your resonance.
e started out with a negative charge of Oxygen for a net of negative one. Our second resonance structure doesn't appear to have a charge. This is where you have to know and understand your formal charges. If you're not comfortable with it, see my video below. But here's the problem, typically we're looking bonds and atoms to see a formal charge. For carbon, all I see is line structure on a lone pair so here is the trick. If carbon starts out with a pi bond and ends up with a lone electron pair, it has one more electron that it had previously because directly attached to carbon we only had one electron of the pi bond but two electrons of the lone pair. That second electron makes it more negative than it was in the previous step. So if we start with carbon neutral, we wind up with carbon negative one. Oxygen should have 6 valence, it has 6 touching it, six minus six is zero that means the only charged atom is carbon for a net of negative one. Negative one equals negative one, charge is conserved and we are good to go!
To show the resonance hybrid or the “rhino” of this structure, we start with the skeleton, show any electrons that are not moving and then simply show dotted line where the electrons are moving back and forth. Since Oxygen carries a negative charge part of the time, we'll give it a partial negative. Carbon carries a negative charge for part of the time, we'll give that a partial negative and that's our resonance hybrid. Now what happens when you're given a molecule that looks like this and you're asked to show all of the contributing resonance structures. Let's start with the lone electron pair in the negative charge. Let's jot the lone pair in green and start here moving the electrons towards that carbon to carbon pi bond. These electrons will form a double bond but the pi bonds will have to break away otherwise carbon will have an overfull octet and that is not allowed.
To show these electrons moving away, we'll start the arrow at the bond and end the arrow at the carbon because that's where the electrons will collapse. So we'll show a double headed arrow then start with the skeleton copying everything that hasn't change. That makes it so much easier. We have oxygen that hasn't changed and the pi bond on the left. Now let's see what has changed. The green lone pair of electrons turned into a pi bond between that carbon and the one up towards the right. The blue pi bond collapsed as a lone pair on the carbon atom, I remember the trick. If carbon started out with a pi bond, it has a lone pair, it gets a negative charge. Quick conservation of charge, negative in the starting molecule, negative on the next structure we're good to go. Now where can we move it from here? The blue electrons can move counter-clockwise to form a pi bond closing the electrons between carbon and Oxygen to collapse onto the Oxygen atom.
We show the double headed arrows, re-draw the skeleton and anything that hasn't moved and then see what didn't. The blue electrons from a pi bond between carbon to carbon, the pink electron form an extra lone pair on the Oxygen atom. A quick formal charge on Oxygen tells us that it has a charge of negative 1 and that's our resonance structure. But that's not the only thing we could have done. Looking at my initial structure, I could've also taken those green electrons and resonated them towards the left causing these electrons to collapse, that will give us another set of resonance structures with the left side of the molecule resonating rather than the right.
So when you draw a resonance hybrid, you'll notice that the entire cyclohexane has resonance, meaning the electrons moving back and forth. And we also have some resonance going up to the Oxygen atom. So the resonance hybrid would have to show all of that with the partial negative charge on Oxygen and then a partial negative charge on the ring because the negative charge is resonating between all the carbons within that carbon ring. So if you have so many options, how do you know where to start? Do you start with the lone pairs, do you start at the pi bonds, what happens when you have a carbocation? That's exactly what we'll discuss on the next video.
Be sure to watch this entire resonance series. Download the cheat sheet so that you'll have all the rules in front of you at a quick glance and then once you're comfortable with the material, make sure you try to practice quiz and see how well you do. And you can find all of this on my website at leah4sci.com/Resonance.
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