Below is the written transcript of my YouTube tutorial video – Radical Resonance.
If you prefer to watch it, see Video HERE, or catch the entire Resonance Structures in Organic Chemistry series.
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Leah here from leah4sci.com and in this video we'll look at resonance with radical structures. You can find this entire video series along with the practice quiz and study guide by visiting my website
leah4sci.com/Resonance.
In the previous videos in this series we looked at the concept of electrons and bonds moving back and forth so that you have a hybrid intermediate where you have partial bonds and partial charges. To show these resonance structures we used double headed arrows to show where the electrons are moving. Where the double headed arrow has a tail that starts at where the electrons are and a head that winds up where the electrons were going. But more importantly the head is a double headed arrow to show the movement of two electrons and my trick for that is to imagine each of this hooks as holding an electron. When it comes to radicals we're dealing with single unpaired electrons and so with radical resonance we're showing the movement of just one electron which means we need a single headed arrow sometimes called a fish hook because it looks like something that you use fishing.
My trick for this is to think of that single headed arrow as one electron moving and this is what we look at with radical resonance. Radical resonance tends to come up with stability and that means when you have a radical near a pi bond, that radical can be shifted or shared between multiple atoms for stability. The two types of radical resonance that you're going to see are the allylic radical resonance and that's where you have a radical near one pi bond or the benzylic radical resonance where you have a radical near a benzene ring. I'm showing the radical as a big electron just to make it stand out, but the radical electron is just like any other electron in terms of size.
So let's start with the allylic radical. We'll start with a very simple molecule, the red carbons on the chain, a pi bond on one end and a radical on the other. To show the resonance here, the goal is still to move the pi bond from one side of the molecule to the other. But this time it's not the entire pi bond that's moving. This radical will be one of two electrons that form the new pi bond and that means to make the pi bond we only need one of the two electrons in the existing double bond. We'll show that one electron contributing with a single headed arrow to meet the red radical and that will form a pi bond. But this also means that the blue electron, the other electron in the pi bond is now let by itself. So it'll collapse onto the carbon and sit there as a new lone radical. Draw your double headed arrow to show that it's resonance and start by re-drawing the skeleton meaning everything that hasn't changed.
Now let's see what has changed. We have a new pi bond formed between the red electron and the purple electron which used to be in the pi bond. And the blue electron sits by itself as a radical on the other end of the molecule. And that's it! It's very simple if you think about it but the single headed arrow tends to confuse students so make sure you understand, one electron moves at a time and a pi bond will break in opposite directions where one electron meets the radical and one electron breaks away as a radical.
Now let's take a look at a resonance for a Benzylic radical. Just like the allylic radical we'll take that lone electron and draw a single headed arrow in the direction of where we want the new pi bond to form. And in this case I'll take the closest pi bond the only one that can resonate with it. Bring one electron to form a pi bond and break away the other one onto the carbon atom closest to it as a lone electron or as a new radical. Now let's see what happen, we have two pi bonds that haven't moved, the red electron is now sitting as a pi bond with one of the purple electrons, and the other purple electron is sitting by itself as radical. If it's by itself, near another pi bond, it can resonate further. So this purple electron will resonate towards the next pi bond with a single headed arrow. And we'll take the next pi bond showed in blue electrons. The closer electron will come and meet the purple to form a new pi bond. The farther electron will break away so it can set by itself as a new radical.
This brings me to my next structure, the red pi bond at the top hasn't changed. The purple electron now sits in the pi bond with the blue electron and the other blue electron is a radical by itself. But that electron is still near yet another pi bond which means it can continue to resonate. With the single headed arrow we show it towards the pi bond and this pi bond which we'll show in green will now take the closer electron and with the single headed arrow meet that blue one to form a new pi bond and the second green electron collapse by itself to give us a new radical. Double headed arrow to represent a resonance structure, now let's see what hasn't changed and what has. The red pi bond hasn't moved, the purple pi bond hasn't moved, the blue electron is now sitting on a pi bond with the green electron and the other green electron is sitting as a radical by itself.
At this point you can think of it as the green electron sitting near yet another pi bond and so you can show more resonance where the green electron goes to meet that red electron and the other will collapse by itself. But you'll notice that this is nearly the structure we started with. A benzene ring has alternating pi bonds that'll constantly resonate and so when you do the last resonance you technically get back to where you started for a total of 4 resonance structures for the benzylic radical.
This concludes the resonance video series, you can catch this entire series plus the practice quiz and study guide by visiting my website, leah4sci.com/Resonance.
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