About this video
This video predicts the regiochemistry of a Diels-Alder reaction when the diene carries an electron-donating methoxy group. Resonance structures show which diene carbon is electron-rich and which dienophile carbon is electron-poor. Lining up opposite charges predicts the major product. The video notes the reaction is pericyclic, not ionic, so the charge argument is a predictive shortcut.
You'll be able to
- identify the electron-rich and electron-poor carbons using resonance structures.
- predict the regiochemistry of a Diels-Alder reaction using opposite charges.
- trace the electron movement of a Diels-Alder cycloaddition to draw the product.
Key words
- Diels-Alder reaction
- A concerted cycloaddition in which a diene and a dienophile form a cyclohexene ring.
- Regiochemistry
- The preferred orientation of a reaction, which determines which atoms bond to which.
- Electron-donating group
- A group, such as methoxy, that pushes electron density onto the diene.
- Resonance structure
- An alternative way to draw a molecule that shows where electron density moves.
- Pericyclic reaction
- A reaction that proceeds through a concerted, cyclic movement of electrons.
Transcript
- [Voiceover] For this Diels-Alder reaction, I've added on an electron donating group to the diene. So here's the diene, and notice there is a methoxy group attached to this carbon. That means there are two possible regiochemical outcomes for this Diels-Alder reaction.
So we could form this product, or we could form this product. So which product is favored? To figure that out, we need to draw resonance structures for both the diene and the dienophile, and let's start with the diene. We could take this lone pair of electrons on this oxygen and move them into here, and then push these electrons off onto this carbon.
So let's draw that resonance structure. So now this oxygen would have a double bond to this carbon, and then this carbon would have a lone pair of electrons on it. Let me go ahead and finish drawing in the rest of these bonds, and this oxygen still has one lone pair of electrons on it which gives this oxygen a plus one formal charge, and this carbon would get a negative one formal charge.
Let me highlight that carbon, so this carbon right here gets a negative one formal charge. Let me draw in my resonance brackets here. Let's look at the dienophile next. So we know that this oxygen is electronegative, so electron density is going to flow towards that oxygen.
I could take these electrons and move them into here and these electrons come off onto the oxygen, so let's draw that resonance structure. We would have a double bond here, and our oxygen would have three lone pairs of electrons around it, which gives that oxygen a negative one formal charge.
Let me put in my resonance brackets, and notice we took a bond away from this carbon, so this carbon gets a plus one formal charge, so our diene has a carbon that is electron rich, and our dienophile has a carbon that is electron poor, and we know that opposite charges attract, so we could just line up these two carbons, and that allows us to predict the regiochemistry for this reaction.
Technically the Diels-Alder reaction is not an ionic reaction. It's a pericyclic reaction. But this trick does allow you to predict the product, so let's go ahead and use it. Let's get some more space down here, and let's redraw our dienophile first. So let me draw this in, so we had our double bonds, and then we had our carbonyl.
We know that this carbon down here is the one that is electron poor, so I'm saying that's partially positive, so we need to line up that electron poor carbon with the electron rich carbon on the diene, so we want to make this carbon the electron rich one, and that means the methoxy group must come off of this carbon, and now we can see the regiochemistry for our Diels-Alder reaction.
Remember the Diels-Alder reaction is a concerted movement of six pi electrons. So these pi electrons gonna move into here to form a bond between these two carbons. These pi electrons move into here to form a bond, and these pi electrons move down. So we form our cyclohexene ring, and then we would have our methoxy group coming off of this carbon, and we would have our ketone coming off of this carbon, so following our electrons, so these electrons in red formed this bond, and then our electrons in blue formed this bond, and our electrons in magenta formed this bond.
So this turns out to be the product of our Diels-Alder reaction, and let me go back up to here, so that's our product as opposed to this one.
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