In this video, we're going to go over electrophilic aromatic substitution reactions, specifically bromination, chlorination, and iodination. So let's talk about the general mechanism for these reactions. So here we have benzene, and we're going to react it with an electrophile. Now the first thing that's going to happen is the benzene ring is going to behave as a nucleophile attacking the electrophile.
And so the first step is an addition step and this step is a very slow step because the benzene ring is losing its aromaticity. So we're going to go from a stable compound to a unstable compound. So this first step is an endothermic step, because we're going from a molecule with low energy to a molecule with high energy.
And that's why it's low. And as you can see, the benzene ring no longer has a continuous pi electron cloud. Now we have a carbocation intermediate.
In the second step, a base is going to come in, take away the hydrogen, and then regenerate the aromatic ring. Now this second step is fast because we're going from an unstable compound to a stable compound. And so the net result is that we replaced a hydrogen atom with an electrophile.
So this is called an electrophilic aromatic substitution reaction. By the way, the second step for this process is FAS, because we're going from an unstable compound to a stable compound. And also, that step is exothermic. Energy is going to be released because we're going from a high energy compound to a low energy compound.
So those are some things to keep in mind. But now let's go over the bromination of benzene. So here we have a benzene ring.
and we're going to react it with Br2 and with a Lewis acid catalyst called FeBr3 iron bromide and so we're going to replace the hydrogen atom with an electrophile in this case the electrophile is the bromine atom and so that's how we can convert benzene into bromobenzene. So let's write a mechanism for this reaction. So in the first step, bromine is going to react with the Lewis acid catalyst.
Right now each bromine atom has three lone pairs. And so bromine adds itself to FeBr3 in a reversible reaction, and we're going to get this compound. Whenever the Fe atom, or the iron atom, has four bonds, it's going to have a negative formal charge.
And bromine, which now has two bonds, has a positive formal charge. So this is the species that we're going to react the benzene ring with. So like always, in an EAS reaction, that is, in an electrophilic aromatic substitution reaction, the benzene ring is going to behave as the nucleophile.
And a bromine atom at the far left is going to behave as the electrophile. By the way, this is in resonance with, you can view it as being in resonance with Br+, and FeBr4-. So the benzene ring will attack this bromine atom, and then these electrons will go to this bromine atom. And so we have a hydrogen atom, a bromine atom, and we have a positive charge. on that carbon.
So between these two carbons, if you add a bromine atom to the top carbon, the positive charge will be at the bottom carbon. So it has to be on the other carbon that you add the bromine atom to. Now, the next step is to use the base.
In this case, we can use the solvent or this molecule to abstract a proton. So I'm going to use this molecule. So we have a bromine atom attached to FeBr3. And so FeBr4-, which is this whole thing combined, it's going to act as a base and abstract the proton. This hydrogen is highly acidic because this compound is unstable.
And so this... Sigma complex, or arenium ion as some textbooks call it, really wants to dish out this proton because once it gets rid of it, it can regenerate the aromatic ring. And so even a bromide ion will suffice as a base to get rid of that proton. So once it's gone, we're going to have the aromatic ring. So once again, we have a stable compound, and this is going to be the final product.
So that's the mechanism for the bromination of benzene. So the net effect is that we are replacing a hydrogen atom with an electrophile, a bromine atom. And so it's called an electrophilic aromatic substitution reaction.
Now let's move on to our next example, and that is the chlorination of benzene. So if we react benzene with chlorine gas and AlCl3, aluminum chloride, this will generate chlorobenzene, which looks like this. By the way, in addition to using AlCl3, you could also use FeCl3. So both of these Lewis acid catalysts can work in this reaction.
But for this example, I'm going to use AlCl3. So in this mechanism, which is similar to the last one, the chlorine molecule will react with the Lewis acid catalyst. So basically it's going to add itself to that catalyst, giving us this intermediate, where the aluminum atom has a negative charge, that is a negative formal charge, and the chlorine atom attached to it has a positive formal charge.
And so the Banting ring... is going to attack this species, just like it did before. So it's going to attack this chlorine atom, expel in this group. So here is the chlorine atom, and we're going to get a sigma complex, which is basically a carbocation inside of what used to be a benzene ring. And then in the next step, the base is going to come in, which is AlCl4, and it's going to grab a hydrogen, regenerating the benzene ring.
So we're going to get chlorobenzene as our product. And then we're also going to get HTL as another side product. And we're also going to regenerate. the aluminum chloride catalyst.
So because AlCl3 was regenerated in a reaction, it wasn't consumed in a reaction, it is therefore a catalyst. Now we're going to go over one last reaction and that is the iodination of benzene. So if we react benzene with iodine using an oxidizing agent under acidic conditions like nitric acid, we can get iodobenzene.
Now there are some other reagents that we could use as well. For example, another one that can accomplish this transformation. is using iodine with hydrogen peroxide and sulfuric acid.
I've also seen that example too. So that's how you can convert benzene into iodobenzene. And so what happens is that the oxidizing agent oxidizes iodine from its neutral state to a positive one oxidation state. And so that is the electrophile that reacts with the benzene ring. And here is the general mechanism for it, which is very similar to the other mechanisms that we've been doing in this video.
So the benzene ring is going to behave as a nucleophile, and it's going to react with this electrophile. And so I'm going to add the iodine atom to the same position. I'm going to follow the same process.
which means we're going to have a positive charge on this carbon. So that's our carbocation intermediate. And then some base in the solution, it could be the solvent or it could be something else, is going to basically take away that proton. And so we're going to regenerate the aromatic ring, thus giving us iodobenzene, which looks like this.
And so that's it for this video. So now you basically have the generic mechanism for the halogenation of an aromatic ring, such as benzene. So thanks for watching.