EAS: Nitration Mechanism

Now let's take a deeper look into the mechanism of EAS nitration. So as mentioned earlier, nitration always has to proceed through the creation of a strong nitronium ion electrophile. A nitronium ion electrophile looks like this. It's going to be an N with a double bond O at the top, double bond O at the bottom and a positive charge.
Why do you think that's going to be a strong electrophile? Guys, it has a full positive charge. That's one of the strongest electrophiles possible. That's the perfect type of molecule that benzene wants to react with.
Remember that we said there's two different common ways to generate this nitronium. You could use concentrated nitric acid by itself. By the way, heat never hurts. Heat is going to help this reaction regardless of what reagents you're using. Or you could also just use nitric acid and sulfuric acid together. I included both of these in our mechanism just so that you guys can see how they're really the same exact thing.
In this mechanism, we have one equivalent of acid reacting with another. Regardless of which acid it is, they really do the same thing. One is going to be a proton donor and one is going to be the base. With the sulfuric acid and the nitric acid that makes sense. Sulfuric acid is a much stronger acid than nitric acid. It makes sense that sulfuric acid is going to be the proton donor.
Now for nitric acid, we would just imagine that at equilibrium some of this nitric acid is going to be donating protons to the other.
Let's go ahead and see exactly which part of the nitric acid would be basic. It's going to be this oxygen here with the lone pairs. The reason being that you're going to see later on it's going to become a great leaving group. We would have that if you're proceeding through sulfuric acid, that this oxygen would grab one of the hydrogens on sulfuric acid. But you could also do the same thing guys for nitric acid up here. It's going to have the same net result.
What we're going to get is we're going to wind up getting, here's our benzene ring. I'm just bringing it down. And now we're going to get nitric acid that looks like this. It's protonated. It's now going to have two hydrogens on that oxygen. That has just created a water leaving group.
So in order to generate the nitronium ion, all I have to do is eliminate with the O-. What I can do is I can do an elimination reaction. Bring down – make a new pi bond and kick out the water leaving group. What this is now going to give me is a nitronium ion plus water. Cool?
Now I can go ahead and I can do the rest of my mechanism. At this point, benzene, had to bring it down a few times now. I'm drawing too much. Benzene is going to attack my nitronium ion. What is that going to look like? It's going to do this. By the way, positive. It's going to attack the nitro or the N. And then it's going to kick out one of these pi bonds and make them into a lone pair because you have to break a bond.
This is going to lead us to our sigma complex. So let's draw our sigma complex. I know it's a little annoying, but you guys should get practice with it. So now this is NO2. We've got double bond here, double bond here, positive charge, and now we've just got to draw the whole complex. I'm going to move it over. I'm going to move it to this location. And now this is my last resonance structure. All right, so we're done with the sigma complex. There we go. That's our full resonance structure.
Now, what are we going to use as the conjugate to eliminate this hydrogen? Remember we have to do basically what's a beta-elimination on this hydrogen. Beta to the carbocation. Even though you totally could use the conjugate of sulfuric acid, so you could use the negatively charged OSOH4. I think I said that right. Whatever, I could write it down correctly. But actually, we're not going to use that because typically most textbooks and most professors are actually going to use the water that left in the nitro group. It really doesn't matter guys. You could use the water. If you want, you could use the conjugate base of the acid that you used. I don't really care, but just typically it's the water that's used in these reactions. It doesn't matter because at the end of the day this would make HNO3, I'm sorry HO3+, which is just basically aqueous acid. So it doesn't matter.
I'm going to go ahead, come down and eliminate my H and put the double bond there and we get our final product. What our final product is going to be is it's going to be our nitric acid. I'm sorry, it's going to be our nitro group, nitrobenzene plus we're going to get H3O+ and then you would get I guess the conjugate of your sulfuric acid if you had used that which is OSO3H. Awesome guys.
That's it for the nitration mechanism. So let's move on to the next reaction. 

Guys, it's worth noting that nitro groups or nitrobenzene is often used as a precursor to get to aniline. So remember that aniline is an amino group on a benzene ring. That's called an aniline molecule. Nitro groups can be easily reduced to aniline. As you can see a reduction reaction would remove oxygens and add hydrogens and make aniline.
Even though we're going to discuss this more in your amines chapter, I do want to go through it right now and just kind of clue you guys in to some of the most important reducing agents that can make this conversion happen.
Now, the way we always want to start with and probably want to be our default whenever we think reduction is lithium aluminum hydride. That's just because this is the most common reducing agent of all organic chemistry. It's also one of the strongest. Lithium aluminum hydride will absolutely get the job done and it will absolutely turn a nitro group into aniline.
But there are a few other types of reagents that can do the same thing that you also might see. Do you guys recognize H2 and a palladium catalyst? This also goes for nickel or a platinum catalyst. These would be the reagents used in catalytic hydrogenation. I'm just going to put here, these are the reagents for catalytic hydrogenation and that will definitely reduce your nitro group to an aniline.
Now one that's actually really special, kind of important here is Tin, two chloride, and water, or what's also known as Stannous chloride. Benzene is just going to have to get written on because I don't have that much room. Stannous chloride. Now this one is particularly special here because we're going to talk a little bit more about this later. This is actually your only chemoselective reducing agent.
What does that mean? What it means is that, by saying that it's chemoselective, what I'm saying is that it has a tendency to only reduce nitro groups and nothing else. It's kind of talented at doing that. It really doesn't like to reduce many other types of groups so that's going to be important when we have other groups that are vulnerable to reduction, Stannous chloride is a great choice because it really just hones in on the nitro groups and turns them into aniline.
Finally, really common reducing agents are either iron or zinc in the presence of HCL. You'll see this all the time. These reagents turn into strong reducing agents that will reduce a nitro group into anilines.
Really, the exact reducing agent that you're going to wind up using the most is going to probably be up to your professor more than anything else, but bear in mind that all these reagents could be used in some way or another to reduce a nitro group to an aniline. My personal favorite is going to be the tin, two chloride, the Stannous chloride. That's the one I'm going to use the most often in this course because I know that it's chemoselective specifically for the nitro group, so it has very high yields of aniline when we use it.
Let's move on to the next topic.