Concept: Concept: Replacement Reactions7m
Now I want to switch gears a little bit and talk about a type of reaction that happens on benzene called a diazonium replacement reaction.
It turns out that aniline, remember an NH2 on a benzene, when it reacts with nitrous acid, NO2 over HCl, you can form a diazo functional group in a reaction called a diazotization. What is a diazo functional group? It’s not one that we see a whole lot. A diazo group is an N triple bond N-group with a positive charge. It’s also called an N2 group, N2 positive. These diazo groups are really good at switching out and reacting with certain types reagents.
Here first of all, I have a really common pathway to make a diazo. If you start off with benzene, what’s the fastest way to get to a diazo group? As of this point, we don't have any way to directly add aniline. We could just use a nitration. We could say you could do a nitration, then you could reduce it using lithium aluminum hydride or any other reducing agent you want. You guys know which ones do I love, the stannous chloride. We could also use the stannous chloride but that’s up to you. You’ve got your aniline. I’ll do my diazo reaction. I have NaNO2 over HCl. That’s going to give you my diazo.
What’s so special about this? Once you react and get that diazo, you can do tons of stuff to that diazo group to replace it with another type of compound. This is a big list and we’re just going to go one by one. If you react a diazo compound with CuBr, you're going to get a bromobenzene. That’s it. If you react a diazo compound with CuCl, you’re going to get a chlorobenzene. If you react the diazo compound with CuCN, you’re going to get what's called a benzonitrile or just a CN attached to a benzene ring.
Don't you wish they were all copper reagents? But they're not going to be. The rest of them get a little bit weird. Iodobenze is formed using KI. Fluorobenzene, this one’s interesting because so far you haven't learned how to make fluorobenzene. I never taught you an EAS reaction that worked with fluorine. If you have an F on a benzene ring, you know the only way to put it there, at least in organic chemistry 2 would be using a diazo replacement reaction. That's going to be HBF4. HBF4, you would make fluorobenzene.
Phenol, there’s actually two ways to make it. One way that’s awesome and one way that sucks. The way that’s awesome, which is the older way to make phenol which is usually the one that professors are cool with is literally just water. Let’s just put water here. That's great. That just makes sense. You put water, diazo, you get a phenol. Awesome. I have seen some textbooks that use a much more complicated version of reagents. That is Cu2O, Cu2+ and water. That one sucks. Cu2O, Cu2+ and water would make phenol but most professors are fine with just putting water.
What about benzene? We start off with benzene. Why would you want to make a benzene? What if you want to just go back and reverse the reaction, pretend like there was never a diazo group there. That could be hopeful because maybe you could use it as a blocking group. Remember, we've talked about blocking groups before. You could use H3PO2. H3PO2 will completely remove the diazo group and just replace it with an H.
Then finally, we get coupling reactions with azo dye. Let’s just back up a little bit. This huge molecule is what we call an azo dye. The reason for those two names is pretty simple. Azo is this group right here, that’s an azo compound, when you have an NN double bond. The reason we call it a dye is because these compounds, because they're so highly conjugated turn really awesome colors. There is a bit of red colors and orange colors. They're actually used as very common dyes in food products and stuff. These are actually very modern-day dyes. It’s called an azo coupling reaction because these diazo groups will literally couple with another benzene. It's pretty straightforward. A diazo group will react with another benzene as long as the benzene has inactivating group or an electron-donating group. If it has an electron-donating group, then it will be reactive enough so that it can react with the diazo group. If you don't have that election-donating group present, it's not going to work.
As you can see, what winds up happening is that you wind up getting the coupled benzene with the old diazo compound at the bottom. We're not going to worry about any of the mechanisms for these reactions because we’re just doing a global view, an oversight. But the most important part is that you memorize these reagents and that you know how to use them in synthesis because a huge part this area of the textbook is knowing how to make a complicated molecule using both EAS and diazo reactions together to make the molecule that you're looking for.
Keep in mind, a few of these are unique to diazo. Diazo is the only way to make fluorobenzene. It's one of the only ways to make a phenol. There are some other ways but you can't use EAS for that. You have to use some other stuff. Obviously, it’s the only way that you can make an azo dye. Keep that in mind. Let's go ahead move on to the next topic.
Predict the product(s) for the following reaction.
What is the result of treating m-bromoaniline with HONO/H2SO4 then CuBr/HBr?
a) 3,4 - dibromoaniline and/or 2,5- dibromoaniline
b) 1,3- dibromobenzene
c) 1,3 dinitrobenzene
e) None of the above