18. Reactions of Aromatics:EAS and Beyond
Blocking Groups - Sulfonic Acid
Let's discuss the one reversible EAS reaction and how we could use it to benefit us, specifically by acting as a blocking group.
Forcing Ortho substitution
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in this video, we're going to discuss a synthetic strategy for benzene reactions. That's called blocking. So, guys, there's only one reversible e s reaction. And that's sulfur nation. Remember that sulfa nation to use concentrated sulfuric acid? It produces eso three, and that reacts makes us a so phonic acid group. Remember that we could very easily remove the so phonic acid with heat and acid or even steam. That would be enough to take off the soul phonic acid group. Okay, that's going to make it really special. Give it a real synthetic advantage to us. Okay? And this is why imagine that you're trying to get an Ortho substituted. Remember that we talked about Ortho pair directors favor. They have Ortho in Paris substitution, but really, they're usually gonna favor the pair of position, right. It's difficult to get things to add Ortho. And when you're trying to get high yields, it's not okay to get a 15% yield of Ortho or a 20% yield. Ortho. Imagine if you're trying to get a 80 or 90% yield of Ortho. How do you do that? The only way to do that is to block the parent position. And that's the advantage of using sulfur nation. Because sulfa nation is actually, that's one of the biggest things that's used for its used to block the pair of position so that you can force Ortho substitution. Because, remember, Ortho substitution is usually not highly favored because of Starik hindrance. But if you can block that para position, then you're gonna force Ortho substitution. It's brilliant. Okay, so this is called a blocking group because it's not actually found in the final product. In the end, you're not going to know that the that the so phonic acid was ever there because you're gonna take it off. It just was there to block the second s reaction. So let's go ahead and look at this. Let's say that you're trying to get some kind of substitution in the Ortho position, and that's it. All you want is Ortho products. You want Ortho substitution. Okay, you better not just react the electric files straight away with the Taliban. In this case, we have Ptolemy. Why? Because you're gonna get so much pere that its's gonna be almost worthless. You're gonna get very little Ortho and then you have to separate it and you get very little yield. It sucks. But what you could do is you could do a sulfa nation first. Okay? Ah, sulfa nation reaction, concentrated sulfuric gas and heat. That's gonna make eso three. Right. And what that's going to do is it's gonna make a soul phonic acid here in the pair of position. Why is it gonna be pera guys? Because we know that the pair of position is favorite. In fact, this is gonna be over. 90% is gonna be pair of okay, very, very high yields. Okay, now we do is you can do R E A s reaction. So now you do your second reaction. This is our first reaction. This is your second reaction you do in E A s and what happens? Notice what types of directors do you have? Well, CH three is Ortho Parra Director. So it's gonna direct either Thio here, Thio here or two here. Are you guys following so far? You got the Ortho positions in the para Can the esri agent actually add Pera? No notice. We also have a meta director. Guys notice that my so phonic acid is a meta director, Because you have a very, very strong partial positive here. Okay? In fact, this is one of our strong the activators. So this is gonna be what type of director Mata. So what positions does it direct to? It's gonna direct to here and to hear, see what's going on. We're actually getting both groups direct into the same spots that are Ortho. Oh, okay. So now when I react, my yes, it's gonna add, the electric file is gonna add specifically here, okay? Or the other one. But they're both the same after my electrical has added. Then I can use my dilute acid to diesel Finney. And what I wind up getting is just a hydrogen here. So in the end, it's called a blocking group because it's not found in the final product. It was there, simply toe block the pair of position and allow me to produce Ortho substitution. Alright. So, guys, that's the whole point behind this topic. Let's go ahead into a practice problem. To really reinforce this, so predict the product of the following multi step synthesis, do all three steps and then I'll show you guys how it works.
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So, guys, in the first step, I'm gonna Sultanate. So that's sulfur nation reaction is gonna produce a soul phonic acid group. Okay, so it's gonna be pere because, like I told you, as you get very high yields of the pair of product Para is favorite over Ortho. Now we're going to nitrate concentrated. H N 03 is a nitrate in reaction. So now we're going to do is we're going to get night train Shin. And where is it gonna act? Well, guys, it has to adhere. Training is a different color. Okay, so it has to adhere because the O. H. Is still a north appear director. It's still going to direct Ortho pere, But it can't add pere anymore because there's something there already. So then you have to add Ortho. And then, as you guys know, dilute acid, he is gonna diesel Finn eight, and it can end up with my final product. That looks like this. Ohh. And Enel to Okay, so I get my Ortho substitution. Okay, so I know there might be a few of you out there wondering, Johnny, if you did the nutrition straight on final, wouldn't the major product be Ortho anyway, because actually talked about how this is one of the exceptions how you could get hydrogen bonding so it actually favor Ortho. Okay, you're exactly right. And I'm glad even paying such close attention. Okay, But the problem is, it still wouldn't be that great. It would be like 60% if you just did it by itself. So if all you did was you did concentrated h n 03 right? What you would get is a final with a nitro group in about 60% yield. Okay, but if you do this three step pathway, it's not much harder. You're going to get a 90% plus yield. See what I mean? So even in a situation where it would have been favored to go Ortho, it's still better. Thio use a blocking group so you can get a higher yield using blocking that pair of position. Okay, Does that make more sense now? Awesome guys. Great example. So let's move on to the next topic.
Beginning from Benzene, synthesize the following compound.
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Additional resources for Blocking Groups - Sulfonic Acid
PRACTICE PROBLEMS AND ACTIVITIES (1)