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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

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18. Reactions of Aromatics:EAS and Beyond

Side-Chain Halogenation

Sometimes R-groups on a benzene like to react with other reagents. Here, we will see how a Benzylic Chlorination and Bromination can occur. Btw, you already should know the mechanism!


Side-Chain Halogenations

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Now that we're pretty much professionals at adding things to benzene using E s mechanisms, we're actually gonna have toe learn how are groups that are on the benzene uniquely like to react with other re agents? And these air in general called side chain reactions. Okay, so it turns out that the alcohol group that is directly textual benzene is known as an alcohol side chain. Okay. And these are groups, even though they might seem like it's just like a normal our group, they're actually special guys because they contain what we call a Benz Ilic position. What's the Benz Ilic position? The Benzali position is the position next to the benzene. Okay, and this position is uniquely stable. It's uniquely stable due to conjugation. So if you can put a reactive intermediate on that Ben's Ilic position, it's gonna be more stable than normal because of conjugation because of resonance, Right? So recall that Ben's Ilic, radicals in particular, are actually the most stable radicals out there. A Benz Ilic radical loves to form because when it conform, it can resonate throughout the whole ring. Let's actually investigate that further. Let's draw all the residents structures of the vans, Ilic Radical. So notice, Let's say a former radical through some kind of radical reaction. Okay, what's gonna happen is that that radical isn't just gonna stay there on the primary carbon. Heck, no. It's gonna resonate throughout that whole ring. So let's go ahead and draw this. Remember that whenever we resonate a radical, we use the three half headed arrow. So we get something like this, we would do half headed arrow here, half headed arrow here, one electron left over. So what we would get is now a double bond here, adult Bon here. A dull one here in a radical every. Call it that If you're not, we'll just do it again, OK, so we'll just do another one. So now that radical isn't stuck there. It can still keep going. Right. So now this radical could come here and try to make a double bond. But it needs one more radical, so it's gonna steal one. It is going to steal an electron from the other bond. But now we've got one radical left, so it's gonna come here and now I'm gonna get a resident structure that looks like this, but we're not done. We know what this radical can keep moving along. So then we're gonna do make a double bond, bring it over from that dull bond, and then dumped the extra radical here. And then finally, guys, you know how this ends. This ends the same way it began with this radical making a double bond, this one coming to join it. And then I get one extra radical left over. So as you guys can see, this radical is like in heaven. Right now, that radical is the most stable radical ever. Because it's right next to a benzene to conform all these residents structures. It's awesome. So, guys, looking back to our conjugation chapter remember that whenever conjugation was present, special reactions could take place of those conjugated sites. And that's exactly what happens here. In fact, these reactions are the identical reactions to a Lilic chlorination on the Lilic domination. The only thing is that we're using a Benz Ilic position in set, so you can think of these as simply same as a Lilic reactions. OK, now, if you're wondering Johnny, it's been a long time. I don't remember the little reactions you could always type in the search bar. Guys, we've got a search bar. You could go a Lilic chlorination and the video shows. Okay, but this comes from your conjugation section of your text. Okay? But it's the same thing, guys. Remember that. Oh, man, there's a typo. But you know what, guys? I'm just gonna keep going. So this is supposed to say Oh, I'm sorry. There isn't a type of Never mind. So Ben's Ilic chlorination and Ben's Ilic domination. So Ben's Ilic chlorination would be a die atomic chlorine with heat or light as a radical initiator. Okay, so this is gonna be a radical reaction. It means it's gonna have an initiation step, a propagation step and a termination step. So you're gonna wind up getting C l radicals that wind up making. Remember, there's an H here, right? That wind up making a radical here. Okay. And once you form the radical there, you propagate, you chlorinate and you wind up getting this. Okay? You want getting this product where you get basically a Benz Ilic? Chlorination. Okay, now, if you're looking for the mechanism of this, which really is not the emphasis of this chapter, okay? um this was discussed in your congregation section. So just look up the mechanism in a little it chlorination. Okay, because it's the same exact reaction, Okay. Except that now you're using a benzene instead of a double bond. But it's the same thing where you're going to get initiation propagation and termination all the same as your A Lilic reactions. Okay, Ben's Ilic prom in ation. So Ben's Ilic domination. Remember, guys, you need trace amounts of broening because you don't want any cross reactions. Okay, that's what NBS is. NBS is a source of trace bro mean, and that's why we use it. So NBS with heater light is going to make is going to make PR radicals. And those BR radicals are gonna wind up planting a radical on the Benzali position. Eventually they terminate or they propagate through BR to and they wind up getting a pencil. It bro me. So guys thes air what's called your side chain reactions? Because this actually has nothing to do with benzene, right? This has nothing to do with the A s. This is just a reaction that can occur at the Benzali position because it's so stable. Alright, so hope that made sense. Guys, I hope I didn't throw you off. I got a little bit scared there for a second. But anyway, I hope this makes sense. It's really the same thing that we would have learned in the conjugation section of your text. Okay, so let's move on to the next reaction.