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Although benzene rings have pi bonds like alkenes and alkynes, they cannot undergo addition reactions because of their stability and instead do Aromatic Reactions.
Concept #1: Understanding Aromatic Reactions
Hey guys! In this new video, we're going to take a look at aromatic reactions. We’re going to say although benzene rings have pi bonds just like alkenes and alkynes, they do not undergo addition reactions. That's because benzene rings are too stable. They belong to a class of compounds called aromatic compounds. Aromatic compounds don't want to do addition reactions. We're going to say instead of doing addition reaction, benzene rings as well as other aromatic compounds will instead do a substitution reaction in order to maintain their aromaticity.
We're going to say that the two major types of substitution reactions you’re expected to know right now the first one being halogenation. Basically in halogenation, we’re placing a halogen on benzene. But really, we're not just placing on benzene, we’re switching out an H on benzene for one of the halogens. They're basically swapping. We're going to say in order to do this, we have to use what's called a Lewis acid catalyst. These two things are Lewis acid catalysts.
Remember, we've talked about Lewis acids before. Lewis acids are electron pair acceptors. The central element has less than eight electrons around it, so it can accept additional electrons. These Lewis acid catalysts help this process occur. You'll notice here that we're doing Br2 and Cl2. When we say halogenation, we're talking about bromination and chlorination. We're talking about Br2 and Cl2. Also notice that our Lewis acid catalyst, here we are using Br. The Lewis acid catalyst has Br. Here we’re using Cl, so the Lewis acid catalyst also has Cl. They have to match each other.
In this reaction, all that happens is one of the hydrogens anywhere on benzene switches places with the Br. What we get here, we can draw the double bonds anywhere we want because remember we’ve talked about that as well, resonance in benzene. Both will be technically the same thing. Here we're just substituting in the halogen for the hydrogen. They got swapped. What we just made here is bromo benzene.
Here if we did Cl, it’d be the same exact thing and it could be any hydrogen anywhere on benzene that I'm switching with the CL. I decided to do the top one but I could've done it anywhere. There goes the Cl up here. What did that make here? I made chloro benzene. Chloro benzene would be my answer.
That's all it is. We’re substituting in a halogen. In this case, Br or Cl. Look at how the Lewis acid catalysts match. We used Br2 so you have FeBr3. If you use Cl2, so you have to use FeCl3.
The other major type of reaction you’re expected to know, here this is called Friedel-Crafts alkylation. I know it's a mouthful. Here, this is where an alkyl group is placed on benzene. Alkyl group switches places with a hydrogen. Notice here we’re also using a Lewis acid catalyst. Here, this Lewis acid catalyst is not Fe anymore, it's aluminum here in the center. Notice, see how this is a Cl? So this is a Cl, meaning that if that was a Br, it would be AlBr3.
All that happened in this reaction is that the H and the alkyl substituent or alkyl group, they switch, they swap. What we get now, we're going to get this structure here so there goes a CH around benzene. We can say that this is called methyl benzene. That's its scientific name but there's also a very common name for it as well, or toluene. When you have a methyl on benzene, the common name is called toluene. That's what happens here with halogenation and Friedel-Crafts alkylation.
There's more than just these benzene reactions. When you get to Organic 2, you’ll learn more about the other ones. There sulfonation, nitration. There's a lot of different reactions that benzene can do. Here we're just limited to these two. Now that we've seen those two, let's see if you guys can attempt to do this example question here. Here this is just another example of one of the two reactions we went over. Try to isolate the answer. Come back and see how I approached the question. Good luck guys!
The first major reaction is Aromatic Halogenation where a hydrogen on benzene is substituted by a halogen.
The second major reaction is Friedel Crafts Alkylation where a hydrogen on benzene is substituted by an alkyl group.
Example #1: Determine the major product from the following Aromatic reaction.
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