In this video, we're going to review some of the major important limitations of Friedel-Crafts alkylations. So it turns out that Friedel-Crafts alkylation isn't all it's cracked up to be. The mechanism was simple. The mechanism makes sense. But turns out it doesn't work that well. Why? There are actually several reasons. Let's just go into them one by one. The first one is, well, the first limitation just makes sense. It doesn't react with vinyl or aryl halides. So if you have a halogen directly on a double bond, that carbocation is going to be far too unstable. So you're not going to get that reaction to happen. Here's an example. Let's say that I have a benzene and I'm reacting it with chlorobenzene and AlCl3. We would expect that the first step of this mechanism would be that the chlorine bond gives its electrons to the aluminum and what I wind up getting is a carbocation that looks like this. What do you think about that carbocation? That's a really unstable carbocation because it can't resonate anywhere. It's stuck. It's on a double bond. That's one of the worst, most unstable carbocations. So I'm going to say too unstable. The answer here is that if you're working with an aryl or vinyl alkyl halide, no reaction. You're not going to get a reaction with Friedel-Crafts alkylation. So just the solution would be to avoid these molecules. There's nothing we can do to get around it. Just avoid those. You can't use them. Let's go on to the next limitation. This one makes sense guys. Turns out that aniline derivatives are going to ruin the Lewis acid catalyst because if you guys recall, this is the most basic lone pair really possible on a benzene. AlCl3 is one of the strongest acids. It's a strong Lewis acid. So guess what's going to happen? Usually, we would expect that the bond between the alkyl halide would donate to the empty orbital, correct? But that's not what happens guys because it's going to compete with the lone pair from the nitrogen and the aniline's actually just going to complex with it itself. So what you wind up getting is a ruined molecule because now what you have is here's my benzene, here's my nitrogen, here's my H's. It's actually going to be attached directly to the AlCl3. It's going to make what we call an adduct. But this is a ruined catalyst because now it's not a catalyst anymore. It just got consumed because good luck separating that bond. We've got a positive, we've got a negative. Those things are really attracted to each other. Friedel-Crafts alkylation on a trying to run a Friedel-Crafts alkylation on a benzene, make sure that you avoid aniline at all costs. You cannot use aniline and Lewis Kasichellis. The answer here would actually be the adduct. It would just be the adduct that I showed earlier but it would not be the right reaction. It would be this thing here. It wouldn't be the
19. Reactions of Aromatics: EAS and Beyond
Limitations of Friedel-Crafts Alkyation
19. Reactions of Aromatics: EAS and Beyond
Limitations of Friedel-Crafts Alkyation - Video Tutorials & Practice Problems
Friedel-Crafts Alkylation has several limitations that render it almost useless in the lab. Let's take a look at 4 examples of what these limitations could be.
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Limitations
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ProblemProvide the major product and the correct mechanism for the following reaction.
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ProblemProvide the major product and the correct mechanism for the following reaction.
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