Now let's discuss how Friedel Crafts acylation is much more effective than Friedel Crafts alkylation. Friedel Crafts acylation has several advantages that make it much more synthetically useful than alkylation. A few of these we've already discussed. Essentially, acylation reactions deactivate the ring to further reactions, favoring monosubstitution, which is a big deal in organic chemistry synthesis. We want to make sure that we're only adding one group at a time, not two, not four. Also, we learned that acylation reactions are not susceptible to carbocation rearrangements because the acylium ion can't resonate. Perfect. Here, I just want to show you an example of how two syntheses could go completely different directions depending on which one you use. Let's say that we're trying to add a three carbon chain to the benzene ring. First, I'm going to use acylation. In acylation, what I will end up getting is an acylium ion that C=O+ C3. Everyone cool with that? I'm going to end up attacking, and after all of our arenium ions, don't worry about it too much, we're going to end up getting a product that looks like this. It's going to have a three carbon chain. It's going to be a ketone with a three carbon chain and that's it. We're not going to get a second reaction. We're not going to get a rearrangement. That's it.
Now notice what happens when we try to use alkylation. Alkylation might seem like the better choice because we don't want the ketone. We just want the chain. Our first thought would be let's just use alkylation because alkylation doesn't give us that ketone. But alkylation is going to give us many more problems because of this. What happens when the bond gives its electrons to the aluminum? Remember that primary carbocations are unstable. That means a rearrangement is actually going to happen
