Advantages of Friedel-Crafts Acylation: Videos & Practice Problems

Friedel Crafts acylation is preferred over alkylation due to its ability to favor monosubstitution and avoid carbocation rearrangements. Acylation produces an acylium ion, leading to a ketone without further reactions, while alkylation can yield polysubstituted products due to unstable primary carbocations. To convert acylation products to alkylbenzenes, the Clemensen reduction using zinc amalgam and HCl can be employed, effectively removing the carbonyl group. This method provides a more efficient route to synthesize straight-chain alkyl groups on aromatic rings.

Advantages

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

Here’s what students ask on this topic:

What are the advantages of Friedel-Crafts acylation over Friedel-Crafts alkylation?

Friedel-Crafts acylation has several advantages over alkylation. Firstly, acylation deactivates the aromatic ring to further reactions, favoring monosubstitution, which is crucial for controlled synthesis. Secondly, acylation reactions are not susceptible to carbocation rearrangements because the acylium ion is stable and does not rearrange. This leads to more predictable and cleaner products. Additionally, acylation products can be converted to alkylbenzenes using the Clemensen reduction, providing a versatile route to synthesize straight-chain alkyl groups on aromatic rings.

How does Friedel-Crafts acylation prevent polysubstitution?

Friedel-Crafts acylation prevents polysubstitution by deactivating the aromatic ring after the initial substitution. The acylium ion formed during acylation is less reactive than the carbocations formed in alkylation, which means the ring is less likely to undergo further reactions. This deactivation ensures that only one acyl group is added to the ring, favoring monosubstitution and preventing the formation of polysubstituted products.

Why are carbocation rearrangements not a concern in Friedel-Crafts acylation?

Carbocation rearrangements are not a concern in Friedel-Crafts acylation because the acylium ion, which is the electrophile in this reaction, is stabilized by resonance. The acylium ion has a structure where the positive charge is delocalized over the carbon and oxygen atoms, making it stable and less likely to rearrange. This stability ensures that the acyl group attaches to the aromatic ring without any unexpected changes in the carbon chain length or structure.

What is the role of Clemensen reduction in Friedel-Crafts acylation?

The Clemensen reduction plays a crucial role in Friedel-Crafts acylation by converting the acylated product into an alkylbenzene. This reduction uses a zinc amalgam and hydrochloric acid to remove the carbonyl group from the acylated product, effectively replacing it with hydrogen atoms. This process allows chemists to first use acylation to achieve controlled monosubstitution and then convert the product to the desired alkylbenzene, providing a more efficient and predictable synthesis route.

How does Friedel-Crafts acylation favor monosubstitution?

Friedel-Crafts acylation favors monosubstitution by deactivating the aromatic ring after the initial acyl group is added. The acylium ion formed during the reaction is less reactive than the carbocations formed in alkylation, which means the ring is less likely to undergo further reactions. This deactivation ensures that only one acyl group is added to the ring, preventing multiple substitutions and leading to a more controlled and predictable synthesis.