Limitations of Friedel-Crafts Alkyation: Study with Video Lessons, Practice Problems & Examples

Friedel-Crafts alkylation has significant limitations, including its inability to react with vinyl or aryl halides due to unstable carbocations. Aniline derivatives can deactivate Lewis acid catalysts, forming adducts instead of desired products. Alkylation can lead to carbocation rearrangements and poly-substitution, making Friedel-Crafts acylation a preferable alternative. Acylation generates an acylium ion, which avoids rearrangements and results in mono-substitution, maintaining control over product formation. This method utilizes electron-withdrawing acyl groups, reducing reactivity and preventing excessive substitution on the aromatic ring.

Limitations

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

Provide the major product and the correct mechanism for the following reaction. 

• A.
• B.
• C.
• D.

Provide the major product and the correct mechanism for the following reaction. 

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

Why can't Friedel-Crafts alkylation react with vinyl or aryl halides?

Friedel-Crafts alkylation cannot react with vinyl or aryl halides because the resulting carbocations are too unstable. When a halogen is directly attached to a double bond (vinyl) or an aromatic ring (aryl), the carbocation formed is highly unstable and cannot resonate. This instability prevents the reaction from proceeding. For example, reacting benzene with chlorobenzene and AlCl₃ would form an unstable carbocation, leading to no reaction. Therefore, vinyl and aryl halides are unsuitable for Friedel-Crafts alkylation.

How do aniline derivatives affect Friedel-Crafts alkylation?

Aniline derivatives deactivate the Lewis acid catalyst in Friedel-Crafts alkylation. The lone pair on the nitrogen of aniline is highly basic and can complex with the Lewis acid (e.g., AlCl₃), forming a stable adduct. This adduct consumes the catalyst, preventing it from facilitating the alkylation reaction. As a result, the desired alkylation product is not formed. To avoid this issue, aniline derivatives should not be used in Friedel-Crafts alkylation.

What are the main limitations of Friedel-Crafts alkylation?

The main limitations of Friedel-Crafts alkylation include: 1) Inability to react with vinyl or aryl halides due to unstable carbocations. 2) Deactivation of the Lewis acid catalyst by aniline derivatives, forming adducts. 3) Susceptibility to carbocation rearrangements, leading to unexpected products. 4) Tendency for poly-substitution, where multiple alkyl groups are added to the aromatic ring, making it difficult to control the product formation. These limitations make Friedel-Crafts acylation a preferable alternative in many cases.

Why is Friedel-Crafts acylation preferred over alkylation?

Friedel-Crafts acylation is preferred over alkylation because it avoids several limitations. Acylation forms an acylium ion (R-C≡O⁺), which is more stable and does not undergo rearrangements. Additionally, the acyl group is electron-withdrawing, reducing the reactivity of the aromatic ring and preventing poly-substitution. This results in mono-substitution, providing better control over the product. In contrast, alkylation forms a carbocation (R⁺), which can rearrange and lead to multiple substitutions, complicating the synthesis.

How does carbocation rearrangement affect Friedel-Crafts alkylation?

Carbocation rearrangement affects Friedel-Crafts alkylation by leading to unexpected products. During the reaction, the initially formed carbocation can rearrange to a more stable carbocation, altering the position of the alkyl group on the aromatic ring. This rearrangement can result in a mixture of products, complicating the synthesis and reducing the yield of the desired product. To avoid this issue, Friedel-Crafts acylation is often used, as the acylium ion formed does not undergo rearrangement.