Friedel-Crafts Acylation Reactions in Synthesis

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Aromatic compounds are important biological compounds, but it is difficult to see reactions occur on them. Friedel-Crafts Acylation are one type of reaction that can occur on aromatic compounds.

Importance of Friedel-Crafts

Vitamin D, DNA, and many other important compounds in our bodies all includes an aromatic portion. But it is very difficult for an aromatic ring to react with other compounds because it is so stable. So how do we add an aromatic ring to the rest of the molecule to form these important molecules in our body? One of the methods is through Friedel-Crafts Acylation. Friedel-Crafts Acylation adds a new carbon-carbon bond to an aromatic ring, and the new carbon is an acyl group. In other words, the new carbon (which was just added on) is also double bonded to an oxygen.

Without Friedel-Crafts reactions we wouldn't be able to extend aromatic compounds into larger molecules, with more carbon atoms. So, we would be able to form the individual rings for DNA but we wouldn't be able to connect them together to fully form DNA. We need Friedel-Crafts reactions to build aromatic compounds.

General Reaction

For Friedel-Crafts Acylation to occur we need an acyl halogen, a Lewis acid, and an aromatic compound. The acyl chloride is any carbon doubled bonded to an oxygen and a halogen (typically chloride). The Lewis acid is AlCl3. It can act as a Lewis Acid because the aluminum can hold an extra pair of electrons, allowing another bond to be formed. And an aromatic compound is any fully conjugated ring, this full conjugation is what makes aromatic compounds so stable. The simplest aromatic compound is benzene, C6 H6.

Reactants

The Lewis acid first removes the halogen from the acyl halogen. This puts a positive charge onto the acyl. This positive charge is stabilized by resonance with the oxygen, this compound is called an acylium ion.


Acylium ion formation


Neither the carbon nor the oxygen like to have a positive charge so the acylium ion is highly electrophilic (electron loving or electron deficient). Although aromatic compounds don't like to react they are still very nucleophilic (electron rich), and have lots of extra electrons to share. Typically, aromatic compounds cannot react, even with those extra electrons, because it would decrease the stability a lot, so it would require a lot of energy. But now we have a very unstable compound, the acylium ion. If we give the acylium ion the extra electrons then we will make it more stable, making it so that not as much energy is lost overall. So, the aromatic compound loses a lot of energy (to break its aromaticity and share electrons) but the acylium ion gains a lot of energy (to gain stability) so overall the energy change isn't too high.


Aromaticity broken


But the energy on the aromatic compound is still higher than it would like to be. It now needs to return to its aromaticity. It could simply kick the acylium ion off again, but then we will be back where we started with an unstable acylium ion. Instead the Lewis Acid with the extra halogen can come and take the hydrogen off, reforming the Lewis Acid and making a halogenic acid.


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