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Friedel-Crafts acylation is one of the electrophilic aromatic substitution reactions developed by French chemist Charles Friedel and American chemist James Crafts in 1877. There are two types of Friedel-Crafts reactions that are “Friedel-Crafts Acylation” and “Friedel-Crafts Alkylation.
In both reactions, a strong Lewis acid is used as a catalyst to substitute an acyl group or an alkyl group for an aromatic ring. The Friedel-Crafts acylation is widely used in the industry to synthesize aromatic ketones. Aromatic ketones are intermediates of synthesizing fragrances, dyes and Pharmaceuticals.
Acyl group consists of a carbonyl carbon and an alkyl group. A carbonyl carbon is a carbon atom that is bonded to an oxygen with a double bond.
Figure 1 – acyl group
In Friedel-Crafts acylation, an acid halide (acyl halide) is used as the acyl group carrier and anhydrous AlCl3 as the catalyst. Usually, acid chlorides are used in this reaction. When benzene is treated with acid chloride in the presence of an anhydrous AlCl3 catalyst. It results in acyl benzene which is an aromatic ketone. The acyl group comes from the acid chloride acts as the electrophile here.
Equation 1
However, in the preparation of Benzaldehyde which is also a carbonyl compound, an acid chloride cannot be used. Because the respective acid chloride (Formyl chloride CHOCl) is highly unstable. Therefore, it uses a mixture of dry carbon monoxide and concentrated HCl in the presence of AlCl3 as the catalyst.
Equation 2
The ideal reaction type of benzene is an electrophilic substitution reaction. Therefore, an electrophile should be prepared for this reaction.
AlCl3 is a strong Lewis acid. The valance shell of the Al in AlCl3 is deficient in electrons. It has only 6 electrons in the valence shell and it needs two more electrons to make 8 electrons. On the other hand, the halogen in the acyl halide has three lone pair electrons.
One-lone pair is donated to the empty orbital in the Al, and forms a dative bond. In this process, the halogen in the acyl halide gets a positive charge and Al gets a negative charge. As an example, if CH3-COCl (methanoyl chloride) is used as the acid halide, it forms a complex as follows.
Equation 3
When there is a positive charge on the Cl, the electronegativity of Cl increases. Also, Cl originally is a highly electronegative atom. Therefore, Cl pulls electrons from the C-Cl bond. Then the C-Cl bond undergoes a heterolytic cleavage.
Thus the acid chloride loses the Cl atom and leaves a positive charge on the carbonyl carbon. The eliminated Cl- ion forms a dative bond with AlCl3, resulting in [AlCl4]- ion to the medium. Since the carbonyl carbon gets a positive charge an acylium ion is generated.
An acylium ion is a special type of carbocation since there is a positive charge on a carbonyl carbon. It is more stable than a typical carbocation due to the resonance stabilization.
Figure 2 -resonance structures of acylium ion
The acylium ion acts as an electrophile. The pi electrons in the benzene attack the positively charged in the acylium ion.
Using the kekule structure the reaction mechanism can be explained. According to the Kekule structure of benzene, there are three pi bonds in the benzene ring. Out of these three bonds, the pi electrons from one bond attack the positively charged carbon.
So, the pi bond is broken down and one carbon atom in the pi bond attaches to the acyl group (CH3-CO) then the other carbon gets a positive charge. Thus, the benzoyl acylium ion is formed. The positive charge on the benzoyl acylium ion delocalizes in the benzene ring. In the reaction mechanism, it can be observed that there is a positive charge on the ortho and para carbon atoms in the benzene ring.
Finally, the [AlCl4]- ion in the medium attacks the hydrogen which is bonded to the carbon to which the acyl group is attached. This hydrogen gives electrons from the C-H bond to the benzene ring and is eliminated as an H+ ion. This H+ ion and the [AlCl4]- regenerate AlCl3 and form the respective hydrogen halide. In this example, it would be HCl.
Equation 4
The three resonance structures of the benzoyl acylium ion can be represented in one resonance hybrid as follows.
Figure – 3
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Figure 01: Contains an image by Werni, from Pixabay
Figure 01: Contains an image by Irhash, from Pxhere
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