Difference Between Friedel Crafts Acylation and Alkylation

Main Difference – Friedel Crafts Acylation vs Alkylation

Friedel Crafts acylation and alkylation are two types of chemical reactions that were first introduced by the two scientists Charles Friedel and James Crafts. Therefore the reactions were given the names of the two scientists. These reactions provide us with a mechanism to introduce acyl groups and alkyl groups into chemical compounds. There are other Friedel Crafts reactions as well, but the most common reactions among them are the acylation and the alkylation. Both these reactions occur through electrophilic aromatic substitution. The main difference between Friedel Crafts acylation and alkylation is that Friedel Crafts acylation reaction is used to add an acyl group to a molecule whereas Friedel Crafts alkylation reaction is used to add an alkyl group to a molecule.

Key Areas Covered

1. What is Friedel Crafts Acylation
      – Definition, Reaction Mechanism
2. What is Friedel Crafts Alkylation
      – Definition, Reaction Mechanism
3. What are the Similarities Between Friedel Crafts Acylation and Alkylation
      – Outline of Common Features
4. What is the Difference Between Friedel Crafts Acylation and Alkylation
      – Comparison of Key Differences

 Key Terms: Acyl Group, Alkyl Group, Friedel Crafts Acylation, Friedel Crafts Alkylation

What is Friedel Crafts Acylation

Friedel Crafts acylation reaction involves the attachment of an acyl group to an aromatic ring. This occurs through an electrophilic aromatic substitution mechanism. Here, the acyl group is provided by an acyl halide compound. In order for this reaction to proceed, a catalyst is required. The most commonly used catalyst is AlCl₃.

Reaction Mechanism

The reaction type is electrophilic aromatic substitution and the acyl halide act as the electrophile. Aromatic ring structures are rich with electrons due to the presence of double bonds. The catalyst is used to enhance the reaction by improving the electrophilicity of the acyl halide. Here, the catalyst, AlCl₃ makes a complex with the halide group of the acyl halide. The halide then leaves the acyl halide molecule, taking the bond electron pair. This causes the remaining acyl group to have a positive charge (the carbon atom which the halide was attached will get this positive charge). Then it acts as an electrophile. This electrophile is called the acylium ion. It can get stabilized by resonance structures. Electrophiles tend to find electrons in order to neutralize their positive charge.

Figure 1: Friedel Crafts Acylation Reaction

Aromatic rings are rich with electrons. Therefore, these aromatic rings give electrons to the electrophile. Then, the acyl group gets attached to the aromatic ring. It causes the leaving of a hydrogen atom in the aromatic ring. In other words, one hydrogen atom in the aromatic ring is substituted by the acyl group.

The catalyst-halide complex is not stable. Therefore, it splits apart resulting in a halide ion and the catalyst molecule. This halide ion gets stabilized by reacting with the hydrogen ion released from the aromatic ring.

Figure 2: End Products of Friedel Crafts Acylation when Acyl Chloride is Used

The overall reaction mechanism results in a substituted aromatic ring, hydrogen halide molecule and the catalyst at the end. Since the catalyst is regenerated, it can be reused.

What is Friedel Crafts Alkylation

Friedel Crafts alkylation is the process of adding an alkyl group to an aromatic ring. There, the alkyl group is provided by an alkyl halide. The reaction mechanism is an electrophilic aromatic substitution. The catalyst used here is Aluminum trichloride (AlCl₃).

Reaction Mechanism

This reaction is an electrophilic substitution reaction. The alkyl halide provides the electrophilic alkyl group. The catalyst forms a complex with the halide group in the alkyl halide. Then the halide leaves the alkyl halide, taking the bond electron pair. This gives a positive charge to the alkyl group (the carbon atom halide was attached to will get this positive charge). Then the alkyl group acts as an electrophile. It needs electrons from another compound in order to get stabilized. The formed electrophile is a carbocation. This carbocation tends to undergo rearrangements in order to form a more stable carbocation.

Figure 3: Friedel Crafts Alkylation Reaction

Aromatic rings are rich with electrons due to the presence of double bonds. Therefore these aromatic rings can give electrons to the alkyl electrophile. “This results in the substitution of an alkyl group by displacing a hydrogen atom in the aromatic ring. This hydrogen atom leaves the aromatic ring as a hydrogen ion.

Since the catalyst-halide complex is unstable, the halide group is released from the complex, and it gets attached with the hydrogen ion released from the aromatic ring. The catalyst is now free to be reused.

Similarities Between Friedel Crafts Acylation and Alkylation

• Both reactions are used to add functional groups to aromatic rings.
• In both mechanisms, AlCl₃ act as the catalyst.
• Both reactions use the halide of the functional group that is going to be attached to the aromatic ring. Ex: alkyl halide, acyl halide.
• Both reactions give hydrogen halide as a byproduct.
• Both reaction mechanisms are electrophilic aromatic substitution reactions.

Difference Between Friedel Crafts Acylation and Alkylation

Definition

Friedel Crafts Acylation: Friedel Crafts acylation reaction involves the attachment of an acyl group to an aromatic ring.

Friedel Crafts Alkylation: Friedel Crafts alkylation is the process of adding an alkyl group to an aromatic ring.

Group Exchanged

Friedel Crafts Acylation: Friedel Crafts acylation reaction exchanges an acyl group.

Friedel Crafts Alkylation: Friedel Crafts alkylation reaction exchanges an alkyl group.

Reagents

Friedel Crafts Acylation: Friedel Crafts acylation uses acyl halide, an aromatic compound and a catalyst.

Friedel Crafts Alkylation: Friedel Crafts alkylation uses alkyl halide, an aromatic compound and catalyst.

Electrophile

Friedel Crafts Acylation: The electrophile for Friedel Crafts acylation is a positively charged acyl group.

Friedel Crafts Alkylation: The electrophile for Friedel Crafts alkylation is a positively charged alkyl group.

Rearrangement

Friedel Crafts Acylation: Friedel Crafts acylation involves the formation of resonance structures of the carbocation (acylium ion) formed.

Friedel Crafts Alkylation: Friedel Crafts alkylation involves the rearrangement of the carbocation formed.

Conclusion

Friedel Crafts acylation and Friedel Crafts alkylation are two important reactions among a series of Friedel Crafts reactions. Although the reaction mechanisms seem similar, these are different reactions due to the difference in the electrophiles involved in each reaction. The main difference between Friedel Crafts acylation and alkylation is that Friedel Crafts acylation reaction is used to add an acyl group to a molecule whereas Friedel Crafts alkylation reaction is used to add an alkyl group to a molecule.

References:

1. Hunt, Ian R. “Friedel-Crafts acylation.” Chem.ucalgary, Available here. Accessed 10 Sept. 2017.
2. Hunt, Ian R. “ Friedel-Crafts alkylation.” Chem.ucalgary, Available here. Accessed 11 Sept. 2017.

Image Courtesy:

1. “Friedel-Crafts-acylation-overview” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
2. “Friedel-Crafts-acylation-step-3” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
3. “Friedel Crafts mechanism” By Rifleman 82 – English Wikipedia (Public Domain) via Commons Wikimedia