Friedel Craft Acylation and Alkylation

The Friedel Crafts reactions are a set of reactions developed by Charles Friedel and James crafts in 1877 to attach substituents to an aromatic ring. Friedel crafts reactions are of two main types: alkylation reactions and acylation reactions. Both are preceded by electrophilic aromatic substitution. Benzene undergoes electrophilic aromatic substitution reactions. Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system, usually hydrogen, is replaced by an electrophile. In this article, we learn about two types of electrophilic aromatic substitutions as alkylation and acylation Friedel crafts reactions.

What is Friedel craft acylation?

The Friedel–Crafts acylation is the reaction of an arene with acyl chlorides or anhydrides using a strong Lewis acid catalyst. This reaction proceeds via electrophilic aromatic substitution to form monoacetylated products.

 Friedel craft acylation Reaction: 

Benzene reacts with acid chlorides or anhydrides in the presence of aluminum chloride to give aromatic ketones. The reaction between benzene and an acyl chloride under these conditions is illustrated below:

Mechanism of Friedel craft acylation:

This reaction involves the formation of a complex between lewis acid [ Alcl3] and the chlorine atom of the acid chloride [ CH3COOCl]. An acylium ion is formed by the cleavage of the C-Cl bond of the complex. The acylium ion has a positive charge on the carbon and is resonance stabilized. This acylium ion acts as electrophile and reacts with the benzene to yield the monoalkylated product [ aryl ketone].

The following steps are involved:

STEP 1:   Formation of electrophile.

STEP 2:  The electrophile attacks the benzene ring to form a carbonium ion.

STEP 3: Loss of proton gives an aromatic ketone.

Limitations of Friedel crafts acylation:

• It can be used to produce ketones in which an oxygen atom is joined to a carbon atom by a double bond.

• Deactivated benzenes do not react under Friedel craft conditions, as they are always less reactive than the original mono-halobenzene.

• The Lewis acid AlCl3 usually forms aryl amine complexes and makes them unreactive.

Applications:

• One of the most important uses of this reaction is that it is used in the production of dyes, like xanthene dyes.

• It is also used in Haworth reactions.

• It is used in synthesizing aromatic compounds which have aldehyde and ketones, functional groups.

What is Friedel craft alkylation?

The Friedel–Crafts alkylation is the reaction of an arene with alkyl halides using a strong Lewis acid catalyst. This reaction proceeds via electrophilic aromatic substitution to form monoacetylated products such as toluene.

Friedel craft alkylation reaction:

Benzene reacts with alkyl halides in the presence of aluminum chloride to form alkylbenzenes. The reaction is as follows:

Mechanism of Friedel craft alkylation:

The mechanism of the Friedel-Crafts alkylation occurs in several steps. Initially, the alkyl halide and Lewis acid react to form the carbocation. The carbocation then attacks the aromatic ring breaking one of the ring double bonds, resulting in the formation of a non-aromatic intermediate. Deprotonation of this intermediate then occurs, the ring regains aromaticity and the removed proton forms an acid that restores the Lewis acid catalyst.

STEP 1: Formation of the electrophile [CH3+]

STEP 2: The electrophile attacks the benzene ring to give a carbonium ion.

STEP 3: Loss of proton gives alkylbenzene.

Limitations of Friedel craft alkylation:

• The halide must be an alkyl halide. Vinyl or aryl halides do not react.

• Alkylation reactions are prone to carbocation rearrangements.

• It is difficult to stop the reaction when one alkyl group has entered the ring. Di and tri alkylbenzenes are also formed.

• Deactivated benzenes are not reactive to Friedel crafts conditions, the benzene needs to be as or more reactive than a mono halobenzene.

• The lewis acid catalyst AlCl3 often complexes to aryl amines making them very unreactive.

 Applications:

• It is used in making many different kinds of aromatic compounds with different substituents.

• It is used in the synthesis of dyes called triarylmethane.

• This reaction is related to several classic named reactions like Clemmensen Reduction.

• This reaction is used in testing for aromatic compounds.

Why is Friedel craft acylation better than alkylation?

Acylation is preferred over alkylation in Friedel craft reaction. Because In the acylation, the acyl group being an electron-withdrawing group reduces the electron density hence further electrophilic substitution is prohibited. The carbon chain of acyl halide does not rearrange and hence there is no possibility of the formation of isomeric ketone compounds.

 Whereas, in the alkylation methyl group being an electron donor increases electron density and increases electrophilic reactivity of the substrate. As a result poly alkylation takes place instead of mono alkylation.

Conclusion:

Friedel craft reactions are electrophilic substitution reactions. In which the alkyl halides and acid chlorides in presence of lewis acid catalyst give mono-substituted products. In alkylation polyalkylation also takes place instead of mono alkylation as the methyl group is electron donor it increases electron density and electrophilic reactivity of substrate also increases.