Friedel-Crafts Reaction | Types

FRIEDEL-CRAFT REACTIONS

Friedel-Crafts Reaction is a set of acylation and alkylation reactions. In this article there is brief information about Friedel craft acylation and alkylation. Friedel Crafts reactions are type of electrophilic substitution reactions. These reactions are named after Charles Friedel and James Crafts. These reactions are of two types alkylation and acylation reactions. Friedel Crafts reactions are discovered in 1877 by Charles Friedel and James Crafts.

What are aromatic electrophilic substitution reactions?

Aromatic compounds give electrophilic substitution reactions in which an electrophile attack on the C-H bond resultantly the C-H bond break down and and a new bond C-E (electrophile) is formed. This type of reaction is called electrophilic substitution reaction. The electrophile is an electron deficient specie and attack on the C-H bond .

These reactions are assisted by addition of a catalyst.

The general mechanism of electrophilic substitution reaction is as follow:

Friedel-Crafts Reaction

An example of electrophilic substitution reaction is chlorination.

Friedel-Crafts Reaction

Friedel-Crafts Reaction:

Friedel craft reactions are a type of aromatic electrophilic substitution reactions. The Friedel craft reactions are of two types alkylation and acylation reactions. The mostly used version of the Friedel craft reactions is Friedel craft acylation.

Friedel craft reactions
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Friedel -Craft Acylation:

The type of electrophilic substitution reaction in which the entering or coming electrophile( electron poor specie) is an acyl group RCO-, it is originated from a carboxylic acid derivative , can be a halide or anhydride. In these acids the carbonyl group is basic so that the formation of complex is takes place with the help of a strong Lewis acid such as Aluminum chloride.

  • In this way the carbocation character of carbonyl carbon is increased by adding aluminum chloride. But in many cases the complex formed itself is electrophilic and it react with aromatic ring.
  • In some cases there is an equilibrium state when the complex exist in equilibrium with acylium ion. That is a strong electrophile.

General mechanism of acylation:

The general mechanism for Friedel craft acylation reaction is following. First of all the reaction of acylium ion with benzene is opposite to other electrophilic reagents. The final product is aromatic ketone in which carbonyl group is basic and is complexed completely by aluminum chloride.

Friedel-Crafts Acylation

The complex that is formed is actual reaction product. The workup procedure is involved by treatment with water and dilute Hydrochloric acid in this way the complex is decomposes and aluminum salts are dissolved. Liberated ketone remains in organic layer and it is isolated by the crystallization and distillation. This is because it complexes with product and aluminum chloride must be used in equimolar amounts. And the complexed ketone not react further and high yield of actual product is given readily.

In aromatic chemistry the Friedel craft acylation reaction is important electrophilic substitution reaction.

For example:

Reaction of acetyl chloride with benzene is an example of Friedel craft acylation reaction.

Friedel craft acylation

Limitations of Acylation:

  • In acylation the formyl-chloride decomposes into CO and HCl and only give ketones.
  • The aromatic compounds that are less reactive than mono-halobenzene do not participate in these reactions.
  • Due to formation of complexes the presence of deactivating groups on aromatic ring can affect the working of Lewis acid catalyst.

Friedel-Craft Alkylation:

The type of electrophilic substitution reaction in which the coming electrophile (electron poor specie) is a alkyl group . Benzene undergoes alkylation reaction with alkyl-halide in the presence of Lewis acid catalyst .

The general reaction of alkylation is as follow.

Friedel craft alkylation reaction

Reaction can be explained by putting the example of t-butyl chloride.

Mechanism of the reaction:

In the above reaction the attacking electrophile is t-butyl chloride that is formed by the reaction of t-butyl chloride with FeCl3 .

Mechanism of Friedel craft alkylation

STEP 1:

In the First step electrophilic carbon is formed by reacting Lewis acid catalyst with alkyl halide.

STEP 2:

And in the second step the carbocation is attacking on the aromatic ring resultantly a cyclohexadienyl cation intermediate is formed. In this way the aromatic character of arene is lost temporarily because of breaking of C-C double bond.

STEP 3:

And in the third and last step deprotonation is occurred on the intermediate which results into again formation of C-C double bond. In this step aromatic character is again gained. And HCl or AlCl3 is also regenerated in this step.

Limitations of Alkylation:

There is limitations of Friedel craft alkylation so that the Friedel craft acylation is mostly used version of Friedel craft electrophilic substitution reactions.

  1. Alkylbenzenes are more reactive than benzene itself in electrophilic substitution reactions. Alkylation reaction give highly alkylated and di-alkyl products.

C6 H6 + (CH3 )3 Cl + FeCl3 →C6 H5C (CH3 )3

C6 H5C (CH3 )3 + (CH3 )3 Cl + FeCl3 → C6 H4 [C( CH3 )3 ]2

The way to control these additional reactions is to require benzene in large amount. So this approach is practical with benzene itself and as this is expensive compound. But this is impractical with substituted benzene that are more expensive.

2.The another limitation of Friedel craft alkylation reaction is that is alternative reactions of carbocation in the absence of particular nucleophile this leads to rearrangement of isomeric carbocations. Isopropyl chloride reacts normally with benzene to give isopropyl benzene.

Friedel craft alkylation reaction

Primary alkyl-halides are less reactive than secondary or tertiary halides and require higher temperature. Under some conditions there is partial rearrangement of primary system. So in this case alkyl halide on benzene coordinate a displacement reaction with Lewis acid to competes the carbocation rearrangement. It should be emphasized. But rearrangements occurs always with primary system . So this limit the usefulness of this reaction.

3.Friedel craft alkylation reactions is also given by alcohol with the Lewis acids aluminum chloride and boron tri-fluoride. But here is a limitation to this reaction that these reactions require benzene excess as in case of alkyl halides. And the one of the product of this reaction is water that can coordinates with Lewis acid. And therefore in case of alkylation reactions the Lewis acid is also required in stoichiometric amount.

Friedel -Craft Alkylation by Alkenes:

Friedel craft alkylation is also given by alkenes. The catalyst typically used in case of alkenes are HF, BF3 , and HCl or AlCl3 .

An example of this type of reaction is:

This type of reaction has an industrial use to prepare alkylbenzenes.

Which group will give Friedel craft reactions easily?

The organic compounds that has electron donating groups gives Friedel craft reactions easily. While the nitrobenzene do not give Friedel craft reaction due to strong withdrawal group the nitro group attached to it.

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