Friedel-Craft Reactions

Friedel-craft reactions

Friedel-craft reaction are set of electrophilic aromatic substitution reaction. In 1877 Charls friedel and James craft develop these reactions. In Friedel-Craft reaction substitution occur on aromatic compound like benzene. There are two basic type of friedel craft reaction on the basis of substituent used in the reaction.

  • Friedel-Craft alkylation
  • Friedel-Craft acylation

Friedel-Craft alkylation

Friedel craft alkylation is an electophilic aromatic substitution reaction on benzene ring in which a proton of a benzene ring is substituted by an alkyl group. This reaction occurs when benzene reacts with alkyl halide in the presence of strong lewis acid which acts as a catalyst.

 

Reaction

Alkylation by Friedel craft mechanism

Mechanism

There are three steps involved in this reaction mechanism.

step 1

In the first step, Lewis acid attack on alkyl halide to form electrophilic carbocation.

Step 1 of Friedel Craft Alkylation

step 2

In second step, pi electrons of benzene ring attack on electrophilic carbocation to foam an intermediate. in this step aromaticity of the compound temporary lost due to breakage of double bond.

Step 2 of Friedel Craft Alkylation

Step 3

In the last step, deprotonation of carbon occur  to regenerate the aromaticity of benzene and also regenerate the catalyst.

 

Limitations

Carbocation Rearrangement

Carbocation formed in first step can rearrange to form unexpected products.

e.g

Deactivating Groups

Electron withdrawing group present on benzene ring deactivate it towards electrophilic aromatic substitution.

e.g

 

Reactive Product

The product formed is more reactive than initial reactant, as a result monoalkylated product is very difficult to form.Reactive product.

e.g

 

Limited to Benzene

These reactions only applicable to Benzene. Only electrophilic aromatic substitution reaction follow this mechanism.

 

Friedel-Craft Acylation

Friedel craft acylation is an electrophilic aromatic substitution reaction on benzene ring in which a proton of a benzene ring is substituted by an acyl group. This reaction occurs when benzene reacts with acyl chloride in the presence of strong lewis acid which acts as a catalyst.

Reaction

 

Mechanism

There are three steps involved in this reaction mechanism.

step 1

In the first step, Lewis acid attack on acyl halide to form electrophilic acylium ion.

 

step 2

In second step, pi electrons of benzene ring attack on electrophilic acylium ion to form an intermediate. in this step aromaticity of the compound temporary lost due to breakage of double bond.

 

step 3

In the last step, deprotonation of carbon occur  to regenerate the aromaticity of benzene and also regenerate the catalyst.

 

Limitations

Only Ketone

Friedel craft acylation is used to form ketones only. Because formyl chloride decompose under these reaction condition.

e.g

HCOH          CO    HCl

Deactivating Groups

Electron withdrawing group present on benzene ring deactivate it towards electrophilic aromatic substitution.

e.g

 

O and N acylation

If amine or hydroxyl group is present on benzene ring then O and N acylation occur instead of acylation on the benzene ring.

e.g

 

Comparison between Friedel-Craft Alkylation and Acylation

  • In alkylation the product formed is very reactive because of the substitution of alkyl group on benzene ring which act as activating group for electrophilic aromatic substitution. Whereas in acylation the product formed is less reactive due to presence of acyl group on benzene ring which act as deactivating group for electrophilic aromatic substitution.
  • In alkylation the carbocation rearrange to form stable carbocation. Whereas in acylation the acylium ion is stabilize by resonance

Concepts Berg

What is a Friedel-Crafts Reaction?

It is electrophilic aromatic substitution reaction which is use to substitute different groups on aromatic compound like benzene.

How is a Lewis acid used in Friedel Crafts acylation?

Lewis acid is used as a catalyst in friedel craft acylation. It help to generate a strong elctrophile.

Is Friedel Crafts acylation reversible?

What are the advantages of Friedel Crafts acylation?

Following are some advantages of friedel craft acylation

  • Carbocation formed is stabilized by resonance and cannot rearrange. So, long chain of alkyl group can be substituted
  • As acyl group deactivate the benzene ring. Hence, poly substitution is inhibited

What is Friedel Craft reaction with example?

What is alkylation of benzene?

when Alkyl group is substituted on benzene ring, it is called alkylation of benzene. Also known as friedel craft alkylation.

What is Friedel Craft alkylation and acylation reaction

How do you identify Friedel-Crafts reactions

Why anhydrous AlCl3 is used in Friedel-Crafts reaction

Anhydrous AlCl3 is used in friedel craft reaction because it electron deficient and act as lewis acid. If we add water, it foam aluminium hydroxide which will precipitate out and cannot act as a catalyst.

What is Friedel-Crafts alkylation mechanism

Why does pyridine not give a Friedel craft reaction

Pyridine have nitrogen with lone pairs, when Lewis acid like AlCl3 is added  in the reaction mixture. It foam a complex by donating lone pair of nitrogen to the Lewis acid. this deactivate pyridine towards electrophilic substitution reaction.

What are the limitations of Friedal Craft reactions

Where does the HCl come from in the Friedel Crafts acylation

in friedel craft reaction HCl is formed as a by product. benzene ring deprotonated to regain its aromaticity. this proton attache with the chlorine in Lewis acid complex ( AlCl4 ) to foam HCl and AlCl3 is regenerated.

Is nitrobenzene a suitable solvent for Friedel Crafts alkylation while benzene is not

What is alkylation of phenols

when we substitute alkyl group on phenol. Alkyl group substituent on oxygen atom of phenol instead of benzene ring. This is known as O alkylation.

Why can benzaldehyde not be synthesized by Friedel Crafts acylation

benzaldehyde cannot be synthesis by friedel craft acylation because the formyl chloride formed is very unstable and an not be isolated.

Why nitrobenzene does not undergo alkylation

Nitro group is electron withdrawing group and deactivate benzene ring towards electrophilic substitution reaction. So, nitrobenzene cannot undergo alkylation.

Why does aniline not undergo Friedel-Crafts reaction

Aniline have nitrogen with lone pairs, when Lewis acid like AlCl3 is added  in the reaction mixture. It foam a complex by donating lone pair of nitrogen to the Lewis acid. this deactivate aniline towards electrophilic substitution reaction.

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