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Friedal Craft’s Alkylation, Acylation and their Mechanism

Friedal Craft’s Alkylation:

It is one of the most familiar reactions in organic chemistry. It is an electrophilic aromatic substitution reaction. The alkylation of benzene or other aromatics in the presence of Lewis acid like AlCl3 or FeCl3 as a catalyst is known as Friedal Craft’s Alkylation. Actually, one of the Hydrogen atoms of benzene ring or aromatic nucleus is replaced with alkyl group(-R). This reaction takes place as normal practice. because aromatic substrate or benzene is electron rich and carbonium ion generated is the strong electrophile and there occurs normal interaction between them and substitution takes place.

Mechanism of Reaction:

Generation of electrophile: (step 1)

Electrophile used here is carbonium ion. Carbonium ion can be generated by using alkyl halides, alcohols, esters and alkenes in the presence of different reagents. In Friedal Craft’s Alkylation the reagent used is alkyl halide in the presence of Lewis acid catalyst. Since, carbon of alkyl halide contains partial positive charge due to the presence of more electronegative halogen atoms. However, carbon is not enough electrophilic to be reacted with aromatic compounds which are not stronger nucleophiles. So, reagent like Lewis acid is used to make it good electrophile.

From alkenes, alcohols and esters carbonium ion can be generated by simple protonation using proton acids like phosphoric acid, hydrochloric acid or less likely sulfuric acid (which may cause different sulphonation products).

In case of primary or secondary alkyl halides carbonium ion is not enough stable and remains in the form of complex with AlCl3. However, in case of tertiary alkyl halides carbonium is stable and contains full fledge positive charge. Generation of electrophile is shown as:

Formation of sigma complex: (step 2)

Second step is the formation of sigma complex between carbonium ion and aromatic species after passing through a transition state which is known as pi complex that is just an interaction between aromatic substrate and carbonium ion. Sigma complex formation is the rate determining step of the reaction because it is the slowest step since automatic substrate and carbonium electrophile both are less reactive. Sigma complex is also known as arenium ion or Whealand intermediate. Formation of sigma complex is shown as:

Aromatization: (step 3)

Third step is the regain of aromaticity. (AlCl4) acts as a base and abstracts proton from the ring and retains the aromaticity back again. Regain of aromaticity is the driving force for overall reaction. i.e.

Drawbacks of Friedal Craft’s Alkylation:

There are several drawbacks of Friedal Craft’s Alkylation:

  • Polyalkylation is the major drawback of Friedal Craft’s Alkylation. The reason is that after alkylation aromatic species is more reactive than the reactant and again reacts with alkyl halide and Lewis acid to give further alkylation and it finally results in polyalkylated product.

For example: In case of benzene, first toulene is formed with methyl chloride and AlCl3 and further alkylation occurs and mixture of products including tetra substituted product durene is formed. However, we can obtain monoalkylated product by using aromatic substrate in excess.

  • Activated aromatic rings having electron donating groups with lone pair of electrons do not undergo alkylation easily. For example: Phenol because it reacts with Lewis acid at oxygen and resultant compound is slightly soluble in the reaction mixture and thus, it reacts slowly. However, methyl ethers of phenol give satisfactory yield of alkylation. This issue can also be resolved by protecting oxygen with acetic anhydride.

Aromatic amines does not undergo alkylation because they have high potential to react with Lewis acids. i.e.

  • Deactivated aromatic rings having strongly deactivating groups such as nitro, cyano, carboxyl etc. do not undergo alkylation under normal conditions since Carbocations are week electrophiles. i.e.
  • Rearrangement: Large primary and secondary alkyl halides give carbocation which undergo rearrangement and gives isomerized product which competes with normal product and in this way a mixture of products is obtained making the reaction less useful. i.e.

 We can get large normal product such as n-propyl benzene by introducing first acyl groups and then Clemmenson’s reduction.

  • Alkylation is reversible: Tertiary butyl group can be easily added to the benzene and it can also be removed easily in the presence of hydrochloric acid in addition to aluminium trichloride. Thus, it is a reversible reaction.  Moreover, when ethyl bromide reacts with benzene in the presence of aluminium tribromide we get 1,3,5- tribromo benzene which is in 87% and stable product. So, alkylation is reversible. i.e.

However, t-butyl group is used as a protecting group.

Friedal Craft’s Acylation:

It is an electrophilic aromatic substitution reaction in which acylation of aromatic substrate occurs in the presence of Lewis acid such as AlCl3 as a catalyst.

Mechanism of Reaction:

Mechanism of reaction consists of three steps:

  • First step is the generation of Acylium ion which plays the role of electrophile. I.e.
  • Second step is the formation of Sigma complex intermediate through Pi complex which is the the simply an interaction between aromatic substrate and Acylium ion.
  • Third step is the regain of aromaticity and it is the driving force of overall reaction.

Differences between Friedal Craft’s Alkylation and Friedal Craft’s Acylation:

  • In Friedal Craft’s Alkylation, primary alkyl halides give rearranged products because the formation of product depends upon the stability of carbocation but in Friedal Craft’s Acylation no such rearrangement occurs because all Acylium ions are equally stable.
  • In Friedal Craft’s Acylation, aromatic substrate is needed in slightly more amount as compared to Friedal Craft’s Alkylation since firstly complexation between Lewis acid and acyl chloride occurs and then further reaction.
  • Alkylated Benzene further gives ortho or para product but acylated benzene further gives meta product.

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Muhammad Asif

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