Friedel Craft Alkylation Reaction of Benzene | Mechanism

Benzene forms an alkyl banzene in friedel craft alkylation reaction. When benzene is treated with alkyl halide in the presence of alnhydrous aluminium chloride, alkyl benzene is given as the product. This reaction is called Friedel-Crafts alkylation reaction.

A hydrogen atom in the benzene ring is replaced by alkyl group.

Benzene and methyl chloride in the presence of anhydrous aluminium chloride

Methyl benzene (toluene) is given as the product. As a by-product, HCl is produced in the reaction of benzene and methyl chloride in the presence of anhydrous aluminium chloride.

Benzene and ethyl bromide in the presence of anhydrous aluminium chloride

Ethyl benzene is given as the product. As a by-product, HBr is produced in the reaction of benzene and ethyl bromide in the presence of anhydrous aluminium chloride.

Importance of AlCl₃

In Friedel craft alkylation, AlCl₃ behaves as a catalyst. AlCl₃ helps to start the reaction and after the reaction, it is generated again.

Mechanism of Friedel Craft Alkylation Reaction of Benzene

This reaction mechanism contains three steps. An alkyl carbonium ion is generated as the electrophile to start the reaction. Anhydrous aluminium chloride helps to formation of alkyl carbonium ion (electrophile). This reaction is an electrophilic substitution reaction.

Benzene and methyl chloride reaction mechanism

Steps of mechanism is explained below.

Formation of electrophile

Due to presence of anhydrous AlCl₃, C-Cl bond of methyl chloride (CH₃Cl) is broken and forms AlCl₄^- and methyl carbocation (⁺CH₃). Methyl carbocation is the electrophile which drives the reaction furthermore by reacting with benzene ring.

Benzene ring attacks electrophile

Benzene ring attacks the methyl carbocation (electrophile) to form intermediate carbocation. The positive charge is spread through the benzene ring as in the figure.

Due to spread of positive charge through benzene ring, the carbocation get considerable stability.

Intermediate carbocation loses a proton to AlCl₄^-

Intermediate carbocation loses its proton which is connected to the carbon atom where methyl group also connected, to give methyl benzene. As a by product HCl is given. AlCl₃ is generated again because it is a catalyst in this reaction.

Benzene alkylation with higher alkyl halides

Until now, we have discussed only, alkylation of benzene with ethyl bromide and methyl bromide. Now, we are going to discuss alkylation with alkyl halides which have three or more than three carbon atoms.

In earlier section, you have learnt, in the alkylation mechanism, a primary carbocation is formed as the electrophile. But primary cations are unstable when it compares with secondary and tertiary carbocations.

Why primary carbocation is less stable than second and tertiary carbocations?

You know alkyl groups are electron withdrawing groups. In secondary and tertiary carbocations, there are two and three alkyl groups respectively.

When number of alkyl groups around the carbocation carbon atom increase, the density of positive charge decreases resulting higher stability.

If, primary carbocation is formed at initial stage, it is changed to more stable secondary carbocation (2⁰) or tertiary carboncation (3⁰) according the structure of the compounds through a rearrangement reaction.

So lets see, alkylation between n-Propyl chloride and benzene.

Alkylation of n-Propyl chloride and benzene

2-phenylpropane is given as the product.

Questions

What is the product when toluene reacts with methyl chloride in the presence of anhydrous aluminum chloride?

Toluene (methyl benzene) is an ortho para director. So it can give two products when reacts with methyl chloride in the presence of anhydrous aluminum chloride. As products, 1,4-Dimethylbenzene and 1,2-Dimethylbenzene are given as a mixture.

What will happen in the alkylation of benzene with 2-bromopropane?

Think about the first step of what kind of carbocation is generated in the initial stage. When you study the mechanism, it is clear that, secondary carbocation is generated.

Can this secondary carbocation become a tertiary carboction? When examing the given carbocation there is no chance to become a tertiary carbocation because none of the carbon atom is joint with three carbon atoms.

So. as the product, 2-phenylpropane is given. This is the same product which is given by n-propyl chloride (discussed earlier in this tutorial).

uop alkylation for alkyl benzene?

Alkyl benzene compounds can go for alkylation and give ortho and para products.