Investigate The Relationship Between Structure And Properties Of Hydrocarbons - Session 3

Let’s discuss different types of electrophilic substitution reactions of benzene. But before proceeding, further can you tell what is meant by electrophilic substitution? The electrophile is an electron deficient specie that has attraction for electrons. The word substitution means replacement. So the electrophilic substitution of benzene means introduction of electrophile onto benzene by substitution of one or more atoms or groups in benzene. Substituted benzene is formed as a product.

Friedel Crafts alkylation is an electrophilic substitution reaction that involves the introduction of an alkyl group onto an aromatic compound. It is named after its discoverers, Charles Friedel and James Crafts. The reaction happens through the interaction of an aromatic compound, an alkyl halide and a Lewis acid catalyst. The products formed are alkylated aromatic compound and hydrogen halide. It is an electrophilic substitution reaction. In this reaction the alkyl group acts as electrophile. It substitutes the hydrogen atom of aromatic compound.

Let us take an example of benzene and methyl chloride to understand the mechanism of Friedel Crafts alkylation. We shall react the benzene with methyl chloride and aluminum chloride. Benzene is an aromatic compound. The methyl chloride acts as alkylating agent. Aluminum chloride acts as Lewis acid catalyst. The product formed is Toluene. Hydrogen chloride is also formed as a by product.

Now we shall discuss the detailed mechanism of Friedel Crafts alkylation of benzene. First step involves the activation of methyl chloride. Aluminum chloride reacts with methyl chloride to generate a reactive species called a methyl cation and AlCl₄⁻. Methyl cation acts as electrophile because it is electron deficient.

Next step is the formation of resonance stabilized carbocation. In this step, the generated methyl cation interacts with the benzene molecule. The double bond of the benzene attacks the methyl cation. Methyl cation is added to the benzene-ring. Resonance stabilized carbocation is formed.

Lets take AlCl₄⁻ as an example. It takes up the hydrogen ion from carbon atom of the carbocation to which alkyl group is attached. As a result the carbocation is converted into methylated benzene. This methylated benzene is called Toluene. After removing the hydrogen ion AlCl₄⁻ ion is converted into aluminum chloride and hydrogen chloride.

Friedel Crafts acylation is a reaction that involves the introduction of an acyl group onto an aromatic compound. The reaction happens through the interaction of an aromatic compound, an acyl chloride and a Lewis acid catalyst. R in the acyl chloride represents any alkyl group. The products formed are acylated aromatic compound and hydrogen halide. It is an electrophilic substitution reaction. In this reaction the acyl group acts as electrophile. It substitutes the hydrogen atom of aromatic compound.

Now we shall discuss the detailed mechanism of Friedel Crafts acylation of benzene. The reaction begins with the activation of the acyl chloride. A Lewis acid catalyst which is typically aluminum chloride is added to the reaction mixture. The aluminum chloride reacts with the acyl chloride to generate acylium ion and AlCl4- ion.

Next step is the formation of resonance-stabilized carbocation. In this step, the acylium ion interacts with the benzene molecule. The double bond of the benzene attacks the acylium ion. Acylium ion is added to the benzene-ring. Resonance-stabilized carbocation is formed.

Now lets consider AlCl₄⁻ ion. It takes up the hydrogen ion from carbon atom of the carbocation to which acyl group is attached. As a result the carbocation is converted into acylated benzene. Acylated benzene is a ketone. After removing the hydrogen ion AlCl₄⁻ ion is converted into aluminum chloride and hydrogen chloride. Can you tell which product will be formed if we carry out the Friedel Crafts Acylation of benzene with acetyl chloride CH₃COCl in the presence of AlCl₃?.

The nitration of benzene is a chemical reaction in which a -NO2 is introduced onto the benzene-ring. It is also an electrophilic substitution reaction. The reaction is carried out using a mixture of nitric acid and sulfuric acid This mixture is called nitrating agent. The product formed is nitrobenzene.

The reaction begins with the generation of the nitronium ion. Nitronium ion is the electrophilic species that is responsible for the nitration of benzene. Nitronium ion is produced by mixing concentrated nitric acid and concentrated sulfuric acid. The sulfuric acid gives a hydrogen ion to the nitric acid. As a result, nitronium ion and bisulfate ion are formed.

The nitronium ion acts as an electrophile. It attacks the electron rich benzene-ring. The benzene-ring donates a pair of electrons to the nitronium ion. This results in the formation of a sigma complex, also known as an arenium ion. The arenium ion is a resonance stabilized intermediate in which the positive charge is delocalized over the benzene-ring.

The aromatic system regains its stability by deprotonation of the arenium ion. Bisulfate ion abstracts a proton from the arenium ion. This results in the formation of nitrobenzene. Sulfuric acid is also formed as a by product.

The halogenation of benzene is also an electrophilic substitution reaction. It involves the introduction of halogen onto the benzene. Catalyst used in this process can be iron bromide or iron chloride. For example, during bromination of benzene, benzene is reacted with bromine in the presence of iron chloride. The product formed in this reaction is bromobenzene.

Let us discuss the mechanism of this reaction. First of all bromine reacts with iron bromide to form a complex. In this complex, iron bromide polarizes the terminal bromine atom. The terminal bromine atom becomes bromine cation. The benzene-ring donates a pair of electrons to the bromine cation. As a result carbocation is formed. After that, the FeBr₄⁻ ion takes up the hydrogen ion from carbon atom of the carbocation to which bromine atom is attached. This results in the formation of bromobenzene. Hydrogen bromide is formed as a by product.

The catalytic hydrogenation of benzene is a chemical reaction in which benzene reacts with hydrogen gas in the presence of a catalyst to form cyclohexane. Catalytic hydrogenation of benzene is not an electrophilic substitution reaction. It is an addition reaction. This reaction involves the addition of hydrogen atoms across the carbon carbon double bonds in benzene. Very finely divided nickel can be used as catalyst for this reaction.