In the previous video on JC2 A-Level H2 Chemistry Organic Chemistry, i have discussed about the stability of an aromatic ring and the 3 Steps involved in Electrophilic Aromatic Substitution Reaction Mechanism. Do check them out if you have missed that.
Today, we will take a look at how Halogenation of Benzene proceeds via Electrophilic Aromatic Substitution Mechanism.
The example i am going to use will be Chlorination of Benzene, which involves the following three steps:
Step 1: Production of the electrophile, Cl+ ion, from Cl2 and a Lewis Acid Catalyst (also known as Halogen Carrier) e.g. AlCl3
Common Lewis Acid Catalysts are AlCl3, AlBr3, FeCl3, FeBr3, Fe, Al
Step 2: Formation of the carbocation intermediate from the attack of the Cl+ on the benzene ring
This is the slow rate-determining step since it involves the formation of unstable carbocation intermediate from a stable aromatic compound
Step 3: Expulsion of H+ ion from the carbocation intermediate to form the aromatic chlorobenzene and the delocalised pi (π) system is preserved i.e. benzene ring is re-formed
This is the fast step since it involves the formation of stable aromatic chlorobenzene from an unstable carbocation intermediate
Click on the following link for Video on Halogenation of Benzene via Electrophilic Aromatic Substitution Reaction Mechanism.
Length of video: 6.25 minutes
Similarly, Bromination of Benzene will be carried out in a same way. Benzene reacts with Br2 in the presence of FeBr3 to give bromobenzene and steamy white fumes of HBr can be detected. This is another common A-Level H2 Chemistry examination question when it comes to halogenation of benzene.
Jim Clark from chemguide.co.uk has also written a short and concise blog post on Halogenation of Benzene. I have recommended students from my weekly JC A-Level H2 Chemistry tuition classes to take a look.
In the next video on Organic Chemistry, we will take a look at another common reaction (in Singapore’s JC GCE A-Level H2 Chemistry) involving Electrophilic Substitution Reaction Mechanism of Arenes, namely: Nitration – Substitution reactions with a Nitration Mixture i.e. Conc. H2SO4 and Conc. HNO3.
I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends.
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