Organic Chemistry: 3 Steps in Electrophilic Substitution (Aromatic) Reaction Mechanism

It is important that all JC A-Level H2 Chemistry students understand Electrophilic Substitution (Aromatic) Reaction Mechanism of Arenes. In this YouTube Video, you will learn the step-by-step mechanism with Benzene C₆H₆ as an example. We always emphasised this mechanism in our recommended H2 Chemistry Tuition Classes in Singapore whereby i’m the principal Chemistry tutor.

Click on the following link for Video on Electrophilic Aromatic Substitution Reaction Mechanism.

H2 Chemistry Video on Electrophilic Aromatic Substitution Reaction Mechanism

Length of video: 9.33 mins

Let’s recap what we have learned from the above video.

First, we look at the meaning of arenes: they are aromatic hydrocarbons and the simplest and most common example is benzene ring, C₆H₆.

Benzene molecule is planar, symmetrical and non-polar. In the hexagonal ring, there are 3 alternating C=C bonds. Benzene is stabilised as a result of delocalisation of these π electrons in the C=C bonds.

Hence, benzene is resistant to oxidation reaction and electrophilic addition reaction, because these reactions would destroy the cyclic π resonance stabilised system.

Instead, benzene readily undergoes Electrophilic Substitution Reactions in which the ring system is preserved.

Benzene molecule is “electron-rich” and thus reactions of benzene would involve the attack by electrophiles on the π electrons of the benzene ring.

3 Steps in Electrophilic Substitution Reaction Mechanism:

1. Production of electrophile i.e. electron deficient species which are “electron-loving”

2. Formation of carbocation intermediate from the attack of electrophile on the benzene ring

3: Expulsion of H⁺ ion from the carbocation intermediate to form the aromatic product and the delocalised π system is preserved i.e. benzene ring is re-formed

In the next two blog posts on Organic Chemistry, we will take a look at the two most common reactions (in Singapore’s JC GCE A-Level H2 Chemistry Exam Syllabus Code 9729) involving Electrophilic Substitution Reaction Mechanism of Arenes, namely:

a) Halogenation – Substitution reactions with Chlorine and with Bromine
b) 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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PS: Under related articles below, there are several blog post discussions and questions related to other Organic Chemistry concepts. You can also do a keyword search using the search box at the top right hand corner.

Related Articles:

• Organic Chemistry: Halogenation of Benzene via Electrophilic Aromatic Substitution
• Organic Chemistry: Nitration of Benzene via Electrophilic Aromatic Substitution Reaction Mechanism
• Organic Chemistry: Key Concepts in Introductory Topics, Alkanes, Alkenes & Arenes
• Organic Chemistry: Types of Formulae
• Organic Chemistry: Electrophilic Addition Mechanism Video

Sean Chua

Winners Education Centre Pte. Ltd.

Many know me as the Ten Year Series book author for JC A-Level H2 Chemistry and O-Level Pure Chemistry. My 20+ years of coaching experience (since 1999) with more than 2100 students from almost every JCs and Secondary Schools has allowed me to understand the true reasons why students are not able to perform well in Chemistry. Most importantly, my strength lies in using everyday analogies (even grandmothers can understand!) to simplify abstract concepts. Also, my teaching methodology has been designed to be fun, enjoyable and effective for my students.

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