Back to: Organic Chemistry 200 Level
Welcome to class!
Hello, superstar! It’s great to see you back and eager to learn. You’re already familiar with the basics of aromatic compounds — especially benzene and why it’s so stable. In this lesson, we’re going a little deeper. We’ll be talking about how aromatic compounds react and why those reactions are different from the usual ones you see with other organic compounds. Don’t worry — I’ll walk you through it step by step, with simple examples that make it easy to understand and remember.
Aromatic Compounds II
Why Do Aromatic Compounds React Differently?
Remember from the last class: benzene has a delocalised electron cloud spread evenly around its ring. That’s what gives it aromaticity — an unusual type of chemical stability. Because of this stability, benzene doesn’t like to lose or gain atoms easily. So instead of addition reactions, which would break up its stable ring, it prefers substitution reactions.
Let’s look at some of these substitution reactions.
1. Electrophilic Substitution Reactions
This is the main type of reaction that aromatic compounds like benzene undergo. In this reaction, an electrophile (an electron-seeking species) replaces one of the hydrogen atoms on the benzene ring.
Here are some important types:
a. Nitration of Benzene
Reagent: Concentrated nitric acid (HNO₃)
Catalyst: Concentrated sulphuric acid (H₂SO₄)
Condition: Warm (about 50°C)
Product: Nitrobenzene
Equation:
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
Use: Nitrobenzene is used in making explosives like TNT and in the production of dyes.
b. Halogenation of Benzene
Reagent: Chlorine or bromine gas
Catalyst: Iron (Fe) or iron(III) chloride (FeCl₃)
Product: Chlorobenzene or Bromobenzene
Equation:
C₆H₆ + Cl₂ → C₆H₅Cl + HCl
Use: Halogenated benzenes are used in pesticides and medicines.
c. Alkylation (Friedel-Crafts Reaction)
Reagent: An alkyl halide (e.g., CH₃Cl)
Catalyst: Aluminium chloride (AlCl₃)
Product: Methylbenzene (Toluene)
This reaction is important in the petrochemical industry, where aromatic compounds are modified to make fuels and synthetic materials.
2. Effect of Substituents on the Benzene Ring
Once a hydrogen is replaced with another group (like -OH, -NO₂, -CH₃), it can change how the ring behaves. Some groups activate the ring (make it more reactive), while others deactivate it.
Activating groups (e.g., -OH, -CH₃): Make further substitution easier and direct new substituents to positions 2 and 4 (ortho and para positions).
Deactivating groups (e.g., -NO₂): Make further substitution harder and direct new substituents to position 3 (meta position).
This is important in making more complex aromatic compounds like aspirin, paracetamol, and aniline dyes.
Industrial Applications of Aromatic Compounds
Used in pharmaceuticals (e.g., aspirin, paracetamol)
In dye production (e.g., azo dyes for fabrics)
In agrochemicals (e.g., herbicides, insecticides)
As solvents in laboratories and industries
Summary
- Aromatic compounds undergo electrophilic substitution reactions rather than addition reactions to preserve their stable ring.
- Common substitutions include nitration, halogenation, and alkylation.
- Substituents can activate or deactivate the ring, affecting where new groups attach.
- These reactions have wide applications in medicine, agriculture, fuel, and synthetic industries.
Evaluation
- Why doesn’t benzene undergo addition reactions easily?
- Name the reagents and conditions for the nitration of benzene.
- What type of reaction is used in the Friedel-Crafts alkylation?
- Give one example each of an activating and a deactivating group on the benzene ring.
Amazing work! You’ve just uncovered how aromatic compounds — especially benzene — participate in reactions that make them so valuable to industries and everyday life. With this knowledge, you’re building a solid foundation in Chemistry. Keep going strong, and remember, Afrilearn is always here to guide you every step of the way. See you in the next exciting lesson!