Aromatic Compounds
25min Part 4 / Ch8 / Lesson 1
Prerequisites: 4-7-2
Objectives
- Describe the structure and properties of benzene
- Understand typical reactions of aromatic compounds
- Know properties of phenol, aniline, and benzoic acid
Structure of Benzene
Benzene :
- 6 carbon atoms form a regular hexagon
- All C-C bonds are equivalent (intermediate between single and double)
- Undergoes substitution reactions readily (resists addition)
Classification of Aromatic Compounds
| Compound | Structure | Properties |
|---|---|---|
| Phenol C₆H₅OH | Benzene + OH | Weakly acidic; purple with FeCl₃ |
| Aniline C₆H₅NH₂ | Benzene + NH₂ | Weakly basic; red-purple with bleach |
| Benzoic acid C₆H₅COOH | Benzene + COOH | Acidic (stronger than phenol) |
| Toluene C₆H₅CH₃ | Benzene + CH₃ | Organic solvent |
| Nitrobenzene C₆H₅NO₂ | Benzene + NO₂ | Almond-scented liquid |
Representative aromatic compounds
Properties of Phenol
Phenol C₆H₅OH:
- Weakly acidic (weaker than carbonic acid: dissolves in NaOH but does NOT release CO₂ with NaHCO₃)
- Gives a purple color with solution (detection test for phenols)
- Has antiseptic properties
Reactions of Benzene
| Reaction | Conditions | Product |
|---|---|---|
| Nitration | Conc. HNO₃ + conc. H₂SO₄ | Nitrobenzene C₆H₅NO₂ |
| Sulfonation | Conc. H₂SO₄ (heated) | Benzenesulfonic acid C₆H₅SO₃H |
| Halogenation | Cl₂ + FeCl₃ (catalyst) | Chlorobenzene C₆H₅Cl |
Check Your Understanding
Q1 Benzene preferentially undergoes:
Q2 Which compound gives a purple color with FeCl₃ solution?
Q3 How strong an acid is phenol?
Exercises
Q1. Rank phenol, acetic acid, and ethanol by acid strength, and describe their reactivity with NaOH and NaHCO₃ solutions.
Solution
Acid strength: Acetic acid > Phenol > Ethanol
| Substance | Reacts with NaOH | Reacts with NaHCO₃ |
|---|---|---|
| Acetic acid | Yes (neutralization) | Yes (CO₂ released) |
| Phenol | Yes (neutralization) | No (weaker than carbonic acid) |
| Ethanol | No (neutral) | No |
Acetic acid is stronger than carbonic acid, so it reacts with NaHCO₃. Phenol is weaker than carbonic acid, so it does not.