Lecture on Benzene

Overview of Benzene

• Colorless Liquid: Benzene is a colorless liquid at room temperature.
• Boiling Point: Its boiling point is 80 degrees Celsius.
• Natural Occurrence: Found naturally in crude oil.
• Odor: Smells similar to petrol.

Molecular Structure

• Composition: Made up of six carbon atoms covalently bonded in a ring.
• Hydrocarbon: Each carbon atom is also covalently bonded to one hydrogen atom.
• Unpaired Electrons: Each carbon usually forms four single covalent bonds but in benzene, the fourth electron is conjugated.

Aromaticity

• Definition: Aromaticity refers to the conjugation in ring-shaped molecules where electrons have free movement around the ring.
• Conjugation: The unpaired electron from each carbon is conjugated into the ring, allowing free movement around all six carbons.
• Depiction: Various methods show benzene's aromaticity in textbooks, typically represented for convenience.

Safety Concerns

• Carcinogenic: Benzene can be carcinogenic (cancer-causing) at sufficient doses.

Benzene Reactions

• Addition vs. Substitution:
  • Addition Reactions: Difficult to perform as they destroy the aromaticity of the ring, requiring significant energy.
  • Substitution Reactions: More common where protons are replaced by other species, maintaining aromaticity.
  • Types of Substitution: Can lead to mono-, di-, or tri-substituted benzene.
• Electrophile Requirement:
  • Electrophiles: Required for substitution reactions due to benzene's stable electron-rich nature.
  • Characteristics: Electrophiles are often positively charged species attracted to benzene's electrons.
  • Examples:
    • Chlorine Cation (Cl⁺)
    • Nitronium Ion (NO₂⁺): Used in reactions to form compounds like trinitrotoluene (TNT).

Practical Applications

• Drug Synthesis: Benzene is used in joining rings to other organic molecules to create important drugs such as aspirin.
• Explosives: Reaction with NO₂⁺ forms trinitrotoluene, known as TNT, a powerful explosive.