Lecture 14: Aromatic Chemistry

Aromatic Compounds

Benzene Overview

  • Formula: C6H6
  • Structure:
    • Arranged in a hexagonal ring.
    • Each carbon forms single bonds to two other carbon atoms.
    • Contains six delocalized electrons, contributing to stability.
  • Model: Known as the Kekulé model.
  • Stability: Benzene is a very stable aromatic compound due to its ring of delocalized electrons.

Representation of Benzene

  • Benzene can be represented in various ways, including:
    • Kekulé structure (alternating double bonds)
    • Other structural illustrations emphasizing the delocalization of electrons.

Aromatic Compounds (Arenes)

  • Definition: Compounds containing one or more benzene rings.
  • Examples: Include various derivatives of benzene (e.g., toluene, phenol).

Reactions of Benzene Derivatives

  • Key reactions include:
    • Combustion
    • Electrophilic Substitution
    • Halogenation
    • Nitration

Combustion of Benzene

  • Usage: Found in crude oil and used in various fuels.
  • Complete Combustion:
    C6H6 + 7.5 O2 ightarrow 6 CO2 + 3 H_2O
  • Incomplete Combustion:
    C6H6 + 6 O2 ightarrow 3 CO + 3 CO2 + 3 H_2O
  • Health Risks: Benzene is harmful; long-term exposure can cause severe health issues.

Electrophilic Substitution Reactions

  • General Mechanism: Aromatic rings can react with electrophiles, following a specific substitution pattern. The variation between reactions lies in how the electrophile is generated.
  • Attraction: Electrophiles are attracted to electron-rich parts of a molecule.

Halogenation Reaction

  • General Reaction:
    C6H6 + X2 ightarrow C6H5X + HX where X can be Chlorine (Cl2), Bromine (Br2), or Iodine (I2).
  • Catalyst: An aluminum halide (AlX3) is used as a catalyst.
  • Mechanism Steps:
    1. Making the electrophile
    2. Electrophilic substitution occurs.

Nitration Reaction

  • Reactants: Uses nitric acid (HNO3) in the presence of sulfuric acid (H2SO_4) as a catalyst.
  • Reaction Equation:
    C6H6 + NO2^+ ightarrow C6H5NO2 + H^+
  • Mechanism Steps:
    1. Electrophile generation:
    • HNO3 + H2SO4 ightarrow NO2^+ + HSO4^- + H2O
    1. Electrophilic substitution step follows.