Halogenoalkanes Lecture Notes

Halogenoalkanes

Elimination Reactions

• Definition: When a halogenoalkane is mixed with hydroxide ions and heated under alcoholic conditions, elimination occurs.
• Reaction Mechanism:
  • Hydroxide ion acts as a base, accepting a proton and removing a hydrogen atom from the halogenoalkane.
  • This results in the elimination of a halide ion and the formation of a double bond (C=C).
  • Example Reaction:
  • HBr + NaOH (alcoholic) → C=C + Br⁻ + H₂O

Conditions for Elimination

• Reflux: The reaction is conducted under reflux, ensuring no loss of volume.
  • Equipment includes:
  • Vertical condenser to condense vapours back into the liquid.
  • Anti-bumping granules to ensure even heating.

Nucleophilic Substitution

• Polar Bonds: Halogenoalkanes contain polar bonds due to the higher electronegativity of halogens compared to carbon atoms, creating
  • Regions of partial positive (0C) and negative charge (X).
• Mechanism:
  • Nucleophiles attack the carbon atom of halogenoalkanes, displacing the halogen.
  • Produces alcohols or amines.
  • Example for alcohol formation:
    • H-C(Cl)(C-H) + :OH → H-C(OH)(C-H) + Cl⁻

Bond Enthalpy and Reaction Rate

• Bond Enthalpy: Higher relative atomic mass of halogen = lower bond enthalpy = faster reaction rate.
  • Example: Iodine (I) has a lower bond enthalpy than fluorine (F), making iodoalkanes react faster than fluoroalkanes.

Reflux Process

• Mechanics:
  • Vapours evaporate, condense, and return to the reaction mixture.
• Specific Equipment:
  • Liebig condenser, round-bottom flask, and anti-bumping granules.

Testing for Halogenoalkanes

• Procedure:
  1. Warm halogenoalkane with sodium hydroxide, ethanol, and water for hydrolysis.
  2. Add dilute nitric acid to neutralize excess sodium hydroxide.
  3. Introduce silver nitrate solution to form colored precipitates with halide ions:
  • Chloride: white
  • Bromide: cream
  • Iodide: yellow
  1. Ammonia may be used for further testing if the color is not distinct.

Uses of Halogenoalkanes

• CFCs (Chlorofluorocarbons):
  • All hydrogen atoms replaced with chlorine and fluorine.
  • Used in refrigerants, foamed plastic production, dry cleaning solvents, and aerosols due to low flammability and toxicity.
• Environmental Impact:
  • CFCs absorb UV radiation, leading to the formation of free radicals which contribute to ozone depletion:
  • Overall reaction affecting ozone (O₃):
    • O₃ + UV → O₂ + O•
    • Facilitating harmful reactions that degrade the ozone layer.

Importance of Ozone Layer

• Protects earth from harmful UV radiation (prevents sunburn and skin cancer).
• Ozone formed from oxygen free radicals in the atmosphere.
• Reactions contributing to ozone maintenance and breakdown are critical for environmental health.