Halogenoalkanes Revision Guide

Naming Halogenoalkanes

• Based on original alkane with a prefix for halogen:
  • Fluoro for F
  • Chloro for Cl
  • Bromo for Br
  • Iodo for I
• Examples:
  • 1-bromopropane
  • 2-chloro-2-methylbutane
• Substituents are listed alphabetically.

Classifying Halogenoalkanes

• Primary: One carbon attached to the C-X functional group.
• Secondary: Two carbons attached to the C-X group.
• Tertiary: Three carbons attached to the C-X group.

Reactions of Halogenoalkanes

Nucleophilic Substitution Reactions

• Nucleophile: Electron pair donor (e.g., :OH⁻, :NH₃, CN⁻).
• Halogenoalkanes undergo substitution or elimination reactions.
• Substitution: Swapping a halogen atom for another atom/group.
• Rate of reaction depends on the strength of the C-X bond:
  • Weaker bonds break easier, react faster.
  • C-I (238 kJ/mol) is weakest; C-F (484 kJ/mol) is strongest.
• Mechanisms show the movement of electrons using curly arrows.

Hydrolysis Reactions

• Water as a poor nucleophile reacts slowly with halogenoalkanes.
• Hydrolysis: Splitting a molecule with water.
• Use of aqueous silver nitrate to form silver halide precipitate:
  • AgI forms first (yellow) due to weakest C-I bond.
  • AgBr (cream), AgCl (white) form slower.
• Faster precipitate formation indicates higher reactivity.

Nucleophilic Substitution with Aqueous Hydroxide Ions

• Change: Halogenoalkane to alcohol.
• Reagent: Potassium (or sodium) hydroxide.
• Conditions: Warm, aqueous solution.
• Tertiary halogenoalkanes form a carbocation intermediate.

Nucleophilic Substitution with Cyanide Ions

• Change: Halogenoalkane to nitrile.
• Reagent: KCN in ethanol/water mixture.
• Conditions: Heating under reflux.

Nucleophilic Substitution with Ammonia

• Change: Halogenoalkane to amine.
• Reagent: NH₃ in ethanol.
• Conditions: Heating under pressure.
• Use excess ammonia to minimize further substitution reactions.

Elimination Reaction of Halogenoalkanes

• Elimination: Removal of a small molecule (often water).
• Reagents: Potassium (or sodium) hydroxide in ethanol.
• Produce alkenes; unsymmetrical halogenoalkanes can form isomers.
• Reaction type influenced by solvent:
  • Aqueous favors substitution.
  • Alcoholic favors elimination.

Ozone Chemistry

• Ozone layer filters UV radiation.
• Chlorofluorocarbons (CFCs) damage ozone, forming chlorine radicals.
• Reactions of chlorine radicals deplete ozone:
  • Cl· + O₃ → ClO· + O₂
  • ClO· + O₃ → 2O₂ + Cl·
• Overall: 2 O₃ → 3 O₂
• C l radicals regenerate, catalyzing further ozone degradation.
• Legislation to ban CFCs led to development of HFCs as safer alternatives.

Uses of Halogenoalkanes

• Chloroalkanes and chlorofluoroalkanes used as solvents.