Halogenoalkanes Revision Guide

Naming Halogenoalkanes

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

Classifying Halogenoalkanes

• Primary: One carbon attached to the carbon atom adjoining the halogen.
• Secondary: Two carbons attached to the carbon atom adjoining the halogen.
• Tertiary: Three carbons attached to the carbon atom adjoining the halogen.

Reactions of Halogenoalkanes

Nucleophilic Substitution Reactions

• Nucleophile: Electron pair donator (e.g., :OH⁻, :NH₃, CN⁻).
• Substitution involves swapping a halogen for another atom/group.
• Reactivity depends on the C-X bond strength:
  • C-I (238 kJ/mol) is weakest and fastest to substitute.
  • C-F (484 kJ/mol) is strongest and slowest (unreactive).
• Mechanism involves nucleophiles attacking the positive carbon atom.
• Curly arrows show electron movement.

Hydrolysis Reactions

• Water as a poor nucleophile, reacts slowly in substitution.
• Hydrolysis: Splitting a molecule with water.
• Silver nitrate test: forms silver halide precipitate indicating reaction rate.
  • Iodoalkane forms precipitate fastest.
• Precipitate colors:
  • AgI - yellow
  • AgBr - cream
  • AgCl - white

Nucleophilic Substitution with Hydroxide Ions

• Change in functional group: Halogenoalkane to alcohol.
• Reagent: Potassium (or sodium) hydroxide.
• Conditions: Aqueous solution, warm.
• Mechanism: Nucleophilic substitution.
• Solvent change to ethanol leads to elimination.

Alternative Mechanism for Tertiary Halogenoalkanes

• Carbocation intermediate forms via Br breaking away.
• Hydroxide nucleophile attacks the carbocation.
• Stabilization by methyl groups prevents direct nucleophile attack.

Nucleophilic Substitution with Cyanide Ions

• Change in functional group: Halogenoalkane to nitrile.
• Reagent: KCN in ethanol/water.
• Conditions: Heating under reflux.
• Increases carbon chain length.

Nucleophilic Substitution with Ammonia

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

Elimination Reactions

• Removal of a small molecule (often water).
• Change in functional group: Halogenoalkane to alkene.
• Reagents: Potassium (or sodium) hydroxide in ethanol.
• Mechanism: Elimination.
• Structural isomers may form.
• Solvent affects reaction type.

Ozone Chemistry

• Ozone layer filters UV radiation.
• CFCs cause ozone depletion.
• Chlorine radicals catalyze ozone decomposition.
• Overall reaction: 2 O₃ → 3 O₂
• Legislation to ban CFCs supported by chemists.
• Alternatives: HFCs (e.g., CH₂FCF₃) that do not affect ozone.

Uses of Halogenoalkanes

• Used as solvents in organic reactions.
• Chloroalkanes and chlorofluoroalkanes are common examples.