Halogenoalkanes

• Halogen alkanes are alkanes that have one or more halogens attached
• Naming halogen alkanes


  1. Find the longest carbon chain

  2. The longest carbon chain forms the last part of the name

  3. The positions of the halogens are indicated by prefixes (fluoro, chloro, bromo, iodo)

  4. Prefixes are in alphabetical order

  5. Prefixes like di, tri, tetra are used for multiple halogens of the same type
• Boiling point trends of halogen alkanes

  • Boiling points increase going down the group
  • This is due to stronger intermolecular van der Waals forces as the halogen atoms get larger
• Bond polarity in halogen alkanes

  Halogen alkanes have polar bonds due to the electronegativity difference between the carbon and halogen
• Nucleophiles


  Substances that are electron pair donors, e.g. cyanide, ammonia, hydroxide
• Reaction of halogen alkanes with hydroxide ions

  1. Conditions: warm aqueous sodium hydroxide, under reflux

  2. Mechanism: nucleophilic substitution, hydroxide ion attacks the carbon, displacing the halogen
• The overall reaction of halogen alkanes with hydroxide ions forms an alcohol and a salt
• Reaction of halogen alkanes with cyanide ions

  1. Conditions: warm ethanolic potassium cyanide, under reflux

  2. Mechanism: nucleophilic substitution, cyanide ion attacks the carbon, displacing the halogen
• The overall reaction of halogen alkanes with cyanide ions forms a nitrile
• Reaction of halogen alkanes with ammonia

  1. Conditions: heated with ethanolic ammonia, excess ammonia

  2. Mechanism: nucleophilic substitution, ammonia attacks the carbon, displacing the halogen

  3. Second step: another ammonia molecule acts as a base, accepting a proton to stabilise the intermediate
• The overall reaction of halogen alkanes with ammonia forms an amine
• Ammonia


  Acts as a base, a proton acceptor
• Amine formation


  1. Ammonia acts as a nucleophile with a lone pair

  2. Ammonia acts as a base, accepting a proton

  3. Electrons shuffle to stabilize the nitrogen

  4. Amine is produced
• Amine


  Compound with a nitrogen atom connected to hydrogen atoms or organic groups
• Halogen alkanes


  • Become more reactive as you go down the group
  • Reactivity determined by bond strength/enthalpy, not bond polarity
• Halogen alkane reaction with hydroxide ions


  1. Reaction occurs in ethanol solvent

  2. OH- acts as a base, removing hydrogen adjacent to halogen

  3. Electrons form a double bond, breaking the C-halogen bond

  4. An alkene is produced
• Solvent used


  Determines if reaction is elimination (ethanol) or substitution (water)
• CFCs


  • Chlorofluorocarbons, used as refrigerants and propellants
  • Breakdown ozone layer when exposed to UV radiation
• CFC breakdown mechanism


  1. UV breaks C-Cl bond, forming radicals

  2. Cl radicals react with ozone, catalyzing its breakdown

  3. Termination occurs when two radicals combine
• CFCs are now banned due to ozone depletion
• Alternatives to CFCs


  HFCs and hydrocarbons are safer options