15.1 The chemistry of the haloalkanes

• Haloalkanes have the general formula CnH2n+1X where X is a halogen atom
• Haloalkanes with more than 2 carbon atoms can exist as structural isomers
• A haloalkane has a polar C-X bond because halogen atoms are more electronegative than carbon
• The C-X bond polarity decreases down the group as the electronegativity of the halogen decreases
• The polarity of the C-X bond attracts nucleophiles to the carbocation
• A nucleophile is an atom or group of atoms that donates an electron pair to an electron-deficient centre
• The C-X bond enthalpy decreases down the group
• In the nucleophilic substitution of a haloalkane, a nucleophile replaces the halogen in the haloalkane which is lost as a halide ion
• Nucleophilic substitution mechanism

  • The nucleophile donates a pair of electrons to the slightly positive carbon atom (arrow drawn from nucleophile to the carbon)
  • Arrow drawn from the middle of the slightly positive carbon bond to the slightly negative halogen
  • A new bond forms with the carbon atom
  • The carbon-halogen bond breaks by homolytic fission
  • Alcohol is formed
• Haloalkanes can also be hydrolysed by water in the presence of aqueous silver nitrate to form alcohols