Halogenalkanes 2.6

• Types of halogenoalkanes


  • Primary
  • Secondary
  • Tertiary
• Primary halogenoalkane


  • 1-bromopropane
• Secondary halogenoalkane


  • 2-bromopropane
• Tertiary halogenoalkane


  • 2-bromomethylpropane
• Primary halogenoalkane


  • Maximum of one alkyl group bonded to the carbon atom that is bonded to the halogen
• Secondary halogenoalkane


  • Two alkyl groups bonded to the carbon atom that is bonded to the halogen
• Tertiary halogenoalkane


  • Three alkyl groups bonded to the carbon atom that is bonded to the halogen
• Naming complex halogenoalkanes


  Halogens placed in alphabetical order: bromo (Br) followed by chloro (Cl) followed by iodo (I)
• Complex halogenoalkane


  • 1,2-dibromo-4-chlorobutane
• Electronegativity of halogens


  Halogens are more electronegative than carbon, causing the carbon to have a partial positive charge (δ+)
• Nucleophile


  Species which can donate an electron pair, must have a lone pair. Examples: H2O, NH3, OH-
• Substitution reaction


  Nucleophile replaces the halogen atom in a halogenoalkane
• Alkaline hydrolysis


  1. Reaction of a halogenoalkane with an aqueous base, such as NaOH(aq), heated under reflux

  2. RX + OH-(aq) → ROH + X-(aq)
• Reflux is used when heating a reaction mixture to contain volatile chemicals
• Identifying halide ions


  Ag+(aq) + X-(aq) → AgX(s) (precipitate)
• Transition state


  Lone pair of electrons on the nucleophile attacks the partial positive charge (δ+) on the carbon, forming a bond between oxygen and carbon while breaking the carbon-halogen bond
• Ease of hydrolysis of halogenoalkanes

  iodoalkanes > bromoalkanes > chloroalkanes
• Elimination reaction


  1. Reaction of a halogenoalkane with an ethanolic base, such as NaOH in ethanol, heated under reflux

  2. CH3CH(Br)CH2CH3 + OH- → CH3CH=CHCH3 + Br- + H2O
• Chlorofluorocarbons (CFCs) have been used as refrigerants and in aerosols, but their use has been banned because of their effect upon the ozone layer
• CFCs are inert due to the high strength of the C-F and C-Cl bonds
• Breakdown of CFCs in the upper atmosphere


  UV radiation breaks C-Cl bonds, producing highly reactive chlorine radicals that react with and break down ozone molecules