Haloalkanes

• Haloalkanes


  Much more reactive than alkanes due to the presence of the electronegative halogens
• Carbon-halogen bond

  Polar causing the carbon to carry a partial positive and the halogen a partial negative charge
• Haloalkanes undergo two key types of reaction
• Nucleophilic substitution reactions

  1. A halogen is substituted for another atom or group of atoms

  2. The products formed when haloalkanes undergo this type of reaction are alcohols, amines and nitriles
• Elimination reactions

  A hydrogen halide is eliminated during the reaction and the key product formed is an alkene
• Elimination reactions

  • Nucleophile is the hydroxide, OH ion
  • An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used
  • Halogenoalkanes are generally insoluble in water
  • This means that there would be an organic layer of the halogenoalkane and an aqueous layer of sodium hydroxide
  • The use of ethanol allows the aqueous sodium hydroxide layer and the organic halogenoalkane layer to mix, allowing the reaction to proceed
  • This reaction is very slow at room temperature, so the reaction mixture is warmed
  • This is an example of a hydrolysis reaction and the product is an alcohol
  • The rate of this reaction depends on the type of halogen in the haloalkanes
  • The stronger the C-X bond, the slower the rate of the reaction
  • In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
  • Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction
• Acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
• Precipitate

  A solid that forms from a chemical reaction and settles at the bottom of the test tube
• As the reaction progresses

  Precipitate forms
• Halide ions

  • Chloride ions
  • Bromide ions
  • Iodide ions
• Precipitate colours

  • White precipitate (chloroalkane)
  • Cream precipitate (bromoalkane)
  • Yellow precipitate (iodoalkane)
• 1. I bond


  Has the lowest bond enthalpy
• 1. I bond


  Easiest to break
• I ions

  Form the fastest
• 1. Cl bond


  Has the highest bond enthalpy
• 1. Cl bond


  Hardest to break
• Cl ions


  Form the slowest
• Nucleophile


  An electron-rich species that can donate a pair of electrons
• 'Nucleophile'


  Means ‘nucleus/positive charge loving’ as nucleophiles are attracted to positively charged species
• Nucleophilic


  Reactions that involve a nucleophile
• Nucleophilic substitution reaction

  A nucleophile attacks a carbon atom which carries a partial positive charge and an atom with a partial negative charge is replaced by the nucleophile
• Haloalkanes

  • Undergo nucleophilic substitution reactions due to the polar C-X bond (where X is a halogen)
  • The rate of this reaction depends on the type of halogen in the haloalkane
  • The stronger the C-X bond, the slower the rate of the reaction
  • Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction
• Nucleophile


  An atom or molecule that donates an electron pair to a positive or partially positive charged atom
• Nucleophile in this reaction

  • Hydroxide, OH ion
• Aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used


  This reaction is very slow at room temperature, so the reaction mixture is warmed
• Products


  • An alcohol
• This is an example of a hydrolysis reaction
• Reacting haloalkanes with aqueous silver nitrate solution


  Results in the formation of a precipitate
• Rate of formation of these precipitates

  Can be used to determine the reactivity of the haloalkane
• The formation of the pale yellow silver iodide is the fastest nucleophilic substitution reaction
• The formation of the silver fluoride is the slowest nucleophilic substitution reaction
• Fluoroalkanes


  Least reactive
• Iodoalkanes


  Most reactive
• Haloalkanes
• Most common halogenoalkanes

  • Chlorofluorocarbons (CFCs)
• CFCs

  Compounds that contain carbon atoms with chlorine and fluorine atoms attached to them
• CFCs

  • CCl F
  • CCl F
• CFCs

  • Chemical inertness
  • Non-flammable
  • Non-toxic
• Uses of CFCs

  • Refrigerators
  • Propellants for aerosols
  • Solvents for dry cleaning
• Hydrofluorocarbons (HFCs)


  Compounds that contain a carbon attached to hydrogen and fluorine atoms only