15 - haloalkanes

• What are haloalkanes?


  Compounds containing the elements carbon, hydrogen and at least one halogen.
• Is a carbon-halogen bond polar or non-polar?


  Polar.
• What partial charge does the carbon atom have in a carbon-halogen bond?


  δ+
• What partial charge does the halogen have in a carbon-halogen bond?


  δ-
• Why are carbon-halogen bonds polar?


  Carbon has a lower electronegativity than halogens. The electron pair sits closer to the halogen in the bond.
• What two types of reactions to haloalkanes undergo?

  Nulceophilic substitution and elimination reactions.
• What happens in nucleophilic substitution of a haloalkane?


  - A halogen is substituted for another atom or groups of atoms.

  - The product formed is an alcohol.
• Nucleophile definition


  An electron pair donator.
• What happens in elimination reactions of haloalkanes?


  - A hydrogen halide is eliminated.

  - Key product formed is an alkene.
• What is the reaction of the formation of alcohols from haloalkanes called?


  Hydrolysis.
• What is the nucleophile in the formation of alcohols from haloalkanes?


  Hydroxide, OH⁻, ion.
• What are the reagents used for hydrolysis of haloalkanes?


  Aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol.
• Why is ethanol used in the hydrolysis of haloalkanes?


  - Haloalkanes are generally insoluble in water.

  - This means there would be an organic layer of the haloalkanes and an aqueous layer of sodium hydroxide.

  - A reaction would only occur where the two layers meet.

  - Use of ethanol allows the aquous sodium hydroxide layer and the organic haloalkane layer to mix, allowing the reaction to procede.
• What is a nucloephile?


  An atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond.
• Three examples of nucleophiles


  - Hydroxide ions, :OH⁻

  - Water molecules, H₂O:

  - Ammonia molecules, :NH₃
• What happens when a haloalkane reacts with a nucleophile?


  The nucleophile is substituted for the halogen atom in the hydrocarbon.
• What type of haloalkanes undergo nucleophilic substitution?


  Primary haloalkanes.
• Hydrolysis of a haloalkane - nucleophilic substitution


  1. The nucleophile, :OH⁻, approached the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom.

  
  

  2. A lone pair of electrons on the hydroxide ion is attracted to the δ+ carbon atom.

  
  

  3. The carbon halogen bond breaks by heterolytic fission.

  
  

  4. A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom.

  
  

  5. The new organic product is an alcohol. A halide ion is also formed.

  
• Why does the nucleophile approach the carbon atom, attached to the halogen, on the opposite side of the molecule from the halogen atom?


  This direction of attack by the nucleophile, OH⁻ ion, minimises repulsion between the nucleophile and the δ- halogen atom.
• What conditions are used for the hydrolysis of haloalkanes?


  Heated under reflux.
• Why is the reaction mixture heated under reflux in the hydrolysis of haloalkanes?


  The reaction is very slow at room temperature.
• How does the rate of hydrolysis change with decreasing bond enthalpy of the carbon-halogen bond?


  Increases.
• What's the trend in bond enthalpies of carbon-halogen bonds down the group.


  As you go down the group, bond enthalpies decrease.
• Results of hydrolysis of haloalkanes

  Chloride: white AgCl precipitate forms very slowly.

  Bromide: cream AgBr precipitate forms moderately slowly.

  Iodide: yellow AgI precipitate forms rapidly.
• Whats the trend in rate of hydrolysis of a primary, secondary, and tertiary haloalkane?


  Tertiary haloalkane - fastest

  Secondary

  Primary haloalkane - slowest
• Most common haloalkanes


  Chlorofluorocarbons (CFCs)
• Chlorofluorocarbons (CFCs)


  Compounds containing carbon atoms with chlorine and fluorine atoms attached to them.
• Why do CFCs have many uses?

  Due to their chemical inertness.
• Two key features of CFCs

  Non-flammable and non-toxic.
• Three uses of CFCs

  - Refrigerants.

  - Propellants for aerosols.

  - Solvents for dry cleaning.
• Hydroflurocarbons (HFCs)


  Compounds containing a carbon attached to hydrogen and fluorine atoms only.
• Why do chlorofluorocarbons have a devastating effects on the ozone layer?


  - CFCs absorb a lot of UV radiation in the upper atmosphere.

  - CFCs are then broken down by the UV light causing the formation of chlorine radicals.

  - Chlorine radicals react with ozone and break down the ozone layer.
• What does ozone do?


  It absorbs the majority of harmful UV radiation.
• Reaction mechanism for the breakdown of the ozone layer due to chlorine radicals

  Initation: UV light

  Cl₂ → 2Cl∙

  
  

  Propagation

  Cl∙ + O₃ → ClO∙ + O₂

  ClO∙ + O → Cl∙ + O₂
• What is the overall equation for the depletion of the ozone layer?


  O₃ + O → 2O₂
• Where is the ozone layer?

  Outer edge of the stratosphere, 10-40km above earths surface.
• What is feared about the continued depletion of the ozone layer?


  - It will allow more UV-B radiation to reach the earth's surface, leading to increased genetic damage and a greater risk of skin cancer in humans.
• What radiation is most commonly linked to sunburn?


  UV-B radiation
• How are nitrogen oxide radicals formed?


  - Naturally during lightning strikes.

  - Result of air travel in the stratosphere.
• Propagation steps of nitrogen oxide radicals breaking down ozone


  NO∙ + O₃ → NO₂∙ + O₂

  NO₂∙ + O → NO∙ + O₂