Haloakanes

• Haloalkanes


  Organic compounds containing at least one halogen atom bonded to a carbon chain
• Forming Haloalkanes - Free-Radical Substitution


  1. Initiation: Halogen molecule splits into two free-radicals

  2. Propagation: Free-radical takes hydrogen from alkane, forming alkyl radical and hydrogen halide; Alkyl radical takes halogen from halogen molecule, forming haloalkane and regenerating free-radical

  3. Termination: Two free-radicals combine
• UV light is required to provide the energy to break the halogen-halogen bond and start the free-radical substitution reaction
• Free-radical


  A species with an unpaired electron
• Mechanism for reaction between methane and chlorine

  • Initiation: Cl2 → 2Cl•
  • Propagation: Cl• + CH4 → •CH3 + HCl; •CH3 + Cl2 → CH3Cl + Cl•
  • Termination: •CH3 + Cl• → CH3Cl; 2Cl• → Cl2; 2•CH3 → CH3CH3
• Mechanism for reaction between fluorine and butane to produce 2-fluorobutane

  1. Initiation: F2 → 2F•

  2. Propagation: F• + CH3CH2CH2CH3 → CH3CHCH2CH3 + HF; CH3CHCH2CH3 + F2 → CH3CHFCH2CH3 + F•

  3. Termination: F• + CH3CHCH2CH3 → CH3CHFCH2CH3; 2F• → F2; 2CH3CHCH2CH3 → C8H18
• Further substitution can occur, producing a wide range of products
• To reduce further substitution, use excess alkane
• Overall reaction for free-radical substitution

  Include appropriate halogen as reactant and appropriate halogen halide as product, with number depending on how many halogen atoms have been substituted
• Overall reactions for free-radical substitution

  • CH4 + 3F2 → CHF3 + 3HF

  CH3CH3 + 5F2 → CF3CHF2 + 5HF

  CH3Cl + 2Cl2 → CHCl3 + 2HCl
• Ozone (O3)


  Pale blue poisonous gas that forms a layer 12-50 km above the earth's surface, acting as a barrier to harmful UV-B radiation
• Ozone can be broken down by a reaction with chlorine radicals, which are released by chlorofluorocarbons (CFCs)
• Chlorofluorocarbons (CFCs)


  Haloalkanes that contain both chlorine and fluorine, historically used as coolants in refrigerators
• In the 1980s, scientists found evidence of a 'hole in the ozone layer' above Antarctica, caused by the breakdown of CFCs in the upper atmosphere
• Stratosphere


  The earth's surface called the stratosphere and acts as a barrier to the harmful UV-B radiation
• UV-B radiation reaching the earth's surface


  Leads to skin cancer, cataracts and plant leaf damage
• Ozone


  Can be broken down by a reaction with chlorine radicals, which are released by chlorofluorocarbons (CFCs)
• Chlorofluorocarbons (CFCs)


  Haloalkanes that contain both chlorine and fluorine
• Uses of CFCs


  • Trichlorofluoromethane was used as the coolant in refrigerators
• In the 1980s scientists produced evidence for a decrease in ozone levels in the atmosphere above Antarctica, described as a 'hole in the ozone layer'
• Further tests showed the presence of CFCs breaking down and facilitating the breakdown of ozone itself
• CFCs are one cause of depletion in ozone concentration in the upper atmosphere
• An international agreement was drawn up and ratified in Montreal (1989) banning the use of CFCs to slow down depletion of ozone layer
• Ozone Breakdown Mechanism


  1. Initiation: CCl3F → Cl• + •CCl2F

  2. Propagation: Cl• + O3 ⟶ ClO• + O2

  3. Propagation: ClO• + O3 ⟶ 2O2 + Cl•
• CFCs were so damaging to the ozone layer as the breakdown of ozone can be caused by a single chlorine free-radical
• Cl● is regenerated in the final propagation step, causing a chain reaction in the decomposition of ozone
• Newer molecules to replace CFCs

  Do not contain a chlorine atom and, therefore, cannot release a chlorine radical
• Molecule used as a refrigerant in place of CFCs


  • 1,1,1-trifluoroethane (CF3CH3)
• Why 1,1,1-trifluoroethane does not lead to ozone depletion

  • It does not contain Cl / does not release Cl
  • C-F bonds are strong and do not break
• Curly arrows


  Used to show the movement of electrons in chemical reactions (other than free-radical substitution)
• Nucleophile


  An electron-pair donor
• Nucleophilic substitution


  • Haloalkanes are more reactive than alkanes due to polar C-halogen bonds
  • Nucleophiles are attracted to the C atom bonded to the halogen
  • The halogen atom leaves as the nucleophile forms a bond to the C atom
• Nucleophiles that react with haloalkanes

  • OH−
  • CN−
  • NH3
• Nucleophilic Substitution by OH− Ions

  1. Reagent: NaOH or KOH

  2. Conditions: Aqueous solvent

  3. Product: Alcohol
• Nucleophilic Substitution by CN− Ions

  1. Reagent: KCN

  2. Conditions: Water and ethanol solvent

  3. Product: Amine
• Nucleophilic Substitution by NH3

  1. Reagent: Excess NH3

  2. Conditions: Ethanol solvent, heat, and pressure

  3. Product: Amine
• Hydrolysis


  Nucleophilic substitution of a haloalkane using hydroxide ions
• Hydrolysis does not occur at the same rate for all haloalkanes
• Method to determine rate of hydrolysis

  1. Place measured samples of haloalkanes in water bath at 50°C

  2. Add ethanol, water and aqueous silver nitrate solution to another test tube in water bath

  3. Once at equal temperature, mix solutions and time how long it takes for precipitate to form

  4. Calculate rate as 1/time taken for precipitate
• Precipitates formed and their rates

  • Chloropropane (AgCl, 150 s, 0.066 s^-1)
  • Bromopropane (AgBr, 90 s, 0.011 s^-1)
  • Iodopropane (AgI, 60 s, 0.016 s^-1)