Haloalkanes

• What is a haloalkane?

  An alkane with one or more hydrogen atoms replaced by halogen atoms.
• Why are carbon-halogen covalent bonds polar?

  · Halogens are more electronegative than carbon

  · They pull the shared pair of electron towards them

  · Giving the carbon a partial positive charge, and the halogen a partial negative charge.
• What is bond enthalpy?

  · Bond enthalpy is the energy required to break one mole of covalent bonds between two atoms in the gaseous state.
• Trend in bond enthalpy of the carbon-halogen bond as we descend group 7?

  As we descend group 7, the carbon-halogen bond enthalpy decreases.
• What is a nucleophile?

  A nucleophile is an electron pair donor, e.g. that attacks a carbon atom which carries a partial positive charge.
• What happens to the halogen atom when a haloalkane undergoes nucleophilic substitution?

  The halogen atoms is replaced by a nucleophile.
• Nucleophilic Substitution Mechanism


  1. Nucleophile (OH-) approaches the carbon attached to the halogen on the opposite side

  2. Direction of attack by the OH- ion minimises repulsion between the nucleophile and the partially negative halogen atom

  3. Lone pair of electron on the oxygen of the hydroxide ion is attracted and donated to the partially positive carbon atom

  4. Carbon-halogen bond breaks by heterolytic fission

  5. New bond is made between the oxygen of the hydroxide ion and the carbon atom

  6. New organic product is formed, which is an alcohol, and a halide ion is formed
• Three types of reactions that produces haloalkanes:

  • Free radical substitution of alkanes by a halogen in UV light.
  • Electrophilic Addition of alkene with a halogen or a halogen halide at room temperature.
  • Alcohol substitution reaction in the presence of diluted H2SO4 and sodium halide.
• Experiment that compare hydrolysis rates of 1-chlorobutane , 1-bromobutane, 1-iodobutane

  1. Set up 3 test tubes with ethanol and drops of the haloalkanes in each different tubes.
  2. Heat the tubes in a water bath at 50 degrees along with a tube of silver nitrate solution.
  3. Add silver nitrate solution to each of the test tube and start timing. The water in the solution hydrolyses the haloalkanes.
  4. Observe the time taken for the silver halide precipitate to from in each tube.
  5. 1-iodobutane is the fastest, followed by 1-bromobutane, followed by 1-chlorobutane
• The order of the rate of hydrolysis of haloalkanes:

  The lower the carbon-halogen bond enthalpy, the quicker the rate of hydrolysis.
• Why is ethanol used in the hydrolysis of haloalkane reactions?

  Haloalkanes are insoluble in water; as ethanol is a universal solvent, it acts as a solvent for haloalkane , allowing the haloalkane and water mix and react.
• Why is water able to act as a nucleophile?

  There is a lone pair of electrons on the oxygen atom of the water molecule, so it can act as an electron pair donor
• Physical Properties of Haloalkanes:

  • Colourless
  • Odourless
  • Insoluble in water
  • Flammable
• Classification of Haloalkanes:

  • Can be classed as either primary, secondary or tertiary.
  • Depends on the number of alkyl groups attached to the carbon atom joined to then halogen atom.
  • 0/1 alkyl group - Primary
  • 2 alkyl groups - Secondary
  • 3 alkyl groups - Tertiary
• What is a CFC <--- Chlorofluorocarbon

  • Type are a type of haloalkane.
  • They contain no C-H bonds because all the hydrogen atoms have been substituted by chlorine and fluorine.
• Environmental issues of the uses of CFCs:

  • Widely used as coolants in fridges and as solvents in industry, and these CFCs often escape into the atmosphere where they deplete/ destroy the ozone layer, because the carbon-chlorine bond breaks and gives off chlorine free radicals.
  • They also contribute to global warming as they are a greenhouse gas.
• • Ozone (O3) in the atmosphere acts as a protective barrier by absorbing harmful UV radiation from the sun. This ozone layer prevents damage to humans and ecosystems.
• • CFC molecules are very stable due to the strength of their carbon-halogen bonds; this means that CFCs have a long residence time in the troposphere (lowest level) - therefore it would take them years to reach the stratosphere.
• • Once CFCs are in the stratosphere, strong UV radiation provides sufficient energy to break a carbon - chlorine bond in CFCs by homolytic fission.
  • A carbon - chlorine bond is broken as it has the lowest bond enthalpy.
  • As UV radiation initiates the breakdown, the initiation process is called photolysis/ photodissociation.
• The photodissociation of CF2Cl2:

  CF2Cl2 -----> CF2Cl* + Cl*
• • The chlorine radical formed is very reactive and therefore it reacts with an ozone molecule (----> breaks down the ozone molecule into oxygen)
• • These chlorine radicals works as a homogenous catalyst - reacts with reactants to form an intermediate; the intermediate breaks down to give the product and the catalyst is regenerated.
• • P1: *Cl + O3 ----> ClO* + O2
  • P2: ClO* + O ----> O2 + *Cl
  • Overall Equation : O3 + O ----> 2O2
• • The two propagation steps would repeat in a cycle over and over again - the regenerated chlorine radical in P2 would attack and remove another ozone molecule in propagation 1.
• • Safer alternatives for CFCs were developed, like hydrofluorocarbons (HFCs) and hydrocarbons, which have no effect on the ozone layer.
• • However, CFCs are not entirely responsible for all ozone depleting reactions, other radicals also catalyse the breakdown of ozone.
  • Nitrogen Oxide radicals are formed naturally during lightning strikes, and also the result of aircraft travel in the stratosphere.
• • P1: *NO +O3 ----> NO2* + O2
  • P2: NO2* + O ----> O2 + *NO
  • Overall Equation: O3 + O ----> 2O2