Module 4.2.2- Haloalkanes

• Naming haloalkanes example

  2-bromo-1-chlorobutane

  
• Nucleophile


  An electron pair donor
• Examples of nucleophiles

  OH-, H2O, NH3, CN-
• Nucleophilic substitution


  A reaction in which a nucleophile is attracted to an electron-deficient carbon atom, and replaces an atom or group of atoms on the carbon atom.
• Hydrolysis


  A chemical process that splits a molecule by adding water.
• Hydrolysis of haloalkane mechanism


  lone pair of electrons on OH- is attracted and donated to the carbon atomcarbon-halogen bond breaks by heterolytic fissionCURLY ARROW TO C (from lone pair of OH-)CURLY ARROW TO Cl (from C-Cl bond)

  
• Haloalkanes to alcohols


  Nucleophilic substitution with aqueous NaOH

  Heated under reflux
• Strength of carbon-halogen bond (strongest to weakest)

  C-F

  C-Cl

  C-Br

  C-I
• Reaction rates of haloalkanes


  Iodoalkanes react faster than bromoalkanes

  Bromoalkanes react faster than chloroalkanes

  Chloroalkanes react faster than Fluoroalkanes

  Fluoroalkanes are unreactive because a large quantity of energy is needed to overcome the strong C-F bond
• Measuring the rate of hydrolysis of primary haloalkanes


  Add ethanol solvent and haloalkane

  Add silver nitrate (heated all to a constant temperature 60 degrees Celcius)

  Chlorine- white precipitate (slowest)

  Bromine- cream precipitate (slower)

  Iodine- yellow precipitate (fastest)

  
• Why is ethanol solvent added to the haloalkane when measuring rates of hydrolysis


  Haloalkanes are insoluble in water

  Ethanol allows water and the haloalkane to mix and produce a single solution (rather than two layers)
• The ozone layer


  A layer of the stratosphere with a high concentration of ozone; absorbs most of the Sun's harmful ultraviolet radiation

  
• What breaks down an O2 molecule into oxygen radicals


  Ultraviolet radiation
• Ozone layer equilibrium


  O2 + O <=> O3

  
• Uses of CFCs


  refrigerants and aerosols
• How are chlorine radicals formed from CFCs


  CFCs are stable due to the strength of the carbon-halogen bond however as they reach the stratosphere they begin to break down by homolytic fission and UV radiation forming chlorine radicals
• Why are chlorine radicals in the stratosphere deemed as not good


  they catalyse the breakdown of the ozone layer
• Photodissociation of CF2Cl2 by UV light


  CF2Cl2 -> CF2Cl• + Cl•
• Breakdown of CFCs- Propagation step 1


  O3 + Cl• --> ClO• + O2
• Breakdown of CFCs- Propagation step 2


  ClO• + O --> Cl• + O2Chlorine radical is regenerated and the two propagation steps are in a constant cycle
• Breakdown of CFCs- Overall equation


  O3 + O --> 2O2
• Nitrogen oxide radicals (from lightning strikes or aircraft travel) breakdown of ozone summary


  Propagation step 1: NO• + O3 --> NO2• + O2

  Propagation step 2: NO2• + O --> NO• + O2

  Overall equation: O3 + O --> 2O2