chapter 15 - haloalkanes

• nucelophiles are species which donate a lone pair of electrons
• when a haloalkane reacts with a nucelophile, the nucleophile replaces the halogen in a substitution reaction

  this is called nucleophilic substitution
• in the hydrolysis of a haloalane, the halogen atom is replaced by an OH- group
• Hydrolysis of a haloalkane:

  1. the nucleophile, OH-, approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
  2. this direction of attack by the OH- ion minimises repulsion between the nucleophile and the polar negative halogen atom
  3. a lone pair of electrons on the hydroxide ion is attracted and donated to the polar positive carbon atom
  4. a new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
  5. the carbon - halogen bond breaks by heterolytic fission
  6. the new products are an alcohol and a halide ion
• Hydrolysis of haloalkanes

  nucleophilic substitution

  
• haloalkanes can be converted to alcohols using aqueous sodium hydroxide

  the mixture is heated under reflux
• the rate of hydrolysis depends upon the strength of the carbon-halogen bond in the haloalkane
• the C-F bond is the strongest and the C-I bond is the weakest
• iodoalkanes react faster than bromoalkanes
• bromoalkanes react faster than chloroalkanes
• fluoroalkanes are unreactive
• you can measure the hydrolysis of primary haloalkanes using silver nitrate to measure the rate of reaction

  as the reaction occurs, a precipitate of silver halide is formed

  the nucleophile in this reaction is water
• halogen ions are insoluble in water

  so an ethanol solvent is needed for them to react with water
• Hydrolysis of haloalkanes:

  1. set up 3 test tubes with 2 drops haloalkane, mixed with 1 cm3 ethanol
  2. stand the test tubes in a 60 degree water bath
  3. place a test tube containing 0.1 moldm-3 silver nitrate in the water bath, and allow all test tubes to reach a constant temperature
  4. add 1cm3 of the silver nitrate quickly to each of the test tubes, start a stop clock
  5. observe the time taken for each precipitate to form
• rate of hydrolysis increases as the strength of the carbon-halogen bond decreases
• organohalogen molecules that contain atleast one halogen atom joined to a carbon chain
• organohalogen compounds are used in many pesticides

  they are not broken down naturally in the environment
• the ozone layer is the layer found at the edge of the stratosphere
• ozone absorbs most of the UV-B light, which is damaging
• continued depletion of the ozone layer will allow more UV-B radiation to reach the earth's surface
• ozone is continually being formed and broken by UV rays, however CFC's upset this equilibrium
• at normal ozone, the rate that ozone is broken down is the same as the rate it is formed

  O2 + 0 -> 03
• CFCs are very stable because of the strength between the carbon-fluoride bonds
• CFCs remain stable until they reach the stratosphere, where they break down, and form chlorine radicals

  these chlorine radicals are thought to catalyse the breakdown of the ozone layer
• UV radiation provides sufficient energy to break the carbon-halogen bond by homolytic fission

  the C-Cl bond has the lowest enthalpy, so is the bond that breaks
• photodissociation is where radiation initiates the breakdown of the CFC molecule

  CF2Cl2 -> CF2Cl. +Cl.
• CF2Cl2 propogation

  CF2Cl2 -> CF2Cl. + Cl.

  Step 1 - Cl. + O3 -> ClO. + O2

  Step 2 - ClO. + O -> 2O2

  these propagation steps repeat in a constant cycle

  overall equation - O3 + O -> 2O2
• nitrogen oxide radicals can also catalyse the breakdown of ozone molecules

  propagation step 1 - NO. + O3 -> NO2. + O2

  propagation step 2 - NO2. + O -> NO. + O2