Haloalkanes

• Hydrolysis


  Reaction with water/aqueous hydroxide ions that breaks down a chemical compound
• Hydrolysis


  • A source of heat is required, therefore it is sometimes asked that this reaction is carried out under reflux as this is a source of heat
  • This is a substitution reaction, and the products are an alcohol and a halide (and an H+ ion if the reaction was done with water)
• Hydrolysis with aqueous hydroxide ions

  Hydrolysis equation
• Hydrolysis with water


  Hydrolysis equation
• Aqueous sodium hydroxide


  The most common source of aqueous hydroxide ions for this experiment
• Nucleophilic substitution mechanism (occurs in hydrolysis)

  1. Halide atoms such as chlorine tend to be more electronegative than hydrogen or carbon

  2. The bond between the halide and the carbon in the halogenoalkane is polar, with the shared pair of electrons closer to the halide atom than the carbon atom

  3. The hydroxide ion has a lone pair of electrons which causes it to be attracted to the electron deficient carbon (they are nucleophiles) to join with it

  4. As the hydroxide ion moves closer due to the attraction, its lone pair repel the shared electrons between the carbon and halogen atoms

  5. The shared electrons are thus repelled completely onto the halogen, breaking the bond via heterolytic fission
• Nucleophile


  Electron pair donor
• Relative rates of hydrolysis of halogenoalkanes


  1. Measure out equal volumes of 3 halogenoalkanes into three separate test tubes

  2. Add 5cm^3 of ethanol to each test tube

  3. To 3 further test tubes add 5cm^3 of silver nitrate solution

  4. Place all test tubes in a water bath at 50 degrees celsius for 5 minutes

  5. Mix the contents of the pairs of test tubes (silver nitrate with the halogenoalkane and ethanol containing tube) and time how long it takes for the silver halide precipitate to become visible

  6. Rate= 1/time

  
• Relative rates of hydrolysis of halogenoalkanes

  • When comparing reaction rates between halogenoalkanes, ensure that they are all the same chain length
  • Silver nitrate is used in this reaction as the silver ions reacts with the halide ions in solution to form an insoluble precipitate, showing when the reaction has occurred as the products (halide ions) have been formed

  
• Relative rates of hydrolysis of halogenoalkanes

  • Iodobutane is fastest in reacting, followed by the bromobutane, then the chlorobutane

  
• Electronegativity


  • As you go down group 7, the electronegativity decreases
  • The greater the electronegativity, the greater the bond enthalpy
  • So this means that despite the fact that the carbon in the iodobutane only weakly attracts the water, because its bond enthalpy is low, the bond is easy to break, leading to a faster reaction
• Bond enthalpy has a greater part in determining the rate of hydrolysis than bond polarity
• Ozone


  The formula for the ozone in the ozone layer is O3
• If there wasn't ozone in the stratosphere

  All of the UVb from the sun would reach the earth, the earth would absorb more UVb and so emit more as infrared and so get hotter

  
• UVb is a very damaging form of radiation and very closely linked to skin cancer, so we would be more prone to it
• Ozone absorbs UVb, so that only about 5% reaches the earth

  
• All of UVa gets to earth, but because it's at a longer wavelength meaning that it doesn't have as much energy and so isn't as damaging

  
• UVc has the shortest wavelength, so is fully filtered out by the ozone layer

  
• How ozone is formed

  1. In the stratosphere we have oxygen molecules which are exposed to high levels of UV

  2. This UV splits the bond between the 2 oxygens in the diatomic molecule (homolytic fission), resulting in two very reactive oxygen atoms

  3. One of these oxygen atoms will then react with an oxygen molecule to form ozone (O3)
• This would be dangerous if it went on indefinitely as we would run out of oxygen, as ozone is as toxic to the human body in a similar way to carbon monoxide
• However, when ozone absorbs UV, it breaks back down into an oxygen molecule and an oxygen atom (so the reaction is reversible)

  
• Over millions of years, the rates of the forward and backward reaction have become equal, and so the reaction has been at equilibrium
• Recently the human race have introduced new chemicals into the stratosphere


  That are breaking down the ozone faster than it forms, causing the ozone layer deplete (get thinner)

  
• Chlorofluorocarbons (CFCs)


  Some of the most damaging chemicals to the ozone layer
• In the 1950s-70s they were used as propellants in aerosols, refrigerants in fridges, blowing agents to make polymers spongy
• They were chosen for this purpose because of their inertness
• When CFCs are released into the atmosphere, and as they aren't even water soluble, aren't washed away by the rain


  They eventually end up in the stratosphere, where they are exposed to UV

  
• When CFCs are exposed to UV in the stratosphere

  One of the chlorine-carbon bonds is split via homolytic fission, resulting in two free radicals (initiation step- stable molecule forming two free radicals), a chlorine radical and a CFCl2 radical
• Propagation 1


  The chlorine radical will then react with the ozone, taking one of the oxygens to form a ClO radical and an oxygen
• Propagation 2


  The ClO radical will react with a singular oxygen atom to form an oxygen molecule and a chlorine free radical
• An overall equation can be produced for this by cancelling out anything that appears on both sides of the combine equation, leaving O3 + O -> 2O2, an overall equation showing that ozone is being destroyed

  
• Because the chlorine radical is used in the first propagation steps and reformed in the second, it is acting as a catalyst, so these two reactions can be referred to as the catalytic breakdown of ozone
• 1 chlorine radical can destroy 100,000 ozone molecules
• Termination


  Eventually the chlorine radical will combine with another chlorine radical and become stable again
• Helping the ozone to recover


  • Scientists can measure and monitor levels of ozone and ozone-depleting chemicals in the atmosphere
  • That data could then be fed to politicians in governments who would meet at protocols (gatherings of many countries) where they would make some promises as to moving forward by cutting levels of or banning certain chemicals
  • Scientists can also develop alternatives to CFC's, i.e: CO2 has now replaced CFCs as the blowing agent for foam polymers, and HFC's (hydrogen replaces the chlorine)

  
• At the Montreal protocol, countries promised to ban CFC's, to stop using CCl4 (a widely used solvent), and to stop using 'halon' fire extinguishers
• However, these replacements are greenhouse gases, so whilst they solve this problem, they add to climate change
• Nitrogen oxides are as bad as CFCs for ozone


  • .NO + O3 -> .NO2 + O2
  • .NO2 + O -> .NO +O2
• Greenhouse effect


  • Without the greenhouse effect, the global temperature would be about -20 degrees celsius
  • The greenhouse effect has built up over millions of years to give us our average global temperature of around 18 degrees celsius
• How the greenhouse effect works

  1. The sun provides us with UV radiation, which enters the atmosphere and reaches earth

  2. The earth absorbs the UV and remits it as infrared, which has a longer wavelength

  3. The stratosphere contains CO2, water vapour, and methane, and others

  4. When the infrared reaches the part of the stratosphere where the greenhouse gases are, they will absorb the infrared, raising the energy of the gases and causing the bonds to vibrate

  5. Eventually the molecules will relax back down into a lower energy state and the energy is re-emitted in all directions, some passing out through the atmosphere, and lots is re-emitted back down to earth

  6. It is these infrared rays that warm up the atmosphere