3.03 haloalkanes

• What are the alternatives to CFCs?

  HCFCs - the presence of H increases there reactivity so they stay around for less time and are less likely to reach the stratosphere. However if these do reach the stratosphere, they are more ozone depleting.

  
  

  HFCs are not ozone depleting but are GHGs.

  
  

  Scientists estimate ozone levels to be returned to pre 1980 stats before 2075.
• What is the Montreal protocol?

  In 1987 68 nations signed a national agreement to reduce the production of CFCs.
• What are the three stages in free radical substitution?

  Initiation - breaking halogen bond to form free radicals

  
  

  Propagation - chain part of the reaction where products are formed but free radical remains

  
  

  Termination - free radicals removed, stable products formed
• How do we get from a alkane to an haloalkane?

  Free radical substitution with the presence of UV light.
• What are the uses of cfcs?

  Coolants in fridges, dry cleaning solvents, propellants in aerosols.
• What are CFCs?

  Chlorofluorocarbons (alkanes containing chlorine, fluorine and carbon) because they are very stable they do not react and exist for a very long time.
• What is the problem with ozone being found in the troposphere?

  Creates smog that causes global dimming.
• What is the problem with UV passing through the stratosphere into the troposphere?

  Can cause skin cancer and damage DNA
• Which levels of UV are absorbed by the ozone layer?

  UVC is fully absorbed, UVB is partially absorbed, UVA passes straight through.
• What is the use of ozone in the stratosphere?

  Absorbs UV radiation from the sun.
• Where is ozone found?

  Stratosphere
• How is ozone identified?

  Pale blue gas with a sharp smell.
• When using the reagent OH- what is produced?

  Primary haloalkanes produce primary alcohols via nucleophilic substitution.

  
  

  Secondary haloalkanes produce both alcohols via nucleophilic substitution and alkenes via elimination.

  
  

  Tertiary haloalkanes produce alkenes via elimination.
• How do you know if a haloalkane will undergo Nucleophilic substitution or elimination.

  Elimination conditions are ethanolic where the OH acts as a base and temperatures are high. Nucleophilic substitution conditions are aqueous where the OH acts as a Nucleophile and temperatures are low.

  
  

  If the reaction occurs in a mixture of water and ethanol solvent, both products (alcohol and alkene) will be produced.
• How do we get from a haloalkane to an Alkene?

  Elimination with hot ethanolic sodium hydroxide under reflux.
• What determines the reactivity haloalkanes?

  The bond strength between C and halogen. The weaker the bond the more reactive. The reactivity increases going down the group so an Alkene with fluorine is less reactive than one with bromine.
• How do we get from a haloalkane to an amine?

  Nucleophilic substitution with excess ethanolic ammonia and heat.
• How do we get from a haloalkane to a nitrile?

  Nucleophilic substitution with warm ethanolic potassium cyanide under reflux.
• How do we get from a haloalkane to an alcohol?

  Nucleophiles substitution in either primary or secondary haloalkanes.

  
  

  With warm aqueous sodium hydroxide and reflux conditions.
• Which haloalkanes can nucleophilic substitution occur in?

  Primary and secondary as tertiary are too stable.
• What happens during nucleophilic substitution?

  A halogen atom is replaced for another atom.
• Name the four nucleophiles in order of best to worst.

  OH-, CN-, NH3, H2O
• Why can haloalkanes be attacked by nucleophiles?

  Halogens are more electronegative than the carbon so pull the electron pair closer to the halogen, leaving the carbon slightly positive.
• What is the trend in boiling point of haloalkanes?

  Increases going down the group because IMFs are stronger as Mr increases meaning there are more electrons available to form VDWs
• What are nucleophiles?

  Anions that can donate a lone pair of electrons.
• What have scientists done to combat ozone depletion?

  They've banned the use of CFCs and developped alternative chlorine-free compounds.

   For example, HFCs are now used for refrigerators and air conditioners. They don't contain the C-Cl bonds. 
• Why is the C-F bond not affected by UV?

  It is stronger than the C-Cl bond
• Why is the naturally occuring ozone layer beneficial?

  It filters out much of the sun's UV radiation 
• What is a free radical?

  A reactive species with possess an unpaired electron 
• Describe the Initiation Step of FRS

  In the precense of UV light, the halogen is broken down. 

  This is an example of homolytic fission - each atom gets one electron from the covalent bond. 
• Describe the Propagation step of FRS

  A hydrogen is replaced and the Cl free radical is reformed as a catalyst. 
• Describe the Termination step of FRS

  Two radicals join to end the chain reaction and form a stable product. 
• Why have CO2 levels risen significantly in the last years?

  due to increasing burning of fossil fuels 
• Describe the mechanism of the greenhouse effect

  UV wavelength radiation passes through the atmosphere and heats up Earth's surface

  The Earth radiates infrafred long wavelength radiation

  The C=O bonds in CO2 absorb IR radiation, so it can't escape

  Energy is transferred to other molecules in the atmosphere via collisions hence the atmosphere is warmed 
• What do catalytic converters contain?

  A ceramic honeycomb coated with a thin layer of a catalyst metals, platinum, palladium, rhodium, to give them a large surface area.