Alkanes

• what is a radical

  A species with an unpaired electron
• what type of bond breaking is radical substitution reaction

  homolytic fission
• when excess bromide is, further substation takes place 


  organic compound + BR2 ---> compound with br + HBR
• properties of alkanes

  Variation of boiling points 

  • Increases boiling point as carbon chain length increases (straight chain alkanes). 
  • London forces are involved  
  • Longer chains have stronger London forces  
  • Surface area is greater points of surface contact increases  
  • As carbon chain length increases, surface area of molecules increase, greater points of surface contact between the molecules, London forces are greater and more energy is needed to overcome these forces. 
• Effect of branching of alkanes

  • Increases branching lowers boiling point, fewer points of surface contact reduces/ decreases London forced, less energy required to overcome forces.  
  • Reactions of alkanes = complete combustion  
• Fission = bond breaking
• Heterolytic fission -  

  A – B --> A+ + B-  

  • Both electrons in the bond (covalent) moves onto one atom  
  • We get positive and negative charged ions  
• Homolytic fission  

  • A-B --> A. + B. 
  • One electron from the covalent bond moves out onto each atom and from a radical. 
  • Radical – is a very reactive species- contains an unpaired electron.  
• Step 1 – Initiation  

  Cl2 --> 2cl.  

  Using UV light - UV light is essential condition for radical substitution 

  Molecule becomes into radical  

   

  Step 2 – Propagation  

  Can go on for a long time (x 10,000 min) 

  Molecule + radical ---> radical + molecule  

  CH4 + Cl. ---> CH3. + HCl 

  CH3 + Cl2 ---> CH3Cl + Cl. 

  The chain will continue on  

  Reaction ends when 2 radicals binds together (termination step) 

   

  Step 3 – Termination  

  2 radicals combine together 

  CH3. + CH3. --> C2H6 

  Cl.+ Cl. --> Cl2 

  CH3. + Cl. --> CH3Cl
• Products of this reaction are haloalkanes. The method for synthesising haloalkanes is limited  

  1. substitution in different positions along the chain
  2. further substitution (mixture of products)