F332 reaction mechanisms for the atmosphere

In the spec it says that you should be able to: 

  • describe the difference between homolytic and heterolytic bond fission and recognise examples; 
  • recall the formation, nature and reactivity of radicals and explain the mechanism of a radical chain reaction involving initiation, propagation and termination;
  • use a radical mechanism as a model to explain the reaction of alkanes with halogens a radical chain reaction in the presence of UV radiation to form halogenoalkanes);
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Difference between homolytic and heterolytic fissi

Heterolytic fission

The bond breaks unevenly where the electron bond pair can stick with one fragment and a positive and negative ion form.


where as in homolytic fission the bond breaks evenly here the bonding pair of electrons are equally divided between two highly reactive fragments called free radicals.

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What is a radical?

Free radicals are characterised by having an unpaired electron not involved in a chemical bond.

It wants to pair up with another electron to form a stable bond - that's why free radicals are so reactive!

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In Initiation free radicals are formed.


UV light provides enough energy to break the bond-photodissociation


The bond splits equally and each atom gets to keep one electron (homolytic fission)

The atom becomes a highly reactive free radical, because of its unpaired electron

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  • the free radicals form a new bond
  • as well as producing a new free radical
  • it continues the reaction in a chain reaction.
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  • Highly reactive free radicals
  • 2 unpaired electrons form a new bond
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I can't find a decent diagram but when drawing this be sure to:

  • use single headed arrows to show the movement of one electron.
  • draw the arrows starting from the bond to the atom
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