Formation of halogenoalkanes Flashcards
What are the three stages to a chain reaction?
initiation, propagation and termination
What is the initiation step?
breaking the halogen bond to form two halogen atoms
How is the initation step initiated?
the halogen molecule abosrbs the ernergy of a single quantum of UV light, the energy of the UV light is greater than the bond so it will break
What is homolytic fission?
when a bond with two of the same atoms breaks, one electron goes to each atom
What is special about the chlorine atoms
they are free radiacal, written as e.g. Cl•
these free radicals are highly reactive
Why does the C-H bond in the alkane not break?
the bond energy is higher than then energy available in the UV light
What is the propagation stage?
takes place in two steps
e.g. Cl• + CH4 -> HCl + •CH3
leaves a methyl free radical
•CH3 + Cl2 -> CH3Cl + Cl•
these steps may take place thousands of times before the radical are destroyed in the termination steps
What is the termination stage?
where the only products are stable molecules
formed from 2 free radicals
What are the other products formed of the chain reaction?
many products can be formed
for example, in the previous example, ethane and dicloromethane may be made at the propagation stage
many isomers will be produced because the Cl• can replace any of the hydrogen atoms
chain reactions are not very useful because the form a mixture of products
What causes ozone depletion?
chloroflurocarbons
Why is the ozone layer important?
protects the earth from the harmful exposure to too many UV rays, too much UV can affect plankton at the bottom of the sea food chain for example, and it also can cause skins cancer
How does ozone depletion occur?
Chlorine free radicals because the C-Cl bond breaks homolytically in the presence of UV
the chlorine is not destroyed in the process, it is a catalyst
What are the steps for ozone depletion
Cl• + O3 -> ClO• + O2
ClO• + O3 -> 2O2 + Cl•
ovarall equation - 2O3 -> 3O2
