the formation of halogenoalkanes
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how is a halogenoalkane formed
when you put a mixute of an alkane and a halogen into bright sunlight, or shine a photoflood lamp onto the mixture, the alkane and the halogen will react to form a halogenoalkane the ultraviolet component of the light starts the reaction
what are the three stages of free-radical substitution
initiation propagation termination
define the initiation stage
the initiation step of the reaction is breaking the covalent bond to form two free radicals
explain how the covalent bond is broken
the molecule absorbs the energy of a single quantum of ultraviolet light the energy of one quantum of UV light is greater than the covalent bond energy, so the bond will break since both atoms are the same, the covalent bond breaks homolytically. so, one electron goes to each atom this results in two separate atoms they are called free radicals
why does the C-H bond not break during the intiation step
the C-H bond in the alkane needs more energy to break than is available in a quantum of ultraviolet radiation, so this bond does not break
give an example equation of the initiation stage
Br-Br ——-) Br. .Br
define the propagation reaction
free radicals are used up and created in a chain reaction
describe the propagation reaction
the chlorine free radical takes a hydrogen atom from methane to form a stable compound. this leaves a methyl free radical the methyl free radical is also very reactive and reacts with a molecule. this produces another free radical and a stable compound
explain the effect of the two steps of propagation
the effect of these two steps produces a stable compound and a new free radical. this is ready to react with more methane and repeat the two steps
define the termination step
the free radicals are removed. if two free radicals join together they make a stable molecule. the two unpaired electrons from a covalent bond
what are the three possible termination reactions using chlorine as an example
two chlorine free radicals react together to give chlorine Cl. + Cl. = Cl2 two methyl free radicals react together to give ethane CH3. + CH3 = C2H6 A chlorine free radical and a methyl free radical react together to give chloromethane
how is dichloromethane made at the propagation stage
Dichloromethane may be made at the propagation stage, if a chlorine radical reacts with some chloromethane that has already formed
what occurs when longer-chain alkanes are used in the chain reaction
there will be many isomers formed because of the Cl. can replace any of the hydrogen atoms
why are chain reactions unuseful
they are not very useful because they produce such a mixture of products. they will also occur without light at high temperatures
what destroys the Ozone layer
CFCs chloroflurorocarbons
define Ozone
ozone is a molecule made from three oxygen atoms, O3 it decomposes to oxygen
what can too much ozone at ground level cause
causes lung irritation and degradation of paints and plastics
why is the Ozone layer important
it protects the earth from harmful exposure to too many ultraviolet rays
give an example of what would happen without the Ozone layer
plankton in the sea, which are at the very bottom of the food chain of the oceans, need protection from too much Uv radiation also, too much UV radiation causes skin cancer in people by damaging DNA
explain why chain reactions are important using chlorine as an example
chlorine free radicales are formed from CFCs because the C-Cl bond breaks homolytically in the pressure of UV radiation to produce chlorine free radicals, Cl. . ozone molecules are then attacked by these the resulting free radicals also attack ozone and regenerate cl. chlorine free radical is not destroyed in this process. it acts as a catalyst in the breakdown of Ozone to oxygen