12.5 The formation of halogenalkanes Flashcards
When you put a mixture of an alkane and halogen into bright sunlight, or shine a photoflood lamp (UV) into the mixture,
what happens?
They react to make a halogenalkane
As the UV component of light starts the reaction
So alkanes don’t react with halogens in the dark at room temp
What happens when hexane and bromine are mixed in a test tube in the dark
What about in light
Give substitution equation for this
What is the name of this type of reaction
In the dark, it stays red/brown which is the colour of bromine
In the presence of UV light, it becomes colourless and misty fumes of HF appear
C6H14 + Br2 —> C6H13Br + HBr
So one or more hydrogen atoms in alkane are replaced by a bromine atom
It is a free radical substitution, which starts off a chain reaction
Why don’t alkanes and halogens react in the dark
There needs to be UVto break the halogen-halogen bond for them to react, which comes from sunlight or photoflood lamps
However with very high temperatures they will react without light
What 3 stages do chain reactions take place in
. Initiation
. Propagation
. Termination
What is the mechanism for any reaction between a halogen and alkane
Eg Methane and chlorine
CH4 + Cl2 —> CH3Cl + HCl
Describe the first step of a chain reaction with chlorine
Initiation
Why doesn’t the UV affect the alkane bonds
. First, the Cl-Cl bond is broken to make two chlorine free radicals by…
. The chlorine molecule absorbs energy of a single quantum of UV light, and the energy is greater than Cl-Cl bond energy so it breaks bond
. Since both atoms are the same, the
Cl-Cl bond breaks so one electron goes to each chlorine atom
. This results in two separate chlorine atoms written Cl .
The dot is the unpaired electron
.Free radicals are highly reactive
. The C-H bond in the alkane needs more energy to break than is available in the UV so their bond doesn’t break
Describe stage 2 of the chain reaction
of chlorine with methane
Propagation
1) The chlorine free radical takes a hydrogen atom from the alkane (methane) to give hydrogen chloride which is a stable compound.
This leaves a methyl free radical .CH3
Cl. + CH4 –> HCl + .CH3
2) The methyl free radical is also very reactive so reacts with a chlorine molecule.
This produces another free radical of chlorine and a molecule of chloromethane which is stable
.CH3 + Cl2 —> CH3Cl + Cl.
The result is a new .Cl radical which is ready to react with more methane and repeat the 2 steps.
Chain part of reaction.
What is the final stage of chain reaction
Termination
. This is where the free radicals are removed
It can happen in 3 ways
Cl. + Cl. –> Cl2
So two chlorine free radicals react together to give chlorine
.CH3 + .CH3 —> C2H6
Two methyl free radicals react together to give ethane
. Cl + .CH3 —> CH3Cl
A chlorine free radical and a methyl free radical react to give chloromethane
What are some other products of this chain reaction to make hydrogen chloride and chloromethane
. Some ethane is produced at termination stage when two .CH3 free radicals react
. Dichloromethane may be made at propagation stage, if a chlorine radical reacts with some chloromethane that has already formed
CH3Cl + .Cl —> .CH2Cl + HCl
Followed by
.CH2Cl + Cl2 —> CH2Cl2 + Cl.
Longer alkane chains make more isomers because the .Cl can replace any hydrogen atoms
. Chain reactions are not very useful because they produce loads of products, and will also occur without light at high temperatures
What is it in the atmosphere that is destroying the ozone layer
Chlorofluorocarbons (CFCs)
What is the ozone layer made of and what problems does it have if at ground level
Ozone is O3 so made of 3 oxygen atoms
It decomposes to oxygen
Too much ozone at ground level causes lung irritation and degradation of paint and plastic
Why is the ozone layer important
It protects earth from harmful exposure of too many UV rays
Without it, life on earth would be very different
Too much radiation causes skin cancer in people by damaging DNA
How does the chain reaction work at ozone layer to destroy it
- Chlorine free radicals are formed from CFCs (Chlorofluorocarbons) because
C-Cl bond breaks homolytically in presence of UV radiation to produce two free chlorine radicals .Cl
Ozone molecules are attacked by these
Cl. + O3 —> ClO. + O2
The resulting free radicals also attack ozone and generate Cl.
ClO. + O3 —> 2O2 + Cl.
So chlorine free radical is not destroyed in process., it acts as a catalyst to break down ozone to oxygen
2O3 –> 3O2
