Chem Unit 2 (Haloalkanes) Flashcards
Haloalkane is?
Alkane where one or more of the hydrogen atoms have been reeplaced by a halogen atom (e.g. CL, Br or F)
Naming Haloalkanes
1) carbon -stem name
2) Br, Cl, F in alphabetical order (2-di, 3-tri)
3) add methyl groups and positions
Free radical substitution (chloroalkanes)
1) start with a hydorcarbon e.g. CH4 +Cl2 = CH3Cl + Cl2 (take away one hydorgen atom, add HCl (see if there are any chlorine left to add on to new haloalkane) Keep repeating with each new haloalkane.
Formation/Breakage of ozone
present in small amounts at trosophere
broken down/ formed in large concentrations in stratosphere
02 > 0. + 0. (UV radiation breaks down oxygen into two free radicals)
0. + 02 > 03 (oxgyen radical collides with oxygen atom = ozone)
03 > 02 + 0. (UV breaks down ozone too)
Why is UV radiation bad?
UV damages biological life on earth (skin cancer)
Natural breaking down of ozone stops more reaching the earth’s surface
Overall its constant rate of breaking down = rate of forming
What is the problem with there being other radicals that can break down ozone?
Problem: radicals like chlorine radicals are in compounds that are made in large quantiites becuase they sre useful (CFC’s)
When these diffuse into the stratosphere, the UV radiation breaks the C-Cl bond to produc e achlorine radical which can react with ozone.to produce oxygen
Cl. + 03 > Cl0. + 02
This can react with anoterh ozone molecule
Cl0. + 03 > 202 + Cl.
add 2 equations to show ozone > oxygen
203 > 302
Why is chlorine breaking down ozone a problem?
Becuasee it ahs a lower activation energy than chlorine, so breaks down ozone way faster than oxygen breaks down and FORMS ozoen so the rate is not constant (destroying the ozone)
chlorine radical is a cat…?
catalyst so speeds up the rate fo reaction without being sued up itself
Order of reactivity for haloalkanes?
C-F (unreactive- too much energy needed)
C-Cl
C-Br
C-I
What are CFC’s and 3 uses of them?
gaseous haloalkanes
blowing foam plastics, AC, refridgerators
Do CFC’s occur naturally?
No
CFC’s naming?
C- 1) 2) 3)
1) Number of carbon atoms minus 1 (if 0 leave)
2) Number of hydrogen atoms plus 1
3) Number of fluroine atoms
Why were CFC’s considered so useful?
stable- didn’t react n aerosal cans/ or with blow-foam plastics/ low boiling point/loads were released into atmosphere
What is the problem with CFC’s being stable so they diffuse into stratosphere instead of trosophere?
problem: uv radiation breaks down the C-Cl bond into a chlorine radical (photodissocaition)
CF2Cl2 > (uv) C. F2Cl + Cl. (now reacts with ozone)
Initiation step of haloalkanes?
Cl2 > Cl. + Cl.
Homolytic fission?
when a covalent bond breaks each atoms gets one electron (unpaired)
makes it very reactive
radical= atom with at least one unpaired electron
First propagation step:
hydrocarbon + radical > hydrocarbon -hdrogen (as a radical- donated an electron) + HCL/HF/HBr (radical has gained an electron and joined with the donated hydrogen)
e.g CH4 + Cl. > CH3. + HCl
Second Propagation step:
The hydrocarbon radical reacts with Cl2/F2/Br2 to a haloalkane with one atom from Cl2/F2 and the other one becomes a radical.
If the reactant is a haloalkane and a radical..
haloalkane radical and a HCl/HBr ect
If the reactant is a haloalkane radical and a Cl2 / br2 ect?
haloalkane and a free radical
Termination step:
free radicals react with each other (rarely radical haloge atoms on their own)
- unpaired electrons cancel each other out
1) Look at a haloalkane radical product you’ve formed and react with a radical to make product they’re asking for
Overall Equation:?
1) Look at last haloalkane product you’ve formed (asked in the question) (could be from termination step or a propagation step) and the product they want and balance the equation.
use La Chatelier’s principle to show how an increase in termperature causes increase in yield of ozone?
As its an endothermic reaction, equilbrium shifts to absorb energy (moves to the right) > yield increases
