12.5- THE FORMATION OF HALOGENOALKANES Flashcards
When will an alkane react with a halogen? (2)
put mixture of alkane and halogen into bright sunlight
or
shine photoflood lamp onto mixture
What will the alkane and halogen react to form?
halogenoalkane
What component of light starts the reaction to form halogenoalkenes?
ultraviolet component
At what condition do alkanes not react with halogens?
in the dark at room temperature
What happens when you shine UV light onto a mixture of hexane and bromine? (hint- change)
changes from red-brown colour to colourless and misty fumes of hydrogen bromide appear
What type of reaction takes place when UV light is shone on a mixture of hexane and bromine?
substitution reaction
Why is it called a substitution reaction when UV light is shone on a mixture of hexane and bromine?
one or more of the hydrogen atoms in the alkane has been replaced by a bromine atom
What is the equation of the main reaction when UV light is shone onto a mixture of hexane and bromine?
C6H14 (g) + Br2 (l) = C6H13Br (l) + HBr (g)
What is the halogenoalkane formed when UV light is shone on a mixture of hexane and bromine?
bromohexane
What type of substitution is the reaction between the alkane and halogen with UV?
free-radical substitution
What does free-radical substitution start off?
chain reaction
How many stages are there in the chain reaction from free-radical substitution?
3 stages
What are the 3 stages in the chain reaction from free-radical substitution?
initiation
propagation
termination
What is the initiation steps? CH4 (g) + Cl2 (g) = CH3Cl (g) + HCl (g)
breaking the Cl-Cl bond to form two chlorine atoms
Why does the chlorine molecule break in initiation?
chlorine molecule absorbs energy of single quantum of UV light
energy of one quantum of UV light greater than Cl-Cl bond energy, so bond will break
How does the Cl-Cl bond break?
homolytically
Why does the Cl-Cl bond break homolytically?
both the atoms are the same
What happens when the Cl-Cl bond is broken homolytically?
one electron goes to each chlorine atom
What does the Cl-Cl breaking homolytically result in?
two separate chlorine atoms
What does the . on Cl. represent?
unpaired electron
What is the Cl. called?
free radical
How reactive are free radicals?
highly reactive
Why is the C-H bond in the alkane not broken in initiation?
needs more energy to break than is available in quantum of UV radiation
In how many steps does propagation take place?
2
What is the first stop of propagation?
CH4 (g) + Cl2 (g) = CH3Cl (g) + HCl (g)
chlorine free radical takes hydrogen atom from methane to form hydrogen chloride- stable compound
leaves methyl free radical .CH3
Cl. + CH4 = HCl + .CH3
What is the second step if propagation?
Cl. + CH4 = HCl + .CH3
methyl free radical also very reactive + reacts with a chlorine molecule
produces another chlorine free radical + a molecule of chloromethane- stable compound
.CH3 + Cl2 = CH3Cl + Cl.
What is the effect of the two steps in propagation?
CH4 (g) + Cl2 (g) = CH3Cl (g) + HCl (g)
produce hydrogen chloride, chloromethane, and new Cl. free radical
What is the Cl. free radical ready to react with?
more methane and repeat the two steps
What is the chain part of the chain reaction?
Cl. free radical reacting with more methane and repeating step 2
How many times can the steps within propagation happen?
thousands of times
What happens in termination?
free radicals are removed
How can the free radicals be removed?
.CH3 + Cl2 = CH3Cl + Cl.
Cl. + Cl. = Cl2
two chlorine free radicals react together to give chlorine
.CH3 + .CH3 = C2H6
two methyl free radicals react together to give ethane
Cl. +.CH3 = CH3Cl
chlorine free radical + methyl free radical react together to give chloromethane
What do two free radicals react to form?
stable compound with no unpaired electrons
What other products are formed as well as the main ones, chloromethane and hydrogen chloride? (4)
CH4 (g) + Cl2 (g) = CH3Cl (g) + HCl (g)
some ethane produced at termination stage
dichloromethane may be made at propagation stage, if chlorine radical reacts with some chloromethane that has already formed
CH3Cl + Cl. = .CH2Cl + HCl
followed by .CH2Cl + Cl2 = CH2Cl2 + Cl.
What can be produced in free-radical substitution with longer-chain alkanes?
will be many isomers formed as .Cl can replace any of the hydrogen atoms
Why are chain reactions not useful?
they produce such a mixture of products
What can chain reactions occur without?
without light at high temperatures
What from the stratosphere is believed to be destroying the ozone layer?
chlorofluorocarbons (CFCs)
What is ozone?
molecule made from three oxygen atoms, 03
What does ozone decompose to?
oxygen
What does too much ozone at ground level cause?
lung irritation + degradation of paints + plastics, but high in atmosphere has vital role
Why is the ozone layer important?
it protects the Earth from the harmful exposure to too many UV rays
Examples of what will happen without the ozone layer? (2)
plankton in the sea, which are at the bottom of the food chain of the oceans, need protection from too much UV radiation
too much UV radiation causes skin cancer in people damaging DNA
What can chlorine free radicals be formed from?
CFCs
How are chlorine free radicals formed from CFCs?
C-Cl bond breaks homolytically in the presence of UV radiation to produce chlorine free radicals, Cl.
What are ozone molecules attacked by?
chlorine free radicals
Equation for chlorine free radical attacking ozone molecule
Cl. + O3 = ClO. + O2
What does the resulting free radical from Cl. + O3 = ClO. + O2 do?
ClO. attacks ozone and regenerate Cl.
Equation for ClO. attacking ozone and regenerating Cl.
ClO. + O3 = 2O2 + Cl.
What does adding the two equations: Cl. + O3 = ClO. + O2 and ClO. + O3 = 2O2 + Cl show?
chlorine free radical not destroyed in this process
What does the chlorine free radical act as in the breakdown of the ozone to oxygen?
catalyst
Overall equation for ozone being broken down into oxygen
203 = 302
