Reactions of alkenes
1- Combustion reactions [ complete and incomplete]
2- Addition reactions on the double bond.
Addition of H2
Addition of Cl2
Addition of HX (HBr, HCl)
addition of steam, H2O
Flow chart of addition reactions of alkenes (ethene)
CC
CC
Br Br
H
Minor product
CC
CC
n
OH
Br2 /H 2O
Bromine
water
Br OH
heat H 2O / H 
Major product
Bromo alcohol
CC
 H2
 Br2
CC
Ni /  200 C
o
CC
H H
Br Br
Alkane
1,2-dibromoethane
KMnO 4 /dilute H2SO4
 HBr
CC
CC
Br H
HO OH
Alkane-1,2-diol
Symmetric molecule gives only one product
Asymmetric molecule gives two products, [major ( more stable) and minor products( less stable)] because it produces
two types of unstable intermediate carbocations
primary carbocation
H
H
C
+
secondary carbocation
H
H
H3C
C
+
Tertiary carbocation
H
H
Carbon with positive charge is bonded to only one carbon
or no carbons
H3C
C
+
CH3
CH3
Carbon with positive charge is
bonded to two other carbons
Relative stability of carbocations: Tertiary > Secondary > Primary
The more stable carbocation contributes to the major product
H3C
C
+
CH3
Carbon with positive charge is
bonded to three other carbons
Flow chart of addition reactions of asymmetric alkene (propene) to steam and hydrogen bromide
Major product
Minor product
|
|
|
|
|
C
C C
|
|
H
|
C C
|
OH
|
|
C
|
OH H
H 2O / H 
heat
C
C C

HBr
|
|
|
|
|
|
|
|
|
|
C
|
Br H
Br
H
|
C C
C
C C
Major product
Minor product
The mechanism of Electrophilic addition of bromine to alkenes (ethene)
Intermediate
carbocation
Electrophilic addition
CC
|
|
C
C
|

Br


Br

Nucleophilic addition
: Br 

|
|
C
C
|
|
Br
Br
Br
The mechanism of Electrophilic addition of bromine to asymmetric alkene (propene)
Intermediate
carbocation
H
C
|
CC
|

Br :


Br
|
C C

Br


C
|
Br
carbocation
|
|
C C
|
|
|
C
|
Br Br
1,2-dibromopropane
(the only product)
The mechanism of Electrophilic addition of bromine to symmetric alkene (cis or trans but-2-ene)
: Br 
CH3
CH3
CC




Br

|
C
|
Br
CH3
C
CH3
CH3
|
|
C
C
CH3
|
|
Br Br
H
1,2-dibromobutane
carbocation
(the only product)
Br
The mechanism of Electrophilic addition of hydrogen bromide to Ethene
Electrophilic addition
CC
|
|
C
C
Nucleophilic addition

|
H



: Br 

|
|
C
C
|
|
H
Br
Br
H
The mechanism of Electrophilic addition of hydrogen bromide to asymmetric alkene (propene)
|
|
|
C C
|
|
H
C

: Br 
C
|
|
H
Br
1-bromopropane
(Minor product)
 Less stable


|
|
C C
|
primary carbocation
CC

|
C

Br
H
|
|

Br :
|
|
C C
C
|
H

|
|
|
C C
|
|
C
|
Br H
secondary carbocation
 More stable
2-bromopropane
(Major product)
The mechanism of Electrophilic addition of HBr to symmetric alkene (cis or trans but-2-ene)
carbocation
CH3
CH3
CC

H

CH3

Br

|
|
C
|
H
C

|
CH3
CH3 C
|
: Br 
H
|
C CH3
|
Br
2-bromobutane
(the only product)
Testing for alkenes (double bond)
1st test: using bromine water
1- Test : add orange bromine water
2- Result: orange turn colourless
2nd test: using acidified purple potassium managanate (VII)
1- Test : add purple potassium managanate (VII)
2- Result: decolorized / turn from purple to colourless
In this reaction, the purple managanate (VII) ions are is reduced to the colourless manganese (II) ions
Addition reactions of alkenes
1- addition of hydrogen




type of reaction: addition because it gives only one product
Reaction name: hydrogenation ( type = addition)
Conditions: finely divided nickel catalyst, and 200 oC
Product: alkane
Ni


alkane
Alkene + hydrogen
Cn H 2 n +
Ni

H 2 
(word equation)
(general equation)
Cn H 2 n 2
H H
C C
 H2
Ni

|
|
|
|
H  C C H
(using structural formulae equation)
H H
2-
addition of halogens ( X 2  Br2 , Cl2 , I 2 )




type of reaction: addition because it gives only one product
Reaction name: halogenation
Conditions: room temperature and pressure
Product: dihalogenoalkane ( disubstituted halogenoalkane)
Alkene + halogen

dihaloalkane
(word equation)
Cn H 2 n +

Cn H 2 n Br2
(general equation)
Br2
dibromoalkane
Cn H 2 n +
Cl2

(general equation)
Cn H 2 n Cl2
dichloralkane
C C

Br2 
brown
H
H
|
|
|
|
H C CH
Br Br
1,2-dibromoethane
(Colourless)
(Using structural formulae)
3- addition of hydrogen halides( HX = HBr , HCl , HI )




type of reaction: addition because it gives only one product
Reaction name: hydrohalogenation
Conditions: room temperature and pressure /very rapid
Product: monohalogenoalkane ( monosubstituted halogenoalkane)
Alkene + hydrogen halide
Cn H 2 n +
HBr 

monohaloalkane
(word equation)
(general equation)
C n H 2 n1 Br
monobromoalkane
Cn H 2 n +
HCl 
(general equation)
Cn H 2 n1Cl
Monochloroalkane
Remark: the addition of HX to asymmetric alkene such as propene, produces two types of carbocations,
generally primary and secondary.
The product from secondary carbocation more stable and called major product because
secondary carbocation is relatively more stable than the primary carbocation.
H H H
C
C C
asymmetric
alkene
 HBr
|
|
|
|
|
|
H - C C  C H
H Br H
2-bromoalkane
Major product because the produced intermediate
product, secondary carbocation, is more stable
than primary carbocation.
H H H
|
|
|
|
|
|
H - C  C  C  H minor product because the produced intermediate
H H Br
product, primary carbocation, is less stable
than secondary carbocation
1-bromopropane
Comparison between atom economy and percentage yield of producing monosubstituted alkane
an alkane by free radical substitution and from alkenes by electrophilic addition

higher atom economy from addition reaction of an alkene because in alkenes the
monosubstituted alkane is the only one product, no by-products such as HBr, HCl,..

Higher yield from alkenes because no di-, tri- etc
4- addition of water ( H 2O )




type of reaction: addition because it gives only one product
Reaction name: hydration
Conditions/reagents: sulphuric acid and heat
Product: alcohol

Alkene + water (steam)
Cn H 2 n +
H  OH 
alcohol
(word equation)
(general equation)
Cn H 2 n1OH
alcohol
C
C C
 H  OH
H
H
H
|
|
|
|
|
|
H - C C  C H
Major product
H OH H
Asymmetric
alkene
Propan-2-ol
H H
H
|
|
|
|
|
|
H - C  C  C  H minor product
H H OH
Propan-1-ol
5- addition of bromine water (orange) ( H2O,Br2 mixture )
 used as a test for alkenes / double bond
 Conditions/reagents: H 2O, Br2 mixture
 Products: 2-bomoalkanol (major) and 1,2-dibromoalakane(minor)
C C
 Br2
 H 2O
OH Br
|
|
|
|
H
H
H C  CH
Major product
2-bromoethanol
Br Br
|
|
H C CH
|
|
H
H
1,2-dibromoethane
minor product
6- Reaction of alkenes with acidified potassium manganate (VII)
 used as a test for alkenes / double bond
 Conditions/reagents: purple KMnO4 and dilute sulphuric acid
 Product: alkane-1,2-diol or alkane-2,3-diol [ depending on the position of the C=C]
H
OH OH
H
KMnO 4 /dilute H2SO4
C C
H
|
|
H C  C H
H
|
|
H
H
Ethane- 1,2- diol
7- Polymerization
Polymer: a long chain molecule made up by joining small units (molecules) together. These small molecules
that are joined together called monomers
Addition polymerisation

Definition: joining small unsaturated identical molecules (monomers) to form large saturated molecule (polymer)
The double bonds break and all monomers join together forming the polymer.

X Y
|
|
7 C  C 

|
|
Z W
X Y
| |
 C C 
| |
Z W
Monomer
one repeat unit
-
X Y
| |
C C 
| |
Z W
X Y
| |
C C 
| |
Z W
X Y
| |
C C 
| |
Z W
X Y
X Y
| |
| |
C  C  C C 
| |
| |
Z W Z W
X Y
| |
C C 
| |
Z W
part of the polymer showing 7 repeat units
one repeat unit
X Y
| |
 C C 
| |
Z W
General equation of addition polymerisation :
X Y
|
|
100o C


n C  C 100atm, traces ofair
|
|
Z W
X Y 
| | 
 C C 
| | 
 Z W

n
Where X, Y, Z and W are any atoms or groups of atoms
Features of addition polymerisation:
-
The monomer contains
CC
The polymer is the only product
The repeat unit is the same as the monomer; except that the repeat unit has single bonds only, while
the monomer has one C  C .
It is called addition polymerisation because the polymer is the only product and the monomer contains a double bond
Empirical formula of all poly(alkenes) is CH2 because their smallest whole number ratio of C:H is 1:2
Big note:
- all addition polymers are synthetic
- synthetic polymers are non-biodegradable
nonbiodegradable = cannot be broken down by bacteria and microorganisms
Hint: Whenever you are given an unsaturated monomer, it is better to represent it in this form
| |
CC
| |
A- Poly(alkenes)
n alkene  poly(alken e)
monomer
1-
polymer
Poly(ethene)
- monomer: ethene gas
- Product: poly(ethene)
Conditions are not required
Equation of polymerisation
H
|
H
|
200o C

100atm, traces of air
n C C
|
|
H H
ethene
monomer
gas
H
|
repeat unit:
H
|
H
|
|
H
|
H
CC
n
poly(ethene)
polymer
solid
H
|
H
|
|
H
|
H
CC
CC
|
H
H
|
|
H
H
|
H H
| |
H
|
|
H
| |
H H
|
H
CC CC
part of the polymer showing 3 repeat units
Uses of ethene:
To manufacture 1.poly(ethene), 2.ethanol, 3.dry clean liquid (dichloroethane) and as 4.fuel
2- Poly(propene)
H
|
CH3
|
nC C
|
|
H H
propene
gas solid
monomer
200o C
H
|
CH3
|
|
H
|
H
CC

100atm, traces of air
poly(propene)
One repeat unit
n
H
|
CH3 H
|
|
|
H
|
H
CC
polymer
CH3 H
|
|
CC
|
H
|
H
CH3
|
CC
|
H
|
H
part of the polymer showing 3 repeat units
3- Poly(but-1-ene)
o
100 C
n CH3  CH 2  CH  CH 2 

100 atm
H
H
|
|
C
C
|
|
C2H5 H n
H
H
|
|
C
C
|
|
C2H5 H
H
H
|
|
C
C
|
|
C2H5 H
4- Poly(but-2-ene):
o
100 C
n CH3  CH  CH  CH3 

100 atm
H
|
C
|
CH3
H
|
C
|
CH3 n
H
|
C
|
CH3
or
H
|
C
|
CH3
H
|
C
|
CH3
H
|
C
|
CH3
Uses of poly (alkenes)
To manufacture plastic bags, bins, buckets, bowls, insulators and reagent bottles
B- Other addition Polymers
(a) Poly(phenylethene): polystyrene
n C6 H 5  CH  CH 2 

100 atm
phenylethene
H
H
|
|
C
C
|
|
C6H5 H
H
H
|
|
C
C
|
|
C6H5 H
H
H
|
|
C or
C
|
|
C6H5 H n
o
100 C
2 repeat units
Used in making disposable cups, plates, etc…
H
H
|
|
C
C
|
|
C6H5 H
One repeat unit
(b) Poly(acrylonitrile):
H
|
C
|
CN
o
100 C
n CN  CH  CH2 

100 atm
acrylonitrile
H
H
|
|
C
C
|
|
CN H
H
|
or
C
|
H n
H
|
C
|
CN
H
|
C
|
H
H
|
C
|
CN
H
|
C
|
H
3 repeat unit
Used in making beds, furniture and clothes
H
|
C
|
CN
One repeat unit
(c) Teflon
F F
| |
n CF2  CF2  

C C
100 atm
| |
1,1,2,2-tetrafluoro ethane
n
F F
Teflon
F F F F
o
100 C
or
|
|
|
|
|
|
|
|
 C C  C C
F F F F
Used as nonstick layer in frying pans [ has very high melting point]
(d) Poly(vinyl chloride)
Monomer : Vinyl chloride
CH2  CHCl
o
n CH2  CHCl
100 C
 

100 atm
H
|
C
|
H
H
|
C
|
Cl n
or
Uses: To manufacture plastic pipes, water bottles, insulators and table covers
H
|
C
|
Cl
H
|
C
|
H
H
|
C
|
Cl
H
|
C
|
H
H
|
C
|
H
Using skeletal formula
polymerisation of propene
One repeat unit
n
n
propene
Poly(propene)
two repeat units
polymerisation of cis - butene
One repeat unit
n
n
Cis-butene
Poly(butene)
two repeat units
polymerisation of 1,2-dimethyl cyclohexene
One repeat unit
n
n
Cis-butene
Poly(butene)
two repeat units
Comparison between the properties of the monomer and the polymer
Monomer
“Alkene”
property
-
physical
-
-
chemical
-
monomer
Polymer
Uses of the
polymer
Physical
properties
gas because it is simple molecule; has
weak intermolecular forces that are
broken by only little energy.
reactive because it has C=C,
it undergoes addition reactions
Ethene
Poly(ethene)
Polymer
“poly(alkene)”
-
solid because it has strong
intermolecular forces as it is
large molecule.
-
unreactive because it has
only strong single bonds
Propene
Poly(propene)
Vinyl chloride
Poly(vinyl chloride)
PVC
Packaging,
Films, ropes that
Water pipes and
utensils
don’t rot
storages
Thermoplastics: Can be resoftened and molded into new
shapes when warmed
Tetrafluoroethene
Teflon
In nonstick frying
items
Very high melting point
Low –density poly (ethene)= LDPE: has highly branched chain with
fairly low melting point.
It is soft and malleable.
High –density poly (ethene)= HDPE: has few branched chain with
low melting point.
It is more rigid than LDPE and
melts at higher temperature.
Both types soften as it is warmed
the raw material for all poly(alkenes) is crude oil because alkenes are obtained by cracking of alkanes from crude oil.
Remember that all addition polymers are nonbiodegradable
Environmental problems associated with polymers industry:
 Not easy to dispose
 Non-biodegradable: are not broken down by microorganisms as bacteria and fungi
 Burning plastics produces a variety of toxic gases, like hydrogen chloride from PVC and hydrogen cyanide
which are damaging both to human health and to other living organisms.
 Burning plastics also increase the carbon footprint
 made from valuable finite resources, fossil fuels
 large amount of energy is used in both manufacturing the polymers and the products made from them.
 problems with disosal of waste from polymers
- Increases landfill sites ( as they are non-biodegradable – take long time to decay)
- Harmful to marine life / harmful to wildlife
- when they are burned a variety of toxic gases may be produced, like hydrogen chloride from PVC and
hydrogen cyanide which are damaging both to human health and to other living organisms such as marine life.
 Solutions to the problems of waste from polymers
- Biodegradable plastics are now being developed from lactic acid (2-hydroxypropanoic acid), and
3-hydroxypropanoic acid which forms a polymer known as biopol.
- Soluble polymers such as poly(ethenol), are used to make liquitabs (detergents in water
soluble packets) and laundry bags for use in hospitals, to avoid handling of contaminated laundry.
- Recycling thermoplastics ( plastics that soften on heating and thus can be reshaped into new items)
Advantages of Recycling
1reduces disposal demand,
2 reduces emission of gases such as CO and SO and
2
2
3-both saves energy and fossil fuel finite resources
Using renewable energy sources such as solar and wind energy in the manufacture of polymers
Use fabric or paper bags instead of plastic ones in supermarkets
Recover some of the energy used in producing the polymers by burning plastics and other polymers and use
the energy released to generate electricity
Incineration = disposal of polymers by burning
-
What is incineration?
Ans: disposal of polymers by burning
Adverse effect of Incineration

Incineration of any chlorinated polymer releases HCl and other chlorinated compounds
which are corrosive/toxic /cause acid rain
How to reduce the adverse effect of incineration of polymers
 Ans: pass the gasses through a base such as aqueous NH3, watered CaCO3, CaO,…..), it
reacts with HCl(g) and neutralises it.
Solved important questions
1. Explain why incineration is not a suitable method for disposal of poly(chloroethene)
Ans: incineration produces HCl (1) which is corrosive/toxic /cause acid rain (1)
allow chlorinated molecules
IGNORE carbon dioxide and its consequences as it is product of combustion of any fuel.
don’t allow chlorine [because Chlorine is not a product of incineration]
2.
Domestic waste is also disposed of by incineration. Outline one advantage of this method
(incineration).
Ans: reduction in bulk/ useful energy can be generated
Objective be able to discuss the reasons for developing alternative fuels in terms of sustainability and reducing emissions,
including the emission of CO2 and its relationship to climate change.
Objective: be able to apply the concept of carbon neutrality to different fuels, such as petrol, bioethanol and hydrogen
Terms to know: Nonrenewable fuel, renewable fuel, Biofuel, Anthropogenic, Carbon neutrality,
Nonrenewable fuel: used up faster than it is formed; it is a finite resource/it takes millions of years to
produce/ it will ‘run out’
renewable fuel: doesn’t ‘run out’ / from nonfinite resources
Biofuel = fuel produced from plants such as bioethanol which is ethanol produced by fermentation of sugar
from plants
Anthropogenic = climate change is man-made change.
Examples on anthropogenic

release of CO2 and SO2 gases from combustion of fossil fuels and NO2 from reaction between N2
and O2 from air inside car’s engines at high temperature.
Carbon neutrality = when the CO2 released on combustion is equal to the CO2 absorbed on
formation of the fuel/by plant in photosynthesis
biofuels are unlikely to be completely carbon neutral BECAUSE CO2 is likely to be released
from combustion of fossil fuel during transport and production of biofuel 
A biofuel is cosidered carbon neutral if
the amount of carbon dioxide absorbed by plants in photosynthesis (when plants grow up)
equals the amount of carbon dioxide released when it is burnt. 
hydrogen is unlikely to be completely carbon neutral BECAUSE CO2 is likely to be released from
combustion of fossil fuel during transport and production of hydrogen by electrolysis or cracking 
Measures by which the chemical industry could reduce its carbon footprint.
Use biofuels (as they have very small carbon footprint)
- Use catalysts to reduce consumption of energy from fossil fuels
- Use nuclear power energy
- Use renewable energy sources such as wind power, solar power and fuel cells
- neutralising with scrubbers such absorbing with alkali/ limestone in water
pass acidic gases such as HCl through aqueous ammonia/ NaO, CaCO3,…
-
harmful consequence to a person of increased exposure to ultraviolet radiation are
- skin cancer
- affects eye retina
a.
Objective know that pollutants, including carbon monoxide, oxides of nitrogen and sulfur, carbon
unburned hydrocarbons, are emitted during the combustion of alkane fuels
particulates and
1. How and why is SO2 gas formed during combustion of fossil fuels?
Crude oil contains small amount of sulfur; 2. on burning alkanes, sulfur in fuel burns forming SO2 gas.
SO2 released to the atmosphere combines with water vapour and forms sulfuric acid that falls as acid rain
Consequences of acid rain
 damages limestone building,
 harms aquatic life,
 damages forests, and ….
-
carbon particulates ( black soot) from unburned carbon are dangerous to health
-
unburned hydrocarbons due to fuel - oxygen unbalance are harmful to health.
How is NO gas formed?
Ans: Under high temperature inside vehicles engines, N2 and O2 in air react forming oxides of nitrogen
Equation of the reaction:
N2(g) + O2(g) → 2NO(g)
Then NO reacts with more O2 in air forming NO2.
2NO(g) + O2(g) → 2NO2(g)
How is the amount of NO and CO released to the atmosphere decreased?
Ans: By passing them through the catalytic converter, it changes the dangerous gases, NO and CO into
non harmful gas N2 and CO2.
It also changes unburned hydrocarbons ( fuel) into CO2 and H2O
The reaction that takes place inside the catalytic converter
2NO(g) + 2CO(g) → N2(g) + CO2(g)
Catalytic converter
However, CO2 is a greenhouse gas that contributes to global warming when its concentration increases in the
atmosphere.
Answered questions
1. When does complete combustion occur?
Ans: enough oxygen/ O2.
2. When are the products of incomplete combustion?
Ans: carbon dioxide and water
3. Write the equation for complete combustion of ethane. State symbols are not required.
Ans: C2H6 + 3½O2 → 2CO2 + 3H2O
Similar equations are written the same way – must be balanced
4.
State and explain the main environmental problem arising from the complete combustion of alkane fuels.
Ans: - Environmental problem: Global warming (1)
1. Explanation: Due to increase in concentration of the greenhouse gas CO2 in the
atmosphere
CO2 traps IR rays radiated from the earth surface (1)
Ignore references to the effects of climate change
5. When does incomplete combustion occur?
Ans: Insufficient oxygen/ not enough O2.
6. When are the products of incomplete combustion?
Ans: carbon, carbon monoxide, carbon dioxide and water
7. An incomplete combustion of methane, CH4, results in the formation of carbon monoxide and water
only. Write the equation for this reaction. State symbols are not required.
Ans: CH4 + 1½O2 → CO + 2H2O
Similar equations are written the same way – must be balanced
8. State two problems that result from the incomplete combustion of alkane fuels.
Ans: - CO is toxic / poisonous (1)
Unburned hydrocarbons react to form compounds which are toxic / harmful (1)
Carbon soot ( particulates) cause global dimming
9. Explain why carbon monoxide released from incomplete combustion of fossil fuels is toxic.
Ans: - the released CO binds with hemoglobin and prevents oxygen from reaching body
cells which leads to death. (causes asphyxia)CO is toxic / poisonous (1)
10. Explain the environmental problem and the adverse effect on health associated with carbon soot
( particulates) from incomplete combustion of fossil fuels.
Ans: - Carbon soot ( particulates) cause global dimming
Carbon soot cause respiratory problems especially in people with asthma
11. Give TWO factors which have to be considered when deciding which material, PVC or metal, contributes to
more sustainable uses of resources in the long term.
Ans: - PVC will last longer than iron due to lack of corrosion (1)
PVC comes from oil which is non-renewable (1)
PVC and metals come from non-renewable sources (1)
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Alkenes have geometric isomerism becuase
C=C prevents rotation
each C in the C=C double bond must be bonded to two different atoms or groups