Unit 3 Chemistry – Reactions of Alkanes and alkenes
Combustion



alkanes are flammable and used as fuels
complete combustion [reaction with excess oxygen from the air] produces water
and carbon dioxide …
there are rules for writing the balanced equation for the combustion reaction …
unbalanced reaction equation … C2H6(g) + O2(g) 
balance the C atoms first
… C2H6(g) + O2(g) 
balance the H atoms next
… C2H6(g) + O2(g) 
finally, balance the O atoms … C2H6(g) + 3½O2(g)
if necessary, multiply by 2
… 2C2H6(g) + 7O2(g)

H2O(g) + CO2(g)
H2O(g) + 2CO2(g)
3H2O(g) + 2CO2(g)
 3H2O(g) + 2CO2(g)
 6H2O(g) + 4CO2(g)
alkenes undergo similar combustion … C3H6(g) + 4½O2(g)  3H2O(g) + 3CO2(g)
Substitution of alkanes

a substitution reaction of an alkane involves one or more of the H atoms on the
alkane being replaced by a different atom or functional group
 substitution reactions involve:1. breaking the strong C–H covalent bond,
2. forming a new bond with the substituting atom
Example: Chlorination of Ethane
 the effect of heat or UV light on a chlorine molecule is to break the Cl–Cl bond,
forming .Cl- free radicals that attack the C–H bonds in the alkane
 one or more of the H atoms on the ethane are replaced by the attacking .Cl-,
resulting in a mixture of chlorinated hydrocarbons.
CH3CH3(g) + Cl2(g)  CH3CH2Cl(g) + HCl(g)
chloroethane
CH3CH3(g) + 2Cl2(g)  CH2ClCH2Cl(g) + 2HCl(g)
1,2-dichloethane
General equation: R–H + Cl2  R–Cl + HCl
------------------------------------------------------------------------Reactions of Alkenes
Properties of ethene [ 1st member of homologous series of alkenes ]
 unsaturated [has C=C];
 non polar molecule
 flammable gas
 insoluble in water
 undergoes addition reactions
Addition Reactions of Alkenes


the C=C double bond makes alkenes more reactive that alkanes
in an addition reaction, a small molecule [ such as Cl2, Br2, H2, H2O, HCl ]
is able to add itself across the double bond, changing the double bond to a
single bond and forming a single product
CH2=CH2 + Br2  CH2(Br)CH2(Br)
H
H
H
H
C=C
H
C
H
C
H
H
Br
Br – Br
Br
1,2 dibromoethane
Example: Addition of water to ethane
 ethane reacts with steam, using a phosphoric acid catalyst to make ethanol
C2H4(g) + H2O(g)  CH3CH2OH(g)
H
H
H
H
H3PO4
C=C
H
C
H
300oC
C
H
H
OH
H
H – OH
ethanol
Addition Polymerisation


a large number of alkene molecules [monomers] can undergo a chain of addition
reactions to form a very large molecule [polymer]
this polymer forming addition reaction is called addition polymerisation
R
n
H
C=C
H
H
R = -H …polyethene;
R = -CH3 …polypropene;
R = -C6H6 … polystyrene
R
H
R
H
R
---– C– C– C– C– C –--H
H
H
H
H
R = -Cl … polychloroethene [PVC]
R = -OH … polyvinylalcohol [PVA]
Reactions of Alkanes and Alkenes
1. Write the balanced molecular equation for the complete combustion of
the following alkane and alkene in excess air:(a) 2,3-dimethyl pentane
(b)
2,3-dimethyl but-1-ene
------------------------------------------------------------------------2. When propane and chlorine gas is mixed and exposed to ultraviolet
light, a mixture of chlorinated alkanes, including 1,2-dichloropropane,
is produced.
(a) Name the reaction type that forms 1,2-dichloropropane.
(b) What role does the UV light play in the reaction?
(c) Draw the structural formula of 1,2-dichloropropane.
(d) Write a balanced molecular equation for the reaction that forms
1,2-dichloropropane.
(e) suggest how the 1,2-dichloropropane could be separated from the
reaction mixture.
------------------------------------------------------------------------3. When propene and chlorine gas is mixed, a single reaction product
1,2-dichloropropane is formed.
(a) Name the reaction type that forms 1,2-dichloropropane.
(b) Write a balanced molecular equation for the reaction that forms
1,2-dichloropropane.
(c) Compare the reactions that occur between propene and chlorine, and
between propane and chlorine, explaining why a single reaction
product is formed in the former and a mixture of reaction products is
formed in the latter.
-----------------------------------------------------------------------4. Explain how derivatives of ethene [XCH=CH2, where X = -OH, -Cl, -C6H5
etc.] are the basis for polymers such as PVA, PVC and polystyrene.
-------------------------------------------------------------------------