Alkanes and Alkenes
Topic 10.2
Alkanes

have low reactivity
 bond enthalpies are relatively strong


low polarity
only readily undergo combustion
reactions with oxygen (very flammable)
and substitution reactions with halogens
in UV light


348 kJ mol-1 to break a C-C bond
412 kJ mol-1 to break a C-H bond
Reactions of Alkanes: Combustion


hydrocarbons (only contain C & H)
complete combustion

alkanes burn in an excess supply of oxygen
to form carbon dioxide and water:

example:

C8H18 (g) + 12 ½ O2 (g) → 8 CO2 (g) + 9 H2O (l)
exothermic (-∆H)

incomplete combustion

if oxygen supply is limited, the gas carbon
monoxide, carbon and water is formed
C8H18 (l) + O2 (g) → C (s) + CO (g) + H2O (l)
(notice left over carbon (black soot) and dangerous CO)
Reactions of Alkanes (methane and
ethane) with Halogens (Cl and Br)

alkanes do not react with halogens in the
dark at room temperature, but will react in
the presence of sunlight (UV)

a substitution reaction will occur where
some or all of the hydrogens will be
replaced with a halogen


C2H6 (g) + Br2 (g) → C2H5Br (l) + HBr (g)
Cl2 + CH4  CH3Cl + HCl

this happens by a process know as free
radical substitution that happens in 3
steps
1.
2.
3.
Initiation
Propagation
Termination

initiation


Cl2  Cl* + Cl*
propagation


initiated by UV light breaking a chlorine molecule into
two free radicals by a process called homolytical
fission (* = unpaired electron)
each resulting atom receives one
unpaired electron, known as free
radicals that have lots of energy
keeps the chain going (radical in reactants and products)
CH4 + Cl*  CH3* + HCl
CH3* + Cl2  CH3Cl + Cl*
termination

this removes free radicals (*) from the system without
replacing them by new ones
Cl* + Cl*  Cl2
CH3* + Cl *  CH3Cl
CH3* + CH3*  CH3CH3
each resulting atom
receives one unpaired
electron, known as free
radicals
Reactions of Alkenes
the general mechanism



alkenes react (more reactive than alkenes)
with many substances to form a new
substance
catalysts, acids or other substances may
be required to complete the reaction:
C2H4 + XY → CH2XCH2Y
process occurs by breaking the double
bond.
Reactions of Alkenes
with hydrogen

alkenes react with hydrogen gas to create
an alkane, using nickel as a catalyst at
150ºC:
C2H4 + H2 → CH3CH3
Reactions of Alkenes:
with halogens

alkenes react readily with chlorine or
bromine to create a dihalogenalkane
(general name)
C2H4 + Cl2 → CH2Cl CH2Cl
H
H
H
H
+
Cl Cl
Cl Cl
H
H
H H
1,2-dichloroethane
Reactions of Alkenes
with hydrogen halides

alkenes react readily with hydrogen
halides to create a halogenalkanes
C2H4 + HBr → CH3CH2Br
H
H
H
H
+
H Cl
H Cl
H
H
H H
Reactions of Alkenes
with water


alkenes do not react readily with water
if sulfuric acid is used as a catalyst, an
alcohol will be created

remember that H2O can be dissociated into H+ and
OHH2SO4
C2H4 + H2O → CH3CH2OH
H
H
H
H
H H
acid
+
H
O
H
H
O
H H
H
Distinguish between alkanes and
alkenes using bromine water

bromine water (a red liquid) tests for
unsaturated hydrocarbons (alkenes)

alkanes → stay redish/orange


no reaction
alkenes → turn clear / colourless

because of reaction with unsaturated hydrocarbon
http://www.youtube.com
/watch?v=NjIuBvod2eM
https://www.youtube.co
m/watch?v=PE1CDR1
S5pk
Reactions of Alkenes:
Polymerization
naming polymers


put “poly-” in front of the name of the
monomer
there are 3 polymerization mechanisms that
you need to be familiar with:

1.
2.
3.
polyethene
polychloroethene
polypropene
Polyethene



monomer: ethene CH2=CH2
undergoes addition reactions with itself to
make a chain
n CH2=CH2  [-CH2-CH2-]n
Polychloroethene



each chloroethene contains 1 chlorine
therefore when the chloroethene
molecules polymerize, every
other carbon will bond to 1 chlorine
this is PVC
Polypropene
+
=
Teflon non-stick pans