alkenes
alkenes
 a group of hydrocarbons with the general formula, CnH2n
where n is the number of carbon atoms in one molecule
 contain the C=C functional group (unsaturated hydrocarbons)
 names of alkenes end with -ene
 are molecular compounds
 have low boiling points; gases under room conditions
a ball-and-stick model
of ethene, C2H4
alkenes
alkene homologous series
 each member of the series differs from the next by an
extra –CH2- group of atoms
No. of Carbon
Atoms in One
Molecule
2
3
4
Alkene
Member
ethene
(C3H4)
propene
(C3H6)
butene
(C4H8)
Relative
Molecular
Mass
Structure of One
Molecule
H
28
H
C
-104
C
H
H
H
42
56
H
H
Boiling
Point
(°C)
H
C
C C
H
H
-48
H
H
H
C
C
H
H H
H
-6
C C
H
alkenes
reactions of alkenes: combustion
 alkenes burn in air to form CO2 and H2O
 incomplete combustion results in the formation of carbon
monoxide, CO and soot, C
 alkenes tend to produce more soot than alkanes when they
burn in air
 example: combustion of ethene
C2H4 + 3O2
2CO2 + 2H2O
alkenes
reactions of alkenes: addition reactions
Alkenes undergo addition reaction
This means that a molecule is added to the alkene to make
one product
This is possible because of the carbon-carbon double bond
(C=C ) in an alkene.
H
H
H
C
C
X
X
H
alkenes
reactions of alkenes: addition reactions
 addition of bromine (bromination)
o
o
bromine molecule adds to the C=C bond of ethene
molecule
example: addition reaction of bromine to ethene
C2H4 + Br2
H
H
C
C
H
H
+
H
C2H4Br2
Br
H
C
Br
Br
H
C
Br
H
 reaction with aqueous bromine is used as a test for alkenes
and other unsaturated compounds
 alkene is shaken with or bubbled through a solution of
bromine in water; alkene quickly decolourises the reddishbrown bromine solution
H
H
H
C
C
Br
Br
H
alkenes
reactions of alkenes: addition reactions
 addition of hydrogen (hydrogenation)
o alkenes react with hydrogen to produce alkanes in the
presence of nickel metal (catalyst)
o example: addition reaction of hydrogen to ethene
o
hydrogen molecule adds to the C=C bond of ethene
molecule
 used in producing margarine from vegetable oil
C2H4 + H2
H
H
C
C
H
H
+
H
C2H6
H
H
C
H
H
H
C
H
H
H
H
H
C
C
H
H
H
alkenes
reactions of alkenes: addition reactions
 addition of water (hydration)
o alkenes react with steam over a catalyst to produce
alcohols
o example: addition reaction of water to ethene
C2H4 + H2O
H
H
C
C
H
H
+
H
CH3CH2OH
OH
H
C
OH
H
H
C
H
H
water molecule adds to the C=C bond of ethene
molecule
 alcohols are commonly used as solvents and to make other
chemicals
o
H
H
H
C
C
H
OH
H
alkenes
saturated and unsaturated hydrocarbons
 alkanes are saturated because it is not possible to add extra
atoms to their molecules
 alkenes are unsaturated because extra atoms can be added
to their molecules in addition reactions
H
H
H
C
C
H
H
C
C
H
H
H
H
H
alkenes
saturated and unsaturated hydrocarbons
 differences between saturated and unsaturated hydrocarbons:
Saturated Hydrocarbons
Unsaturated Hydrocarbons
Contains carbon-carbon single
bonds (C-C)
Contains carbon-carbon
double bonds (C = C)
Extra atoms not added to their
molecules in addition
reactions
have alkane-type formula of
CnH2n+2
Extra atoms added to their
molecules in addition reactions
have alkene-type formula of
CnH2n
Reaction with aqueous bromine Reaction with aqueous bromine :
: do not decolourise aqueous
decolourises aqueous bromine
bromine
alkenes
testing for unsaturated compounds
like alkenes
 for liquids: add a solution of bromine in water to an alkene
and shake them; reddish-brown colour of bromine
disappears in the presence of alkene
liquid
alkene
bromine
solution
shake
testing for liquid alkene
bromine
becomes
colourless
alkenes
testing for unsaturated compounds
like alkenes
 for gas: bubble the gas through a solution of bromine in
water; colour of bromine disappears in the presence of
alkene gas
gaseous alkene
bromine
solution
poly(ethene) bromine
becomes
colourless
testing for gaseous alkene
Demo - to distinguish between an alkane and an alkene
(decolourisation of bromine!).
hexene
hexane
hexane
hexene
bromine
bromine
Qn: What will happen if I bubble the gas from a bottle filled with bananas to
the bromine water?
Demo – while ripening, bananas give off ethene gas!
bromine
bananas
bananas
bromine
cracking
manufacture of alkenes
Alkene molecules can be made by cracking alkanes
 a process to break up big alkane molecules into smaller
molecules; done by passing big alkane molecules over a
solid catalyst at a high temperature
 example: cracking of big petroleum (hydrocarbon)
molecules
CATALYST
big hydrocarbon
molecules
hydrocarbon molecules
break up on the
surface of the catalyst
small hydrocarbon
molecules
cracking of alkanes
cracking
C20H42
big
alkane
C18H38
big
alkane
C12H26 + C8H16
an alkane
molecule
an alkene
molecule
X + C10H20
an
________
molecule
an
________
molecule
cracking of alkanes
cracking
 Uses:
Hydrogen can be one of the products in the cracking of
alkanes
C16H34
H2 + C6H12 + C10H20
Hydrogen is used as fuel for big rockets and to make
ammonia for fertilisers
cracking of alkanes
cracking
 Uses:
o
cracking of big petroleum molecules into smaller
hydrocarbon molecules to produce fuel for motor
vehicles;
• This is important as the amount of petrol produced
from fractional distillation of petroleum is insufficient. So
the extra petrol needed is produced by cracking
polymers
polymerisation
Macromolecules are very large numbers of small molecules
joined together.
Polymerisation is the chemical reaction when large numbers
of similar small molecules (monomers) are joined together
to form a big molecule (polymer).
H
H
HH
H
H
C C C CC
H
H
H
HH
H
HH
HH
CC
C
CC
HH
HH
H
HH
H
H
H
CC C C C
H
H
HH
H
H
polymers
polyethene (polythene)
 one of the simplest addition polymer
 used to make ‘cling film’ for wrapping fresh vegetables
and meat, plastic bottles, plastic bags, buckets and
pipes
 can be made up of 500 to 20,000 ethene molecules
hydrogen
atom
carbon atom
model of polyethene molecule
polymers
polyethene (polythene)
 deriving the polymerisation equation:
o write out the structural formulae of several ethene
molecules in a row
o
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
open the C=C double bonds
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
polymers
polyethene (polythene)
 deriving the polymerisation equation:
o join the molecules together
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
 formula of polyethene can be simply written as:
H
H
C
C
H
H
n
polymers
examples of addition polymer
Chemical Name of
Polymer
Poly(ethene)
or polythene
Poly(chloroethene)
or PVC
Poly(phenylethene)
or polystyrene
Monomer
Polymer
H H
H
H
C
C
C
C
H
H
H
H n
H Cl
H Cl
C
C
C
C
H
H
H
H n
H C6H5
H
C6H5
C
C
C
C
H
H
H
H
Uses
Plastic film for wrapping
fresh vegetables and meat;
plastic bottles; plastic
bags, buckets and pipes
For water pipes, waterproof
plastic sheet, insulation for
electric cables and wires
n
For packaging and
containers for electrical
goods, disposable cups and
food containers used in
‘fast food’ restaurants and
hawker centres
unsaturated foods
 polyunsaturated means that organic molecules contain C=C
bonds
 example: vegetable oils are polyunsaturated oils
 manufacture of margarine by addition reaction; vegetable
oil reacts with hydrogen in presence of nickel catalyst; H2
molecules add to the C=C bonds of vegetable oil
 after addition reaction, vegetable oil has more hydrogen
atoms and hence, greater relative molecular mass and
higher melting point; becomes solid (solidify) at room
conditions
margarine (a healthy product) made
from vegetable oil by addition reaction
Demo - Testing for unsaturation
bromine
After five droplets of each to bromine solution, shaken and stirred…
butter
margarine
ghee
olive oil
Butter - did not decolorized Br soln (very minimal) - mostly saturated
Margarine - decolorized Br soln (largely) - mostly polyunsaturated
Ghee - did not decolorized Br soln(least) - highly saturated
Olive oil -decolorized Br soln(completely) - highly monounsaturated
end of chapter 16 - alkenes