g10alkenes
TRANSCRIPT
Starter
How many fractions can you name from the fractionating
column ?In order
Without looking
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
alkenes
a ball-and-stick model of ethene,
C2H4
alkenes
each member of the series differs from the next by an extra –CH2- group of atoms
alkene homologous series
No. of Carbon Atoms in One
Molecule
Alkene Member
Relative Molecular
Mass
Structure of One Molecule
Boiling Point (°C)
2ethene(C3H4)
28 -104
3propene(C3H6)
42 -48
4butene(C4H8)
56 -6
H
H
C
H
H
C
H
H
C
H
H
H C
H
C
H
H
H C
H
H
C
H
C
H
H
C
alkenes burn in air to form CO2 and H2O
alkenes
C2H4 + 3O2 2CO2 + 2H2O
reactions of alkenes: combustion
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
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.
C C
X X
HH
HH
alkenes reactions of alkenes: addition reactions addition of bromine (bromination)
C2H4 + Br2 C2H4Br2
o example: addition reaction of bromine to ethene
o bromine molecule adds to the C=C bond of ethene molecule
H HC
H HC
+Br
Br CH
H
H
H
C Br
Br
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 reddish-brown bromine solution
C C
Br Br
HH
HH
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
C2H4 + H2 C2H6
H HC
H HC
+H
H CH
H
H
H
C H
H
used in producing margarine from vegetable oil
C C
H H
HH
HH
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
o water molecule adds to the C=C bond of ethene molecule
C2H4 + H2O CH3CH2OH
H
H HC
H HC
+OH
H CH
H
H
C OH
H
alcohols are commonly used as solvents and to make other chemicals
C C
H OH
HH
HH
alkenessaturated and unsaturated hydrocarbons
alkenes are unsaturated because extra atoms can be added to their molecules in addition reactions
C C
HH
HHC C
HH
HH
HH
alkanes are saturated because it is not possible to add extra atoms to their molecules
alkenessaturated 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)
Reaction with aqueous bromine : do not decolourise aqueous bromine
Reaction with aqueous bromine : decolourises aqueous bromine
have alkane-type formula of CnH2n+2
have alkene-type formula of CnH2n
Extra atoms not added to their molecules in addition reactions
Extra atoms added to their molecules in addition reactions
alkenestesting 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
bromine becomes colourless
shake
liquid alkene
bromine solution
testing for liquid alkene
poly(ethene)
alkenestesting 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
testing for gaseous alkene
bromine becomes colourless
gaseous alkene
bromine solution
Demo - to distinguish between an alkane and an alkene (decolourisation of bromine!).
bromine
hexenehexane
bromine
hexene
hexane
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!
bananasbananasbromine bromine
manufacture of alkenes
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
big hydrocarbon molecules
hydrocarbon molecules break up on the
surface of the catalyst
small hydrocarbon molecules
CATALYST
cracking Alkene molecules can be made by cracking alkanes
an ________mo
lecule
C18H38 X + C10H20
an ________molecule
big alkane
cracking
cracking of alkanes
C20H42 C12H26 + C8H16
an alkenemolecule
an alkanemolecule
big alkane
Uses:
C16H34 H2 + C6H12 + C10H20
cracking
cracking of alkanes
Hydrogen can be one of the products in the cracking of alkanes
Hydrogen is used as fuel for big rockets and to make ammonia for fertilisers
Uses:
o cracking of big petroleum molecules into smaller hydrocarbon molecules to produce fuel for motor vehicles;
cracking
cracking of alkanes
• 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
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).
polymerisation
H
H
CC
H
H
H
H
CC
H
H
H
H
CC
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
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
model of polyethene molecule
carbon atom
hydrogen atom
polymers
polyethene (polythene) deriving the polymerisation equation:
H
C C
H H
H H
C C
H H
HH
C C
H H
H H
C C
H H
HH
C C
H H
H
o write out the structural formulae of several ethene molecules in a row
C
H
C
H H
H H
C C
H H
HH
C C
H H
HH
C C
H H
HH
C C
H H
H
o open the C=C double bonds
polymers
polyethene (polythene) deriving the polymerisation equation:
o join the molecules together
H
H
C C
H H
H H
C C
H H
HH
C C
H H
H H
C C
H
H H
C C
H H
HH
C C
H H
H
formula of polyethene can be simply written as:
C
H
C
H
H H n
polymers
examples of addition polymer
Chemical Name of Polymer
Monomer Polymer Uses
Poly(ethene) or polythene
Plastic film for wrapping fresh vegetables and meat; plastic bottles; plastic bags, buckets and pipes
Poly(chloroethene)or PVC
For water pipes, waterproof plastic sheet, insulation for electric cables and wires
Poly(phenylethene)or polystyrene
For packaging and containers for electrical goods, disposable cups and food containers used in ‘fast food’ restaurants and hawker centres
H
H
C
Cl
H
C
H
H
C
C6H5
H
C
H
H
C
Cl
H
C
n
H
H
C
C6H5
H
C
n
H
H
C
H
H
C
H
H
C
H
H
C
n
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