1. KNOCKHARDY PUBLISHING
AN INTRODUCTION TO
THE CHEMISTRY
OF ALKANES
KNOCKHARDY PUBLISHING

2. KNOCKHARDY PUBLISHING THE CHEMISTRY OF ALKANES
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
Navigation is achieved by...
either clicking on the grey arrows at the foot of each page
or using the left and right arrow keys on the keyboard

3. THE CHEMISTRY OF ALKANES
CONTENTS
Structure of alkanes
Physical properties of alkanes
Chemical properties of alkanes
Breaking covalent bonds
Chlorination via free radical substitution
Cracking
Revision check list

4. THE CHEMISTRY OF ALKANES Before you start it would be helpful to…
Recall the definition of a covalent bond
Be able to balance simple equations
Be able to write out structures for hydrocarbons

5. ALKANES General members of a homologous series
general formula is CnH2n for non-cyclic alkanes
saturated hydrocarbons - all carbon-carbon bonding is single
bonds are spaced tetrahedrally about carbon atoms.
Isomerism the first example of structural isomerism occurs with C4H10
BUTANE METHYLPROPANE
Structural isomers have the SAME MOLECULAR FORMULA BUT
DIFFERENT STRUCTURAL FORMULA
They possess different physical properties such as boiling point,
melting point and density

6. HYBRIDISATION OF ORBITALS1 1s 2 2s 2p
The electronic configuration of a carbon atom is 1s22s22p2

7. HYBRIDISATION OF ORBITALS1 1s 2 2s 2p
The electronic configuration of a carbon atom is 1s22s22p2
If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3 1 1s 2 2s 2p
The process is favourable because the of arrangement of electrons; four unpaired and with less repulsion is more stable

8. HYBRIDISATION OF ORBITALS IN ALKANES
The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.
Because one s and three p orbitals are used, it is called sp3 hybridisation
2s22p2
2s12p3
4 x sp3

9. THE STRUCTURE OF ALKANES
In ALKANES, the four sp3 orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5º.
Each sp3 orbital in carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond.
109.5º

10. PHYSICAL PROPERTIES OF ALKANES
Boiling point increases as they get more carbon atoms in their formula
more atoms = greater intermolecular Van der Waals’ forces
greater intermolecular force = more energy to separate the molecules
greater energy required = higher boiling point
CH4 (-161°C) C2H6 (-88°C) C3H8 (-42°C) C4H10 (-0.5°C)
difference gets less - mass increases by a smaller percentage
straight chains molecules have greater interaction than branched
“The greater the branching, the lower the boiling point”
Melting point general increase with molecular mass
the trend is not as regular as that for boiling point.
Solubility alkanes are non-polar so are immiscible with water
they are soluble in most organic solvents.

11. CHEMICAL PROPERTIES OF ALKANES
Introduction - fairly unreactive; (old family name, paraffin, meant little reactivity)
- have relatively strong, almost NON-POLAR, SINGLE covalent bonds
- they have no real sites that will encourage substances to attack them
Combustion - make useful fuels - especially the lower members of the series
- react with oxygen in an exothermic reaction
complete CH4(g) O2(g) ——> CO2(g) H2O(l)
combustion
incomplete CH4(g) ½O2(g) ——> CO(g) H2O(l)
the greater the number of carbon atoms, the more energy produced
BUT the greater the amount of oxygen needed for complete combustion.
Handy tip When balancing equations involving complete combustion, remember...
every carbon in the original hydrocarbon gives one carbon dioxide and
every two hydrogen atoms gives a water molecule.
Put the numbers into the equation, count up the O’s and H’s on the RHS
of the equation then balance the oxygen molecules on the LHS.

12. Processes involving combustion give rise to a variety of pollutants...
POLLUTION
Processes involving combustion give rise to a variety of pollutants...
power stations SO2 emissions produce acid rain
internal combustion engines CO, NOx and unburnt hydrocarbons
Removal
SO2 react effluent gases with a suitable compound (e.g. CaO)
CO and NOx pass exhaust gases through a catalytic converter
Catalytic converters
In the catalytic converter ... CO is converted to CO2
NOx are converted to N2
Unburnt hydrocarbons are converted to CO2 and H2O
e.g. 2NO CO ———> N CO2
• catalysts are made of finely divided rare metals Rh, Pd, Pt
• leaded petrol must not pass through the catalyst as the lead
deposits on the catalyst’s surface and “poisons” it, thus blocking
sites for reactions to take place.

13. BREAKING COVALENT BONDS
There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond.
UNEQUAL SPLITTING
produces IONS
known as HETEROLYSIS or
HETEROLYTIC FISSION
EQUAL SPLITTING
produces RADICALS
known as HOMOLYSIS or
HOMOLYTIC FISSION
• If several bonds are present the weakest bond is usually broken first
• Energy to break bonds can come from a variety of energy sources - heat / light
• In the reaction between methane and chlorine either can be used, however...
• In the laboratory a source of UV light (or sunlight) is favoured.

14. FREE RADICALS TYPICAL PROPERTIES
• reactive species (atoms or groups) which possess an unpaired electron
• their reactivity is due to them wanting to pair up the single electron
• formed by homolytic fission (homolysis) of covalent bonds
• formed during the reaction between chlorine and methane
• formed during thermal cracking
• involved in the reactions taking place in the ozone layer

15. CHLORINATION OF METHANE
Reagents chlorine and methane
Conditions UV light or sunlight - heat is an alternative energy source
Equation(s) CH4(g) Cl2(g) ——> HCl(g) CH3Cl(g) chloromethane
CH3Cl(g) + Cl2(g) ——> HCl(g) CH2Cl2(l) dichloromethane CH2Cl2(l) + Cl2(g) ——> HCl(g) CHCl3(l) trichloromethane CHCl3(l) Cl2(g) ——> HCl(g) CCl4(l) tetrachloromethane
Mixtures free radicals are very reactive - they are trying to pair their electron
with sufficient chlorine, every hydrogen will eventually be replaced.

16. CHLORINATION OF METHANE
Reagents chlorine and methane
Conditions UV light or sunlight - heat is an alternative energy source
Equation(s) CH4(g) Cl2(g) ——> HCl(g) CH3Cl(g) chloromethane
CH3Cl(g) + Cl2(g) ——> HCl(g) CH2Cl2(l) dichloromethane CH2Cl2(l) + Cl2(g) ——> HCl(g) CHCl3(l) trichloromethane CHCl3(l) Cl2(g) ——> HCl(g) CCl4(l) tetrachloromethane
Mixtures free radicals are very reactive - they are trying to pair their electron
with sufficient chlorine, every hydrogen will eventually be replaced.
Mechanism Mechanisms portray what chemists think is going on in the reaction,
whereas an equation tells you the ratio of products and reactants.
Chlorination of methane proceeds via FREE RADICAL SUBSTITUTION because the methane is attacked by free radicals resulting in
hydrogen atoms being substituted by chlorine atoms.
The process is a chain reaction.
In the propagation step, one radical is produced for each one used

17. CHLORINATION OF METHANE The single dots represent UNPAIRED ELECTRONS
Initiation Cl2 ——> 2Cl• RADICALS CREATED
The single dots represent UNPAIRED ELECTRONS
During initiation, the WEAKEST BOND IS BROKEN as it requires less energy.
There are three possible bonds in a mixture of alkanes and chlorine.
Average bond enthalpy kJ mol-1
The Cl-Cl bond is broken in preference to the others as it is the weakest and requires requires less energy to separate the atoms.

18. CHLORINATION OF METHANE
Propagation Cl• + CH4 ——> CH3• HCl RADICALS USED and
Cl2 + CH3• ——> CH3Cl Cl• then RE-GENERATED
Free radicals are very reactive because they want to pair up their single electron.
They do this by abstracting a hydrogen atom from methane; a methyl radical is formed
The methyl radical is also very reactive and attacks a chlorine molecule
A chlorine radical is produced and the whole process can start over again

19. CHLORINATION OF METHANE
Termination l• + Cl• ——> Cl2 RADICALS REMOVED
Cl• + CH3• ——> CH3Cl
CH3• + CH3• ——> C2H6
Removing the reactive free radicals brings an end to the reaction.
This is not very likely at the start of the reaction because of their low concentration.

20. CHLORINATION OF METHANE
OVERVIEW
Initiation Cl2 ——> 2Cl• radicals created
Propagation Cl• CH ——> CH3• HCl radicals used and
Cl CH3• ——> CH3Cl Cl• then re-generated
Termination Cl• + Cl• ——> Cl2 radicals removed
Cl• + CH3• ——> CH3Cl
CH3• + CH3• ——> C2H6
Summary
Due to lack of reactivity, alkanes need a very reactive species to persuade them to react
Free radicals need to be formed by homolytic fission of covalent bonds
This is done by shining UV light on the mixture (heat could be used)
Chlorine radicals are produced because the Cl-Cl bond is the weakest
You only need one chlorine radical to start things off
With excess chlorine you get further substitution and a mixture of chlorinated products

21. CHLORINATION OF METHANE
RADICALS
PRODUCED
Initiation
Propagation
Termination
RADICALS USED
AND REGENERATED
RADICALS
REMOVED

22. CHLORINATION OF METHANE
Further
propagation If excess chlorine is present, further substitution takes place
The equations show the propagation steps for the formation of...
dichloromethane Cl• + CH3Cl ——> CH2Cl• HCl
Cl2 + CH2Cl• ——> CH2Cl Cl•
trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HCl
Cl CHCl2• ——> CHCl Cl•
tetrachloromethane Cl• + CHCl3 ——> CCl3• + HCl
Cl CCl3• ——> CCl Cl•
Mixtures Because of the many possible reactions there will be a mixture of products.
Individual haloalkanes can be separated by fractional distillation.

23. CRACKING THERMAL Converts heavy fractions into higher value products
Involves the breaking of C-C bonds in alkanes
Converts heavy fractions into higher value products
THERMAL proceeds via a free radical mechanism
CATALYTIC proceeds via a carbocation (carbonium ion) mechanism
THERMAL
HIGH PRESSURE kPa
HIGH TEMPERATURE °C to 900°C
FREE RADICAL MECHANISM
HOMOLYTIC FISSION
PRODUCES MOSTLY ALKENES e.g. ETHENE for making polymers and ethanol
PRODUCES HYDROGEN ... used in the Haber Process and in margarine manufacture
Bonds can be broken anywhere in the molecule by C-C bond fission or C-H bond fission

24. CRACKING CATALYTIC Converts heavy fractions into higher value products
Involves the breaking of C-C bonds in alkanes
Converts heavy fractions into higher value products
THERMAL proceeds via a free radical mechanism
CATALYTIC proceeds via a carbocation (carbonium ion) mechanism
CATALYTIC
SLIGHT PRESSURE
HIGH TEMPERATURE °C
ZEOLITE CATALYST
CARBOCATION (IONIC) MECHANISM
HETEROLYTIC FISSION
PRODUCES BRANCHED AND CYCLIC ALKANES, AROMATIC HYDROCARBONS
USED FOR MOTOR FUELS
ZEOLITES are crystalline aluminosilicates; clay like substances

25. What should you be able to do?
REVISION CHECK
What should you be able to do?
Recall and explain the physical properties of alkanes
Recall the use of alkanes as fuels
Recall and explain the different ways to break a covalent bond
Write balanced equations representing combustion and chlorination
Understand the conditions and mechanism of free radical substitution
Recall the conditions and products from thermal and catalytic cracking
CAN YOU DO ALL OF THESE? YES NO

26. You need to go over the relevant topic(s) again
Click on the button to
return to the menu

27. Try some past paper questions
WELL DONE!
Try some past paper questions

28. © 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
AN INTRODUCTION TO
THE CHEMISTRY
OF ALKANES
THE END
© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

29. PETROCHEMICALS
In the past, most important organic chemicals were derived from coal.
Nowadays, natural gas and crude petroleum provide an alternative source.
the composition of crude petroleum varies according to its source
it is a dark coloured, viscous liquid
consists mostly of alkanes with up to 40 carbon atoms, plus water, sulphur and sand
can be split up into fractions by fractional distillation
distillation separates the compounds according to their boiling point
at each level a mixture of compounds in a similar boiling range is taken off
rough fractions can then be distilled further to obtain narrower boiling ranges
some fractions are more important - usually the lower boiling point ones
high boiling fractions may be broken down into useful lower boiling ones - CRACKING
CONTENTS

30. PETROCHEMICALS Boiling C’s per Name of Use(s)
range / °C molecule fraction
< LPG (Liquefied Calor Gas
Petroleum Gas) Camping Gas
GASOLINE Petrol
NAPHTHA Petrochemicals
KEROSINE Aviation Fuel
GAS OIL Central Heating Fuel
> LUBRICATING OIL Lubrication Oil
> FUEL OIL Power Station Fuel
Ship Fuel
> WAX, GREASE Candles
Grease for bearings
> > 50 BITUMEN Road surfaces
Roofing
CONTENTS