1. 3.3.2 Alkanes

2. 3.3.2.1 Fractional distillation of crude oil
Alkanes are saturated hydrocarbons
Definition: Hydrocarbons contain hydrogen and carbon atoms only.
Definition: Saturated means the compound contains only single bonds.

3. Boiling point of hydrocarbons
As the number of carbon atoms increases the boiling point increases. Short hydrocarbons are gases and long hydrocarbons are viscous liquids.

4. Boiling point of hydrocarbons
As the hydrocarbons get longer there are more/ stronger Van der Waals forces between the molecules. This means more energy is needed to separate the hydrocarbon molecules, so the boiling point increases.

5. Fractional distillation
Petroleum (a.k.a. crude oil) is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.

6. Fractional distillation
Petroleum is heated and turned into a vapour (gas).
The column is kept hot at the bottom and cooler at the top (temperature gradient)
The hydrocarbon gases condense at different heights depending on their boiling points.

7. Fractional distillation
When the gases condense, they turn into liquids which can be tapped off (removed). Each different liquid formed is called a fraction.

8. Fractional distillation
Hydrocarbons which are short have low boiling points and are collected at the top of the column.
Hydrocarbons which are long have high boiling points and are collected at the bottom of the column.

10. Fuel for lorries and buses
Fractional distillation
Fraction
Use
Refinery gases
Bottled gas
Gasoline
Fuel for cars
Naptha
Making chemicals
Kerosene
Fuel for aeroplanes
Diesel oil
Fuel for lorries and buses
Fuel oil
Fuel for ships
Bitumen
Road surfaces

11. 3.3.2.2 Modification of alkanes by cracking
The demand for the shorter hydrocarbons (refinery gases and petrol) is greater than the longer hydrocarbons.

12. There is a large supply of long hydrocarbons but a low demand
There is a large supply of long hydrocarbons but a low demand. There is a short supply of short hydrocarbons but a high demand.
To solve this problem we break the long hydrocarbons into shorter hydrocarbons by a process called cracking.

13. Cracking involves breaking C–C bonds in alkanes.
There are two different types of cracking:
Thermal cracking – uses high temperatures and high pressures
Catalytic cracking – uses a catalyst

14. Thermal cracking
It involves breaking the C-C bond so that long alkanes are broken down to produce shorter alkanes and a high percentage of alkenes.
Thermal cracking happens at high temperatures (between 450°C - 900°C) and high pressures (7000 kPa).

15. Thermal cracking
Example: Write an equation to illustrate the thermal cracking of one molecule of tetradecane, C14H30, in which the products are ethene and propene, in the ratio of 2:1, and one other product
Answer: C14H30 → 2C2H4 + C3H6 + C7H16

16. Thermal cracking
Ethene, C2H4 is a valuable product of cracking. It can be used to make polymers such as poly(ethene). Poly(ethene) is used to make plastics.

17. Catalytic cracking
Catalytic cracking takes place at a slight pressure (100 kPa), high temperature (450 °C) and in the presence of a zeolite catalyst.
It is used mainly to produce motor fuels and aromatic hydrocarbons
Catalytic cracking produces branched, cyclic and aromatic hydrocarbons.

18. Catalytic cracking
Branched hydrocarbons are used as motor car fuels as they burn more smoothly than unbranched hydrocarbons.

19. Catalytic cracking
Branched hydrocarbons are used as motor car fuels as they burn more smoothly than unbranched hydrocarbons.

20. Combustion of alkanes
Alkanes are used as fuels. Combustion of alkanes and other organic compounds can be complete or incomplete.

21. When hydrocarbons burn in plenty of air (oxygen) they produce carbon dioxide and water. This is called complete combustion.
hydrocarbon + oxygen → carbon dioxide + water
Example: Complete combustion of propane:
C3H8 + 5O2 → 3CO2 + 4H2O

22. hydrocarbon + oxygen → carbon monoxide + water
When we burn hydrocarbons and there is not enough oxygen they produce carbon monoxide (CO) and water. This is called incomplete combustion.
hydrocarbon + oxygen → carbon monoxide + water
Example: Incomplete combustion of propane:
C3H8 + 3½O2 → 3CO + 4H2O

23. Carbon monoxide (CO) is a poisonous gas
Carbon monoxide (CO) is a poisonous gas. Carbon monoxide attaches to the red blood cells instead of oxygen. Therefore reducing the amount of oxygen the blood can carry to the cells. The body is starved of oxygen and this can lead to death.

24. Carbon is the solid product of incomplete combustion
Example: Incomplete combustion of methane:
CH4 + O2 → C + 2H2O

25. Sulfur impurities
Combustion of hydrocarbons containing sulfur leads to sulfur dioxide (SO2) that causes air pollution. Sulfur dioxide leads to acid rain and damages the environment (plants, animals and buildings).

26. Sulfur impurities
Sulfur dioxide dissolves in water vapour to form sulfurous acid (H2SO3)
SO2 + H2O → H2SO3
The sulfurous acid (H2SO3) is then oxidised by a series of reactions to form sulfuric acid (H2SO4). The sulfuric acid causes rain to become acidic.

27. Sulfur impurities
In power stations, the acidic sulfur dioxide is removed from the chimney (flue) gases by reacting it with an alkaline substance such as quicklime (CaO).

28. The product of the reaction between quicklime (CaO) and sulfur dioxide is calcium sulfite (CaSO3) which can be oxidised to make calcium sulfate (CaSO4), which is used to make plasterboard for lining interior walls.
CaO + SO2 → CaSO3

29. Internal Combustion engines
Nitrogen and oxygen can also react under high temperatures to form nitrogen oxides (NOx). Nitrogen oxides are produced when the fuel-air mixture is sparked and ignites in a car engine. This provides sufficient energy for nitrogen to react with oxygen to form nitrogen monoxide (NO)
N2 + O2 → 2NO

30. Internal Combustion engines
Nitrogen monoxide (NO) reacts further with more oxygen to form nitrogen dioxide (NO2).
2NO + O2 → 2NO2
Nitrogen dioxide (NO2) reacts with water vapour (H2O) and more oxygen. This forms nitric acid (HNO3), which leads to acid rain
2NO2 + H2O + ½O2 → 2HNO3

31. Internal Combustion engines
Some of the hydrocarbons pass through the engine without being burnt. These are called unburnt hydrocarbons
They can react with nitrogen oxides in the presence of sunlight to produce an irritating photochemical smog (haze in the air).

32. Internal Combustion engines
Exhaust systems of cars have been fitted with catalytic converters. These devices help to remove carbon monoxide, nitrogen oxides and unburnt hydrocarbons from car exhausts.

33. Carbon monoxide + nitrogen oxides → carbon dioxide + nitrogen
Internal Combustion engines
The catalytic converter contains transition metals such as platinum, palladium and rhodium (they are the catalysts) which are spread in a thin layer.
Carbon monoxide + nitrogen oxides → carbon dioxide + nitrogen
2CO + 2NO → 2CO2 + N2

34. 3.3.2.4 Chlorination of alkanes
Definition: A covalent bond is a shared pair of electrons
When the covalent bond between two atoms breaks and one electron is transferred to each atom producing two free radicals. This is called homolytic fission.
In general: X-Y → X∙ and Y∙
A radical is a species with an unpaired electron. ∙ = unpaired electron

35. There are three steps to this reaction: Initiation Propagation
Methane and chlorine
 Methane and chlorine do not react together in the dark or at room temperature. But in the presence of ultra-violet light they react forming chloromethane and hydrogen chloride.
CH4 + Cl2 → CH3Cl + HCl
Name of the mechanism: Free radical substitution
There are three steps to this reaction:
Initiation
Propagation
Termination

36. Cl2 → Cl∙ + Cl∙ (or Cl2 → 2Cl∙)
Methane and chlorine
 Step 1: Initiation - Making two free radicals
Cl2 → Cl∙ + Cl∙ (or Cl2 → 2Cl∙)
Condition: UV light (breaks the Cl-Cl bond forming chlorine radicals, Cl•)

37. Methane and chlorine
 Step 2: Propagation - A free radical reacts with a molecule to make a free radical and a new molecule
CH4 + Cl• → •CH3 + HCl
•CH3 + Cl2 → CH3Cl + Cl•
Overall: CH4 + Cl2 → CH3Cl + HCl

38. Methane and chlorine
 Step 2: Propagation - A free radical reacts with a molecule to make a free radical and a new molecule

39. Methane and chlorine
 Step 2: Propagation –
The initial product formed is chloromethane (CH3Cl). However, the free radical substitution reaction can happen again and again producing: dichloromethane (CH2Cl2); trichloromethane (CHCl3) and tetrachloromethane (CCl4)

40. Methane and chlorine
 Step 2: Propagation – Further substitution

41. Step 3: Termination - Two free radicals react to make a molecule.
Methane and chlorine
 Step 3: Termination - Two free radicals react to make a molecule.
Unpaired electrons pair up to form a covalent bond. No radicals are made and this ‘kills’ the radicals and the reaction.
•CH3 + •CH3 → C2H6 Makes an alkane: ethane
Cl• + Cl• → Cl2 Makes chlorine
Cl• + •CH3 → CH3Cl Makes chloromethane

42.  Reaction conditions
Formation of chloromethane (CH3Cl) is favoured by an excess of methane.
More chance of chlorine radical (Cl•) reacting with a methane molecule. The chlorine radicals (Cl•) are used up before further substitution
Formation of tetrachloromethane (CCl4) is favoured by an excess of chlorine.
More chance of chlorine radical (Cl•) reacting with a product molecule as there is a large amount of Cl• 

43.  Question: Write all the equations for the formation of 1-chloropropane (CH3CH2CH2Cl) from chlorine and propane.
Answer:
Mechanism: Free radical substitution
Initiation: Cl2 → Cl• + Cl• (Need UV light)
Propagation: CH3CH2CH3 + Cl• → •CH2CH2CH3 + HCl
•CH2CH2CH3 + Cl2 → ClCH2CH2CH3 + •Cl
Termination: •CH2CH2CH3 + •Cl → ClCH2CH2CH3
(Other termination reactions do not produce 1-chloropropane such as
Cl• + Cl• → Cl2 and
•CH2CH2CH3 + •CH2CH2CH3 → CH3CH2CH2CH2CH2CH3)