1. Reactions of Alkanes 28 November, 2018
Hydrocarbons
Reactions of Alkanes
28 November, 2018
Lesson aim: To describe the reactivity of alkanes.
Keywords: Complete combustion, incomplete combustion, halogenation, initiation, propagation, termination

2. STARTER
Using your GCSE knowledge: Write a definition for complete combustion and a definition for incomplete combustion
Hint: Combustion involves burning something in oxygen. A fuel is a mixture of different hydrocarbons. Think of what could be the products of those two reactants.
Extension: Write word equations for the two types of reaction.

3. STARTER
Using your GCSE knowledge: Write a definition for complete combustion and a definition for incomplete combustion
Complete combustion occurs in a plentiful supply of oxygen where all the elements in the fuel fully react with oxygen to make carbon dioxide and water.
(e.g. Hydrocarbon + Oxygen  Carbon dioxide + Water)
Incomplete combustion occurs when there is a poor supply of oxygen, carbon monoxide is produced instead as a poisonous gas along with carbon.
(e.g. Hydrocarbon + Oxygen  Carbon monoxide + Carbon + Water)

4. Reactivity of alkanes
Literacy task: Follow the instructions on the sheet to complete the task.

5. Combustion Knowledge Check
Write balanced symbol equations for the complete combustion of the following alkanes:
CH4
C4H10
Write a symbol equation for the incomplete combustion of:
C2H6
Hint: Look back over your literacy task where examples of the reactions are given. Make sure your equations are balanced!
Extension: Describe the environmental impact of combustion chemistry and suggest how this impact could be reduced in the use of cars (4 marks)

6. Combustion Knowledge Check
Write symbol equations for complete combustion of the following alkanes:
CH4 + 2 O2  1 CO2 + 2 H2O
2 C4H O2  8 CO H2O
Write a symbol equation for the incomplete combustion of:
2 C2H6 + 5 O2  4 CO + 6 H2O
Extension: Describe the environmental impact of combustion chemistry and suggest how this impact could be reduced in the use of cars.
CO2 produced in complete combustion, harmful to the environment
CO produced in incomplete combustion, poisonous and harmful gas.
Any two sensible suggestions (e.g. using electric cars, cleaner fuels, efficient engines)

7. Reactions of alkanes - Halogenation
Halogenation is the reaction between an organic compound and any halogen.
Alkanes will not react with halogens in the dark, but will in the presence of UV light. They react in 3 stages known as a chain reaction.
The three stages are:
Initiation
Propagation
Termination

8. Example 1 Stage 1 - Initiation
Cl2  2Cl•
The chlorine molecule is split due to the absorption of UV light.
The Cl-Cl bond breaks homolytically (one electron going to each chlorine atom) to form two chlorine atoms.
These chlorine atoms are written as Cl• and are called free radicals. Radicals are species that contain an unpaired electron.
Free radicals are highly reactive.

9. Example 1 Stage 2 - propagation
Cl• + CH4  CH3• + HCl
CH3• + Cl2  CH3Cl + Cl•
This takes place in 2 steps which can repeat many, many times – this is the ‘chain’ part of the reaction
The chlorine radical reacts with an alkane, removing a hydrogen to become HCl
As you can see, the reaction results in the formation of a CH3• radical. This methyl radical reacts with more chlorine to produce another chlorine free radical and a stable compound called chloromethane

10. Example 1 STAGE 3 - termination
Cl• + Cl•  Cl2
Cl• + CH3•  CH3Cl
CH3•+ CH3•  C2H6
The propagation steps continue many times until two radicals happen to meet. The reaction then stops.
Termination could happen in a number of ways.

11. Example 2 Stage 1 - Initiation
Cl2  2Cl•
Chlorine is split homolytically by UV light to form very reactive free radical species.

12. Example 2 Stage 2 - propagation
Cl• + C2H6  C2H5• + HCl
C2H5• + Cl2  C2H5Cl + Cl•
Propagation (I)
C2H5Cl + Cl•  C2H4Cl• + HCl
C2H4Cl• + HCl  C2H4Cl2 + Cl•
Propagation (II)
The radical species causes a chain reaction to form new species.
This can happen multiple times depending on the number of radicals available.

13. Example 2 STAGE 3 - termination
Cl• + Cl•  Cl2
C2H4Cl• + C2H4Cl•  C4H8Cl2
C2H5• + C2H5•  C4H10
When any two radicals meet they form a stable species and the reaction stops.
Termination could involve any combination of radicals involved in the propagation steps.

14. Points to note
The initiation, propagation and termination are the mechanism of the reaction. If the exam question asks for the mechanism, it is these reactions you have to outline.
If the exam asks for the overall equation, for example between ethane and bromine, you outline the initial reactants and final products:
C2H6 + Br2  C2H5Br + HBr
Polysubstitution can occur (where numerous hydrogens on the alkane are replaced by halogens) but this can be reduced by limiting the amount of halogen used.

15. Knowledge check
What stage of a free radical reaction of bromine with methane is represented by the following?
Br• + Br•  Br2
CH4 + Br●  CH3● + HBr
CH3• + Br2  CH3Br + Br•
Br2  2Br•
Define the term ‘radical’ and explain why they are so reactive.
Write the mechanism for the chlorination of propane to form chloropropane.
Hint: Look back over the worked examples and try to spot any similarities.
Extension: Describe how you could limit the amount of waste products produced in the chlorination of propane.

16. Knowledge check
What stage of a free radical reaction of bromine with methane is represented by the following?
Br• + Br•  Br2
CH4 + Br●  CH3● + HBr
CH3• + Br2  CH3Br + Br•
Br2  2Br•
Define the term ‘radical’ and explain why they are so reactive. Radical - A species with an unpaired electron. Reactive because they are unstable and need to form an electron pair.
Termination
Propagation
Initiation

17. Knowledge check Initiation Propagation Termination
Write the mechanism for the chlorination of propane to form chloropropane.
Cl2  2Cl•
C3H8 + Cl•  C3H7• + HCl
C3H7• + Cl2  C3H7Cl + Cl•
Cl• + Cl•  Cl2
Initiation
Propagation
Termination
Extension: Describe how you could limit the amount of waste products produced in the chlorination of propane.
Add a small amount of chlorine at a time to limit the number of free radicals present in the reaction meaning the reaction is more likely to go to completion before any terminations occur to form side products.

18. Past Paper Question

19. Past Paper Question UV light Cl2  2Cl• C3H5• + C3H5•  C6H10
One of the free radicals produced in the chlorination of propane is C3H5•. If two of these radicals were to react they would form hexane (C6H10) in a termination stage.