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The alkanes

Methane (CH
4
)
 Draw a dot and cross diagram to represent the bonding shown here.
 What shape are the bonds around the carbon atom? Why?

Ethane (C
2
H
6
)
 Draw a dot and cross diagram for ethane.
 What shape will the bonds take around each carbon atom now?

Propane (C
3
H
8
)

Butane (C
4
H
10
)

The Alkanes: molecular formula or displayed formula?
 Methane CH
4
 Ethane C
2
H
6
 Propane
 Butane C
4
H
10
C
3
H
8
 Pentane


Hexane
Octane
C
6
H
14
C
8
H
18
C
5
H
12

The alkanes C n
H
2n + 2
General formula
 Methane CH
4
 Ethane C
2
H
6
 Propane
 Butane C
4
H
10
C
3
H
8
 Pentane


Hexane
Octane
C
6
H
14
C
8
H
18
C
5
H
12
Homologous series:
Same general formula, similar chemical properties but a graduation in physical properties

COPY!
The alkanes C n
H
2n + 2
General formula
 Methane CH
4
 Ethane C
2
H
6
 Propane
 Butane C
4
H
10
C
3
H
8
 Pentane


Hexane
Octane
C
6
H
14
C
8
H
18
C
5
H
12
Generally unreactive, but burn exothermically.
Longer chains burn less easily.

Structural formulae

Alkanes are saturated compounds
No double bonds

Isomers
Same molecular formula, but different structures (arrangement of atoms)
Name?
Name of this?
Why does this have lower boiling point?

Isomers
Draw out pentane .
How many other isomers can you draw?
Stretch yourself – can you name them?

Reactions of the Alkanes
 Combustion
 Chlorination

Combustion

Combustion
Hydrocarbons combust well. What does this mean?
What do you predict the products are of this reaction?
They burn producing carbon dioxide and water
Write a word equation for this reaction, then try a chemical equation
For example, with methane:
CH
4
+ 2O
2
CO
2
+ 2H
2
O

Incomplete combustion
 How do you think this reaction is different?
 What do you think the products are this time?
 Why is this reaction so dangerous?
 Who should be careful?
Hurricane Ike link

Incomplete combustion
 When there is insufficient oxygen, carbon monoxide is also formed
It reduces the blood’s oxygen carrying capability

Chlorination of Methane
 Video link

Chlorination of Methane
Alkanes react with chlorine in sunlight:
CH
4
+ Cl
2
CH
3
Cl + HCl
How would you describe this reaction?
A chlorine atom takes the place of a hydrogen atom = substitution reaction.
What would happen if lots more chlorine was present?