Chemical Properties of Alkanes

Chemical Properties of Alkanes

1. Alkanes are chemically less reactive compared to the other carbon compound such as alkene, alcohol or carboxylic acid.
2. Alkanes do not react with chemicals such as oxidizing agents, reducing agents, acids and alkalis.
3. Alkanes are saturated hydrocarbon with strong carbon-carbon (C – C) bonds and carbon-hydrogen (C – H) bonds.
4. All the bonds are single bonds which require a lot of energy to break.
5. Alkane just only undergo:
1. Combustion reaction
2. Substitution reaction (with the presence of ultraviolet light)

Alkanes are chemically less reactive because they contain only strong covalent bonds which need a lot of energy to break.

Combustion of Alkanes

1. All alkanes undergo combustion to produce water and carbon dioxide (or carbon monoxide/carbon in incomplete combustion).
2. There are 2 types of combustion:
1. complete combustion
2. incomplete combustion
Complete Combustion

Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.

Example:
Combustion of methane

CH4 + 2O2 → CO2 + 2H2O

Combustion of ethane

C2H6 + 7/2 O2 → 2CO2 + 3H2O

Combustion of propane

C3H8 + 5O2 → 3CO2 + 4H2O
Incomplete Combustion
1. Incomplete combustion occurs when there isn’t enough oxygen present.
2. In an incomplete combustion, carbon or carbon monoxide will be produced as the product of the reaction.
3. Carbon monoxide is a colourless poisonous gas.

Example:
Incomplete combustion of ethane

C2H6 + 5/2 O2 → 2CO + 3H2O
C2H6 + 3/2 O2 → 2C + 3H2O

Incomplete combustion of propane

C3H8 + 7/2 O2 → 3CO + 4H2O
C3H8 + 2 O2 → 3C + 4H2O
MUST Know!
Enough oxygen → Complete combustion
Not enough oxygen → Incomplete combustion

Trends

1. The hydrocarbons become harder to ignite as the molecules get bigger.
2. Combustion of alkanes with bigger molecule will produce more soot as the percentage of carbon of the molecule is higher.

Percentage of Carbon

1. The amount of soot produced depends on the percentage of carbon in a hydrocarbon molecule.
2. The percentage of carbon in a molecule can be calculated by using the following formula:
$\begin{gathered} {\text{Percentage of Carbon}} \hfill \\ {\text{ = }}\frac{{{\text{Relative mass of carbon in 1 molecule}}}}{{{\text{Relative molecular mass of the molecule}}}} \times 100\% \hfill \\ \end{gathered}$
3. Lets calculate the percentage of carbon in a methane and a hexane.
1. Percentage of carbon in methane (CH4) [Relative Atomic Mass: Carbon: 12; Hydrogen: 1]
$\begin{gathered} {\text{Percentage of Carbon}} \hfill \\ {\text{ = }}\frac{{{\text{12}}}}{{{\text{12 + 4}}\left( 1 \right)}} \times 100\% \hfill \\ = 75\% \hfill \\ \end{gathered}$
2. Percentage of carbon in hexane (C6H14)
$\begin{gathered} {\text{Percentage of Carbon}} \hfill \\ {\text{ = }}\frac{{6\left( {{\text{12}}} \right)}}{{6\left( {{\text{12}}} \right){\text{ + 14}}\left( 1 \right)}} \times 100\% \hfill \\ = 83.7\% \hfill \\ \end{gathered}$
4. Percentage of carbon in hexane is higher than in methane, therefore combustion of propane is sootier than combustion of methane

MUST Know!

The bigger the molecule of an alkane, the higher the percentage of carbon, and hence produces more soot during combustion.

Substitution Reaction
1. The substitution reaction is a reaction where one atom (or a group of atoms) in a molecule is replaced by another atom (or a group of atoms).
2. Substitution reaction of alkanes occurs when an alkane is mixed with a halogen in the presence of sunlight (ultraviolet light).
3. Since the substitution reaction of alkanes is the reaction between alkanes and halogen, hence it is also called the halogenation reaction.
4. The substitution reaction is a slow reaction.

MUST Know!

Substitution reaction of alkanes takes place only in the presence of ultraviolet light (such as sunlight).

Reaction between Methane and Chlorine

1. We can see that, in the reaction, each hydrogen atom in the alkane molecule is substituted one by one by a chlorine atom.
2. The sunlight or UV light is needed to break the covalent bond in halogen and alkane molecules to produce hydrogen and chlorine atom.
3. Alkanes also react with bromine vapour in the presence of UV light, but with a much lower rate.