Alkane

General Formula: CnH2n+2, n = 1, 2, 3, …

Functional Group:

No Functional Group

First 3 Members:

  1. Alkane are the simplest family of hydrocarbons – compounds containing carbon and hydrogen only.
  2. They only contain carbon-hydrogen bonds and carbon-carbon single bonds.
  3. Alkanes are saturated hydrocarbon

Nomenclature of Alkanes

  1. Nomenclature = Naming method.
  2. All organic carbon compounds are named according to the number of carbon in the molecule.
  3. The name is made up of two parts: the stem and the suffix.
  4. The stem tells the number of carbon atoms
  5. The suffix tells the homologous series of the compound.
Example:
Ethane is an alkane, the sten “eth” tell us that it has 2 carbon in the molecule, and the suffix “ane” indicates that it is a member of the alkanes homologous series.

The table below shows the naming code for the stem. In SPM, You will need to remember the codes for the number of carbon atoms in a chain up to 6 carbons.

No of carbonsCodeNo of carbonsCode
1meth7hept
2eth8oct
3prop9non
4but10dec
5pent11hendec
6hex12dodec

Name of the First 8 Alkanes

The table below shows the molecular formula and name of the first 8 members of alkanes.
Number of CarbonMolecular FormulaName
1CH4Methane
2C2H6Ethane
3C3H8Propane
4C4H10Butane
5C5H12Pentane
6C6H14Hexane
7C7H16Heptane
8C8H18Octane

Physical Properties of Alkanes

  1. Alkanes are covalent compounds, hence their physical properties are similar to other covalent compounds.
  2. The atoms in an alkane molecule are bonded together by strong covalent bonds  (intramolecular forces).
  3. The molecules are held together by weak Van der Waals forces (intermolecular forces).

Boiling Points

Boiling Point of Alkane
  1. The boiling point of alkane increase when the number of carbons in the molecule increases.
  2. This is due to the increase of Van der Waal force when the size of the molecule increases.
  3. The boiling points shown are all for the “straight chain” isomers when there are more than one.
  4. The first four alkanes are gas at room temperature.
Solubility
  1. Alkanes are insoluble in water, but dissolve in organic solvents. 
  2. The liquid alkanes are good solvents for many other covalent compounds.

Density

  1. The density of water is higher than density of most alkanes.
  2. When going down the series, relative molecular mass of alkanes is higher due to the higher force of attraction between molecules and alkane molecules are packed closer together.

Electrical Conductivity

  1. All members in alkanes do not conduct electricity.
  2. Alkanes are covalent compounds and do not contain freely moving ions.

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.

Symbol of Elements

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