Alkane Structure

Alkanes are hydrocarbons, that is, compounds that contain carbon and hydrogen only.
Alkanes have only carbon to carbon single bonds (C─C).
Alkanes are considered to be saturated organic compounds, since they have the maximum number of hydrogen atoms.
The general formula for an alkane is
CnH2n+2 .

Low Reactivity of Alkanes

Alkanes tend to be quite unreactive for two reasons:
·         Relative strength of C─C and C─H bonds.
·         Low polarity of bonds.
Note: Alkanes only readily undergo combustion and substitution reactions with oxygen and halogens respectively in the presence of ultraviolet light.

Combustion of Alkanes

GAll hydrocarbons burn in a plentiful supply of oxygen to give carbon dioxide and water. However, if there is an insufficient supply of oxygen, then incomplete combustion occurs and carbon monoxide and carbon are also produced as products. So:
  • products of complete combustion = carbon dioxide + water
  • products of incomplete combustion = carbon monoxide + water (Note: Carbon and hydrogen may also be produced when hydrocarbons burn incompletely.)
Although the C─C and C─H bonds are strong, the C=O bond is even stronger, so the reaction is very exothermic (ΔH < 0).
Alkanes are widely used in industry as fuel. Examples include:
  • Natural gas (methane)
  • Gasoline (petrol)

Substitution Reactions

Alkanes undergo free radical chlorination/bromination to give halogenoalkanes.
Alkanes can react with chlorine (or other halogens) in the presence of UV light to produce hydrogen chloride (HCl), that is hydrochloric acid, and a substituted alkane.
  • Methane (CH4) can react with chlorine to form chloromethane.
  • Ethane (C2H6) can react with bromine to form bromoethane.
If other hydrogen atoms are available on the original hydrocarbon, then polysubstituted chloroalkanes will be formed.
  • e.g. methane (CH4) => chloromethane (CH3Cl) => dichloromethane (CH2Cl2) => trichloromethane (CHCl3) => tetrachloromethane (CCl4)

Mechanism of Chlorination of Methane

  • Mechanism: free radical substitution
  • Steps: initiation (i), propagation (ii), and termination (iii)

Structural Isomers of Alkanes

Structural Isomers of Butane (2) and Pentane (3)
Structural isomers are compounds with the same chemical formula but different structural formulas (go to the Structural Isomers page for more information).
As the number of carbon atoms in an alkane molecule increases, so do the possibilities for isomerism of this kind.
Methane, ethane and propane have no structural isomers. Butane, pentane and hexane have two, three, and five isomers respectively.

Boiling Points of Alkanes