Describing Alkanes (Edexcel AS Chemistry): Revision Note

Describing Alkanes

• Hydrocarbons are compounds containing hydrogen and carbon only

• There are four families of hydrocarbons you should know: alkanes, alkenes, alkynes and arenes

  • Alkanes, alkenes and alkynes can be described as aliphatic

  • Arenes can be described as aromatic

• Alkanes have the general molecular formula C_nH_2n+2 

• They contain only single bonds and are said to be saturated

• Alkanes are named using the nomenclature rule alk + ane

  • The alk portion of the name depends on the number of carbons

    • 1 carbon = meth

    • 2 carbons = eth

    • 3 carbons = prop

    • 4 carbons = but

    • 5 carbons = pent

    • After 5 carbons, the naming of alkanes matches the names of the polygons in Maths

  •  The ane portion of the name suggests single bonds between the carbon atoms

The first six alkanes

• The first six members of the alkanes are:

• Methane

  • Molecular formula: CH₄

• Ethane

  • Molecular formula: C₂H₆

• Propane

  • Molecular formula: C₃H₈

• Butane

  • Molecular formula: C₄H₁₀

• Pentane

  • Molecular formula: C₅H₁₂

• Hexane

  • Molecular formula: C₆H₁₄

Types of alkanes

• Alkanes can be linear, branched or cyclic 

  • The key points are that there are no functional groups and only single bonds between the carbon atoms

• Cycloalkanes have the general formula C_nH_2n

  • This general formula is the same as the alkene general formula but cycloalkanes are still saturated

  • Cycloalkanes are named using the nomenclature rule cyclo + alk + ane

    • The cyclo portion of the name shows that there is a ring structure

    • As with alkanes, the alk portion of the name depends on the number of carbons

    •  The ane portion of the name suggests single bonds between the carbon atoms

Alkanes are compounds made up of carbon and hydrogen atoms only and contain no functional group