Cycloalkanes (SQA National 5 Chemistry): Revision Note

The cycloalkanes

What are cycloalkanes?

• Cycloalkanes are a family of hydrocarbons

  • For a full table of the first cycloalkanes and their structures, see the Full & shortened structural formulae

• Cycloalkanes are a homologous series with four key facts:

1. They are saturated, cyclic hydrocarbons

• This means they contain only carbon and hydrogen atoms

• All of the carbon-carbon bonds in their structure are single bonds (C-C)

• The carbon atoms are joined together to form a ring structure

2. They are represented by a general formula

• The general formula for the cycloalkane homologous series is C_nH_2n

• This is the same general formula as the alkenes, which means that cycloalkanes and alkenes with the same number of carbon atoms are isomers.

  • For example, cyclopropane and propene both have the molecular formula C₃H₆

3. They are used as fuels and solvents

• Like alkanes, they burn well and are used as fuels

  • Complete combustion of cycloalkanes produces only carbon dioxide and water

• They are also effective at dissolving other substances, so they are used as industrial solvents

4. They are insoluble in water

• Just like alkanes, cycloalkanes do not mix with water

Names & formulae of cycloalkanes

• Like other organic compounds, cycloalkanes are named systematically

• There are three skills for cycloalkanes:

  • Naming cycloalkanes from their structure

  • Drawing cycloalkanes from their name

  • Determining the formula of a cycloalkane

• The rules for naming and drawing un-branched cycloalkanes are straightforward

1. Naming cycloalkanes

• The name is made of two parts:

  • The prefix cyclo- to show it is a ring

  • The standard alkane name (e.g., propane, butane) to show the number of carbon atoms in the ring

• The minimum number of carbon atoms required to form a ring is three

  • Therefore, the smallest cycloalkane is cyclopropane

2. Drawing cycloalkanes

• You can work backwards from the name to draw the structure of a cycloalkane

  • "cyclo-" means a ring

  • The alkane name indicates how many carbon atoms are in the molecule

• The rules for drawing cycloalkanes:

  1. Identify the carbon ring

     • Identify the number of carbons in the ring from the alkane part of the name

     • For example, "-pentane" means a 5-carbon ring

  2. Add the hydrogens to complete the structure

     • Make sure every carbon atom has exactly four bonds

3. The molecular formula of cycloalkanes

• The general formula of cycloalkanes can be used to determine the molecular formula