Systematic Carbon Chemistry (SQA National 5 Chemistry): Revision Note

Members of a homologous series

• A homologous series is a family of organic compounds with a set of shared characteristics:

  • They have the same general formula

  • They have the same functional group, which gives them similar chemical properties

  • They show a gradual change (a trend) in their physical properties, such as boiling point

General formulae

• The general formula tells you the composition of any member of a homologous series

• You can use it to work out the molecular formula for any member of a homologous series, just by knowing the number of carbon atoms, n

General formulae of homologous series

• For example:

  • Alkanes have the general formula C_nH_2n+2

  • n represents the number of carbon atoms

  • So, taking the number of carbon atoms in the alkane, doubling it and adding two gives the number of hydrogen atoms in the alkane

Functional groups

• A functional group is the specific group of atoms in a molecule that is responsible for its characteristic reactions

• It is the most important part of the molecule, as it determines how the compound will react

• You need to be able to identify the functional group for each homologous series:

Homologous series and their functional groups

[GRAPHIC: Functional Group Examples]

Trends in physical properties

As you go up a homologous series, the melting and boiling points increase.

Explaining the trend

• As the size of the molecule increases, the strength of the intermolecular forces increases

  • Intermolecular forces are the weak forces of attraction between the molecules

• This means that more energy is needed to overcome these stronger forces

• So, the melting and boiling points increase

Example: The alkanes

• As the molecules get bigger, the boiling points increase from -162 ^oC to -42 ^oC

• This same trend applies to other homologous series, such as alcohols and carboxylic acids