Homologous Series (CIE IGCSE Chemistry)

• A homologous series is a family of organic compounds that have similar features and chemical properties due to them having the same functional group
• The functional group is a group of atoms which are bonded in a specific arrangement that is responsible for the characteristic reactions of each member of a homologous series

Table of compounds & their functional groups

• The general formula tells you the composition of any member of a whole homologous series of organic compound
• For example:
  • All of the alkanes have the general formula C_nH_2n+2, where n represents the number of carbon atoms
  • This tells you that however many carbon atoms there are in the alkane, doubling this number and adding two will give you the number of hydrogen atoms present in the alkane
• General formulae can be used to work out the formula of a compound from different homologous series if the number of carbon atoms present is known

General formula of common homologous series

Extended tier only

• All members of a homologous series have:
  • The same general formula
  • Same functional group
  • Similar chemical properties
  • Gradation in their physical properties, such as melting and boiling point
  • The difference in the molecular formula between one member and the next is CH₂
• These characteristics are shown below for ethanol and propanol, which belong to homologous series, alcohols

Table of characteristics of ethanol and propanol