Homologous Series | Chemistry for Grade 11 (IGCSE) PDF Download

Homologous Series

• This is a series or 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

General Formulae

• This kind of formula reveals the composition of any element within a complete homologous series of organic compounds.
• For instance, all alkanes follow the general formula C_nH_2n+2, where 'n' signifies the number of carbon atoms. This indicates that by doubling the number of carbon atoms and adding two, you get the count of hydrogen atoms in the alkane.
• General formulae are instrumental in determining the compound's formula from various homologous series if the quantity of carbon atoms is specified.

General Formula of Common Homologous Series

Characteristics of Homologous Series

• All members of a homologous series share the following characteristics:
  • They have the same general formula.
  • They contain the same functional group.
  • They exhibit similar chemical properties.
  • There is a gradation in their physical properties, such as melting and boiling points.
  • The difference in the molecular formula between consecutive members is CH₂.
• Let's consider ethanol and propanol, which are part of the homologous series of alcohols:
  • They share the same general formula.
  • Both contain the same functional group.
  • They demonstrate similar chemical properties.
  • There is a gradation in their physical properties, like melting and boiling points.
  • The molecular formula differs by CH₂ between them.