Homologous Series (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

• 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

Family	Functional Group	Name ends in...
Alkane	C-C	-ane
Alkene	C=C	-ene
Alcohol	-OH	-anol

Extended tier only

• All members of a homologous series have:
  • The same general formula
  • Same functional group
• These characteristics are shown below for ethanol and propanol, which belong to homologous series, alcohols

Table of characteristics of ethanol and propanol

	Ethanol	Propanol
Molecular formula	CH3CH2OH	CH3CH2CH2OH
General formula	CnH2n+1OH	CnH2n+1OH
Functional group	-OH	-OH
Boiling point (oC)	78	98

• Members of a hoologous series:
  • Have similar chemical properties due to the presence of the same functional group
  • Show a trend in physical properties such as melting point, boiling point and density because they differ by a -CH₂- group