Homologous Series (Cambridge (CIE) O Level Chemistry): Revision Note

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

This type of 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

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