Homologous Series (CIE IGCSE Chemistry)

• 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

Table of Compounds & their Functional Groups

General Formulae

• 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

General Formula of Common Homologous Series

Homologous Series	General Formula
Alkanes	CnH2n+2
Alkenes	CnH2n
Alcohols	CnH2n+1OH
Carboxylic Acids	CnH2n+1COOH

Answer

Number of carbons = 5

Number of hydrogens (excluding in the functional group) = 2 x 5 + 1 = 11

Formula = C₅H₁₁OH

Answer

There are 12 carbon atoms, so n = 12

There are twice the number of hydrogen atoms than carbon atoms = 2n

Therefore the general formula of the compound is C_nH_2n which means this compound is an alkene

EXTENDED

Characteristics of a Homologous Series

• 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