Homologous Series (Cambridge International IGCSE Chemistry)

Spec point — 11.1.2 · Identify a functional group as an atom or group of atoms that determine the chemical properties of a homologous series State that a homologous series is a family of similar compounds with similar chemical properties due to the presence of the same functional group

Homologous series

• 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

Exam Hint Make sure you can identify the functional group for each homologous series.

General formulae

Spec point — 11.1.2 · Write and interpret general formulae of compounds in the same homologous series, limited to: alkanes, CnH2n+2 alkenes, CnH2n alcohols, CnH2n+1OH carboxylic acids, CnH2n+1COOH

General formulae

• 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

Worked Example Questions

1. What is the formula of an alcohol that contains 5 carbon atoms?
2. What is the formula of an alkene that contains 10 carbon atoms?

Answers:

1. The formula of an alcohol containing 5 carbon atoms is:

• Number of carbons = 5

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

• Formula = C₅H₁₁OH

1. The formula of an alkene that contains 10 carbon atoms is:

• Number of carbons = 10
• Number of hydrogen atoms = 2 x 10 = 20
• Formula = C₁₀H₂₀

General characteristic of homologous series

Spec point — 11.1.2 · Describe the general characteristics of a homologous series as: having the same functional group having the same general formula differing from one member to the next by a –CH2– unit displaying a trend in physical [properties sharing similar chemical properties

General characteristic of 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

Exam Hint Make sure you learn the general formula for each homologous series.