Homologous Series (DP IB Chemistry): Revision Note

Homologous Series

• Organic chemistry is the chemistry of carbon compounds

• Carbon forms a vast number of compounds because it can form strong covalent bonds with itself

• This enables it to form long chains of carbon atoms, and hence an almost infinite variety of carbon compounds are known

• The tendency of identical atoms to form covalent bonds with each other and hence form chains is known as catenation

Examples of catenation using carbon

Catenation in carbon allows an almost infinite variety of chains, branches and rings

• Carbon always forms four covalent bonds which can be single, double or triple bonds

• A functional group is a specific atom or group of atoms which confer certain physical and chemical properties onto the molecule

• Organic molecules are classified by the dominant functional group on the molecule

What is a homologous series?

• Organic compounds with the same functional group, but a different number of carbon atoms, are said to belong to the same homologous series

• One definition of a homologous series is:

A family of similar compounds, having the same functional group, and so similar chemical properties

• Every time a carbon atom is added to the chain, two hydrogen atoms are also added

Homologous Series of Alkanes Table

The characteristics of a homologous series

• The features of a homologous series are:

  • Each member has the same functional group

  • Each member has the same general formula

  • Each member has similar chemical properties

  • Each member differs by  -CH₂-

  • Members have gradually changing physical properties, for example, boiling point, melting point and density

• As a homologous series is ascended, the size of the molecule increases

  • This has an effect on the physical properties, such as boiling point and density

General formulae of different homologous series

• Each homologous series can be described by a general formula
  

General formulae of homologous series table

^** Ethene is the smallest possible alkene as a minimum of two carbons are required to form the carbon-carbon double bond

Physical Trends in a Homologous Series

• As the number of carbon atoms in a straight-chain homologous series increases, the boiling and melting points also increase

Boiling point graph for four different homologous series

• Each successive member differs by a –CH₂– group, known as the homologous increment

• This increases the molecular size and surface area, leading to stronger London dispersion forces (a type of intermolecular force)

• Stronger dispersion forces mean more energy is needed to separate the molecules, so boiling and melting points rise

• This trend is seen across many homologous series, including:

  • Alkanes

  • Primary alcohols

  • Halogenoalkanes

  • Carboxylic acids

• Similar patterns are also observed for other physical properties such as viscosity, density, and melting point