What is an homologous series? Explain with an example.
A homologous series is a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain. The presence of a functional group decides the properties of the carbon compound, regardless of the length of the carbon chain.
For example, the chemical properties of alcohols like CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH are all very similar because they contain the same -OH functional group. Similarly, alkanes (CH₄, C₂H₆, C₃H₈, C₄H₁₀) form a homologous series.
In a homologous series, the formulae of successive compounds differ by a -CH₂- unit. For instance, CH₄ and C₂H₆ differ by -CH₂-, and C₂H₆ and C₃H₈ also differ by -CH₂-. This pattern continues throughout the series.
As the molecular mass increases in any homologous series, a gradation in physical properties is seen. The melting and boiling points increase with increasing molecular mass. However, the chemical properties remain similar in a homologous series because they are determined solely by the functional group.
Each homologous series has a general formula. For example, alkenes have the general formula CₙH₂ₙ, where n = 2, 3, 4.
Explanation
The textbook defines homologous series in Section 4.2.4 as compounds where the same functional group substitutes for hydrogen. Key characteristics include: (1) similar chemical properties due to same functional group, (2) successive members differ by -CH₂- unit, (3) gradation in physical properties with increasing molecular mass, and (4) existence of a general formula. The alcohol series (methanol, ethanol, propanol, butanol) and alkane series are textbook examples.