What Is a Homologous Series? – Types, Examples, Properties

What is Homologous Series?

A homologous series is a collection of carbon compounds with varying numbers of carbon atoms with similar chemical properties and functional groups that vary from the next member by CH2. The organic compounds with the same general formula contain carbon chains of various lengths. 

A series is a collection of organic substances that differ from each other by repetition but have the same overall formula. The most fundamental homologous series in organic chemistry is formed of alkanes, alkenes, and alkynes with general formulas \(C_{n} H_{2n+2}\), \(C_{n} H_{2n}\), and \(C_{n} H_{2n-2}\). 

What Is a Homologous Series? - Types, Examples, Properties

Examples of Homologous Series

Each successive member is distinguished from the rest by a \(C H_{2}\) unit. For example, the \(-C H_{2}\) unit is the difference between \(C H_{4}\) and \(C_{2} H_{6}\), and the difference between \(C_{2} H_{6}\) and \(C_{3} H_{8}\) is also a \(-C H_{2}\) unit. As a result, \(C H_{4}\), \(C_{2} H_{6}\), and \(C_{3} H_{8}\) are homologs. A similar effect may be seen with alkenes, where the first component is ethene, and the subsequent members are \(C_{3} H_{6}\), \(C_{4} H_{8}\), and \(C_{5} H_{10}\). They are distinguished by a \(-C H_{2}\) unit. \(C_{n} H_{2n}\) is the formula for alkene. 

This series’ members all belong to the same functional groupings. They have identical physical qualities that increase in mass in a set gradation. The characteristics of \(C H_{3}OH\), \(C_{2} H_{5}OH\), and \(C_{3} H_{7}OH\) are similar and vary gradually as the molecular mass of the series members increases. This is because as the molecular mass of the compounds grows, so does the number of bonds. As a result, features such as melting and boiling points, solubility, and so on that are dependent on the mass and the total number of bonds in a compound gradually alter as the molecular masses of the compounds rise. Members of a homologous series have the same chemical characteristics because they all contain the same functional groups. 

The Reaction of Homologous Series

A homologous reaction is also known as homologation. A homologation reaction is a process that increases the number of repeating groups in a molecule. As a result, the molecule no longer belongs to the homologous series. 

Take the Ardnt-Eistert homologation procedure as an example. The number of repeating units in carboxylic acid is increased by this method. 

In this case, the process involves a number of distinct reactions. Furthermore, the atoms in the molecule are redesigned and reorganized because of these processes. 

Types of Homologous Series of Hydrocarbons

A homologous series is a set of hydrocarbons that have the same general formula and chemical characteristics that are equivalent to one another. There are three types of hydrocarbons: alkanes, alkenes, and cycloalkanes. 

Characteristics of Homologous Series

  • Two succeeding members differ in terms of\(-C H_{2}\) group or atomic mass 14. 
  • The approach to preparing for a family is nearly the same. 
  • The members share the same chemical and physical characteristics, including melting and boiling points. 

This series has enabled scientists and engineers to conduct systematic research on many chemical substances. 


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