What is a homologous series in organic chemistry?

A homologous series in organic chemistry refers to a group of compounds that share a similar structural feature and exhibit a systematic change in properties as the molecular size increases. The defining characteristic of a homologous series is that each successive compound differs from the previous one by a specific structural unit, which in the case of organic compounds is a -CH₂- (methylene) group.

This means that if you take any member of the series, you can derive the next member by adding a -CH₂- group to the molecular structure. For example, in the case of alkanes, the series begins with methane (CH₄), followed by ethane (C₂H₆), propane (C₃H₈), and so on. Each compound has higher molecular weight and exhibits trends in physical and chemical properties, such as boiling points and reactivity.

In contrast, a group of compounds with varying functional groups does not describe a homologous series since it may not maintain a consistent structural relationship across the entire group. Similarly, a series with different molecular formulas or a classification of inorganic compounds does not align with the concept of a homologous series, which is specifically focused on organic compounds with a consistent difference of a CH₂ unit.