What distinguishes a homologous series in organic chemistry?

A homologous series in organic chemistry is characterized by a sequence of compounds that differ from each other by a consistent pattern, specifically by the addition of one CH2 unit (methylene group). This consistency in structure leads to similar chemical properties among the members of the series. For example, alkanes, alcohols, and carboxylic acids each form their own homologous series where each subsequent compound has one more CH2 than the previous one, contributing to a gradual change in physical properties such as boiling point and solubility.

In addition to this structural similarity, compounds within a homologous series typically exhibit a trend in reactivity and other chemical behaviors, further unifying them as a family of related compounds. This distinctive feature of differing by one CH2 unit and sharing similar properties is central to defining a homologous series in organic chemistry.