What is a Homologous Series

This article gives an explanation for the homologous series and their characteristics. It describes what are called a homologous series, examples of these series, and properties of some of selected homologous series in detail. Most of the homologous series are abundant in Organic Chemistry, but it is not unique only to Organic Chemistry. This article describes some of the homologous series in Inorganic Chemistry, as well. Most of the times in such a series, each member of the group possesses common features and there is a unique variation in their physical properties along the series. For example, after looking at the variation of the first few member’s physical properties, the next can be predicted. This article elaborates some of these characteristics with examples.

What is a homologous series?

A series of compounds where each consecutive member differs from one particular unit is called a homologous series. Members of the homologous series are called homologs. Alkanes, alkenes and alkynes are some examples for this series.

In the above three series, each member differs from its proceeding member by a CH2 unit.

Examples:

Most of these homologous series are found in Organic Chemistry, but there are some members in Inorganic Chemistry too. Oxides of Vanadium, oxides of Titanium, oxides of Molybdenum and Silanes are some examples of Inorganic homologs.

Characteristics of a homologous series

Example 1: Hydrocarbons

For all unbranched hydrocarbons; n-alkanes, n-alkenes and n-alkynes shows similar variation in some of their physical properties along the series. For example, the variation in boiling points increases along the series. This behavior is common for all members in a particular family. As the molecular weight increases, surface area of the molecule also increases. This results in increasing their wanderwaalsforce attractions. It raises their boiling points.

Example 2: Primary alcohols

In this homologous series, the boiling point gradually increases as the molecular weight increases. It follows a smooth pattern with the molecular weight.

Alcohols are soluble in water, b ut the solubility decreases as the molecular weight increases. The solubility of alcohols in water is facilitated by the –OH group. As the non-polar alkyl group increases, the solubility in water gradually decreases.

Homologous Series – Summary

Homologous series present in both Organic Chemistry and Inorganic Chemistry. Alkanes, Alkenes, primary alcohols, monocarboxylic acids, silanes, oxides of Vanadium, Titanium are some examples for a homologous series. In a homologous series, all the members have a common molecular formula and their bonding patterns are identical from one member to another except the length in the structure. There is a common unit adding to their molecular structure along the series. There is a unique variation in some of their physical properties along the homologous series.

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