Homologous Series: Definition, Examples

In order to simplify and systematize the study of organic chemistry, all the organic compounds have been further classified into various families or groups based on their similar structures and chemical behavior. Each of these families or groups is known as a homologous series. It can be defined as:

A homologous series is a family or group of organic compounds which contain the same functional group, similar chemical properties, and exhibit a gradual change in physical properties is called homologous series. In simple words, homologous series is a series of organic compounds in which each successive member differs from the next by a methylene group (–CH₂–).

The individual members of such a series are called homologues, and the phenomenon is known as homology. All the members of a homologous series exhibit similar chemical properties, but their physical properties vary with molecular weight and the shape of the molecules. The different members of the same homologous series can be prepared using the same general method of preparation.

Example of Homologous Series

An example of a homologous series is alkanes. The first six members of this series are:

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)
• Butane (C₄H₁₀)
• Pentane (C₅H₁₂)
• Hexane (C₆H₁₄)

These hydrocarbons are represented by the same general formula, CₙH₂ₙ₊₂, where n is the number of carbon atoms. Furthermore, any two adjacent members of this family differ by a –CH₂– group.

If we replace a hydrogen atom in the above hydrocarbons with a hydroxyl group (–OH), we get a new homologous series called alcohols. The members of this series can be represented by the general formula CₙH₂ₙ₊₁OH. The first five members of this series are:

• Methanol (CH₃OH)
• Ethanol (C₂H₅OH)
• Propanol (C₃H₇OH)
• Butanol (C₄H₉OH)
• Pentanol (C₅H₁₁OH)

Some other homologous series are alkenes, alkynes, alkyl halides, ethers, aldehydes, ketones, amines, esters, carboxylic acids, etc.

Key Characteristics of a Homologous Series

Some of the key characteristics of a homologous series are:

• Each homologous series can be represented by a general formula. For example, C_nH_2n+1COOH is the general formula of carboxylic acids.
• All the members of a given homologous series have the same functional group. For example, the functional group of alcohols is the hydroxyl group (-OH). Similarly, the functional group of aldehydes is the aldehydic group (-CHO).
• The successive members of a homologous series differ by a CH2 group or by 12 + 2 * 1 = 14 atomic mass units (1 carbon = 12, 2 hydrogen = 2).
• The chemical properties of the members of any particular family are almost identical. Though the first member may vary considerably from the rest of the members.
• The physical properties such as melting point, boiling point, density, and solubility of the members of a particular homologous series show a regular gradation with the increase in molecular mass.
• The individual members of a homologous series can be prepared by identical general methods of preparation developed for that series.

Importance of Homologous Series

In the field of organic chemistry, the homologous series is a very important concept because it helps us to understand, organize, and predict the behavior of many organic compounds. Below are the key reasons why the homologous series is important in organic chemistry:

• In order to systematically study of organic compounds, homologous series provides an organized way to group organic compounds. This makes it easier to study the properties, structures, and chemical reactions of compounds without memorizing each one individually.
• All the members of any particular series have the same functional group. Therefore, they undergo similar chemical reactions. This simplifies learning and understanding the reaction mechanisms in organic chemistry.
• The presence of a general formula and a similar structure helps in the systematic IUPAC naming of organic compounds. It becomes easier to name new compounds in a series simply by knowing the number of carbon atoms.
• Physical properties such as boiling point, melting point, density, etc. change gradually within a homologous series. This regular gradation helps in predicting the properties of unknown compounds based on known ones.
• Once the behavior of one compound in a homologous series is known, chemists can predict the behavior of other members without repeating experiments. This saves time and effort for researchers.
• Homologous series is used in various industries like petroleum refining, pharmaceuticals, polymers, plastics, perfumes and alcohols.

How to Identify a Homologous Series?

To identify whether a group of organic compounds belongs to the same homologous series, you need to check some important features. These features ensure that the compounds are structurally related and follow a specific pattern.

• All the members of a homologous series must have the same functional group. For example, ethanol (C₂H₅OH) and propanol (C₃H₇OH) belong to the alcohol series because both have the same –OH functional group.
• All the members of a series must follow a common general formula, which represents the entire series. For example, the general formula of alkanes is CₙH₂ₙ₊₂, while the general formula of alkenes is CₙH₂ₙ. If the organic compounds follow the same formula by varying only the value of n (i.e. number of carbon atoms), they are likely in the same series.
• Each successive member of a homologous series differs by one methylene group (–CH₂–), which corresponds to a difference of 14 atomic mass units. For example, methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) each differ by a –CH₂– group and belong to the same homologous series called alkanes.
• Since all the members have the same functional group, they exhibit similar chemical properties.

Example: Identifying Alcohols in a Homologous Series

Let’s take the organic compounds of alcohol family:

• Methanol – CH₃OH
• Ethanol – C₂H₅OH
• Propanol – C₃H₇OH
• Butanol – C₄H₉OH

Check all conditions:

• Same functional group → ✓ (–OH)
• Same general formula → ✓ (CₙH₂ₙ₊₁OH)
• Difference of –CH₂– → ✓
• Similar chemical reactions → ✓

As all the criteria for a homologous series are met, these compounds are classified under the alcohol family.

Homologous Series of Organic Compounds

Various homologous series, along with their general formulas and examples of compounds, are listed in the table below:

Differences Between Homologous Series and Functional Group

FAQs on Homologous Series