A homologous series is a group of compounds that are similar in structure and properties. Each compound in the series differs from the next by a small repeating unit, usually a CH2 group. These compounds have the same functional group, which gives them similar chemical behaviour. As you move along the series, physical properties like boiling point or melting point change in a regular way. Homologous series make it easier to study and understand organic compounds as they follow a clear and predictable pattern.

Homologus Series

A homologous series is a group of organic compounds that have the same functional group and similar chemical properties. Each compound in this series differs from the next by a fixed unit— a CH2 group. This means they follow a regular pattern and have the general formula.

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In such series, the carbon chain grows by one CH2 unit each time, but the overall behaviour and reactions of the compound stay quite similar. For example, alkanes, alkenes, and alcohols each from their own homologous series.

In this article, we’ll explore what homologous compounds are and look at examples like the homologous series of alkanes and alcohols.

In a homologous series, all the compounds have the same functional group and show similar chemical behavior. Their physical properties–like melting point, boiling point, and solubility–change in a steady way as you move from one compound to the next.

For example, compounds like CH3OH (menthol), C2H5OH (ethanol), and C3H7OH (propanol) all belong to the same homologous series of alcohols. While their chemical nature stays similar, their physical properties increase gradually. This is because as the size of the molecule increases, the number of bonds also increases–affecting things like how easily the substance melts, boils, or dissolves. So even though each compound is slightly bigger than the last, they follow a clear and predictable pattern in both structure and behaviour.

Homologus Series of Alkanes

Alkanes are simple hydrocarbons with only single bonds between carbon atoms. They are non-popular, so they don’t dissolve in water but mix with other non-polar substances like oils.

Physical properties of Alkanes:

Solubility: Alkanes do not dissolve in water because they are non polar.

Boiling Point: Boiling points increase as the size and mass of the alkane molecules increases due to stronger Van de Waals forces.

Melting Point: Melting points also increase with molecular weight, especially in higher alkanes which are solid at room temperature.

Examples of Alkanes in a Homologous Series:

Homologus Series of Alkenes

Alkenes are hydrocarbons that contain at least one carbon–carbon double bond. They follow the general formula CnH2n.

They share similar chemical properties and differ from one another by a CH2 unit.

Examples of Alkenes in Homologus Series:

Homologus Series of Alkynes

Alkynes are hydrocarbons with at least one triple bond between carbon atoms. Their general formula is CnH2n-2.

Examples of Alkynes in a Homologous Series:

Homologous Series of Alcohols

Alcohols contain the –OH (hydroxyl) functional group and follow the general formula ROH. They are named by replacing the “e” in alkanes with—”ol”.

Examples of Alcohols in a Homologous Series:

Examples of Aldehydes in a Homologous Series:

Homologous Series of Aldehydes

Aldehydes contain the -CHO group and are named with the suffix “-al”. They are formed from the oxidation of alcohols.

Properties and Trends in Homologous Series

All compounds in a homologous series behave similarly in chemical reactions due to the same functional group. As the number of carbon atoms increases:

• Molecular weight increases
• Boiling and melting points gradually increase
• Reactivity can change slightly due to size and structure 

Uses of Homologus Series Members

• Methane (natural gas); used for cooking and heating
• Propane: used in LPG cylinders
• Octanes: key component in petrol (gasoline)
• Alkenes: used to make alcohols, plastics, detergents, and fuels
• Acetylene (C2H2): used in welding and for making synthetic products like PVC and rubber

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