Homologous Series - Overview, Stricture, Properties & Uses

Homologous Series - Overview, Stricture, Properties & Uses

Homologous Series: Introduction.

Homologous Series Definition: It is a term used to describe a set of chemical compounds in which the difference between successive members is single structural unit. Such series are most frequent among organic compounds, with the structural variation being a methylene group. The number of methylene groups in each of these homologous series—designated by the subscript n—can have the values 0, 1, 2, 3, etc. Phosphoric acids, silicic acids, and phosphonitrilic chlorides are homologous sequence of inorganic chemicals.

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Define Homologous series

Homologous series includes alkanes, alkenes, and cycloalkanes. A homologous series is a collection of substances that share chemical properties and can be represented by a single formula.

Homologous series examples

Each subsequent member of homologous series differs from the previous by one CH2 unit. The difference between CH4 and C2H6 is -CH2 unit, while the difference between C2H6 and C3H8 is also -CH2 unit. As a result, CH4, C2H6, and C3H8 are all homologous. The same effect may be seen in alkenes with ethene as the first member and C3H6, C4H8, and C5H10 as the subsequent members. By a –CH2 unit, they differ from one another. CnH2n is the formula for alkenes.

The functional groups are the same for all members of this homologous series. They have identical physical qualities that increase in mass in a fixed gradation. The characteristics of CH3OH, C2H5OH, and C3H7OH are similar and vary gradually as the molecular mass of subsequent members of the homologous series increases. This is due to the fact that as the molecular mass of the compounds grows, so does the number of bonds. As a result, qualities such as melting and boiling points, solubility, and other properties that are dependent on the mass and total number of bonds in a compound alter gradually as the molecular masses of the compounds grow.

Homologous Series Properties

A homologous series has four characteristics:

  1. All compounds in the series have the same general formula.

  2. They are both members of the same functional group.

  3. Their physical properties, such as melting point, boiling point, and density, change gradually as the molecular formula in the series increases.

  4. Due to the presence of the same functional group in them, their chemical characteristics are strikingly comparable.

  5. The -CH2- group, also known as the methylene group, distinguishes consecutive members of the series, and their molecular weights differ by 14 units.

Homogeneous series and hydrocarbons

A homologous series is a collection of hydrocarbons that have the same general formula and have similar chemical characteristics.

Homologous series of Alkanes.

The alkanes are the first homologous series. All of their names finish in –ane.

The alkanes have a wide range of applications:

  1. Propane – used in gas cylinders for BBQs, etc.

  2. Methane – (natural gas) cooking, heating.

  3. Octane - a type of gasoline used in automobiles.

The alkanes have the following general formula:

They are water insoluble.

The first eight alkanes must be learned, including their names, molecular formulas, and structural formulas. Learning the names is easier if you use a mnemonic.


Molecular Formula





Homologous series of Alkenes.

The alkenes are the second homologous series. All of their names finish in –ene, such as ethene.

Alkenes are more reactive than alkanes because they all have a carbon to carbon double bond. The general formula for alkenes is

Unsaturated alkenes are often used. This indicates that they have a double bond between carbon atoms. Because they only have single bonds, the alkanes are saturated. Bromine water swiftly decolorizes when it comes into contact with an unsaturated hydrocarbon. Alkanes take a long time to do this. Fuels, solvents, and starting materials for daily items like plastics and alcohol can all be made from alkenes.


Molecular formula






Homologous series of Alkynes.

The compounds with at least one triple bond are known as alkynes.

The general formula for alkynes is , where n is an integer, and the triple bond distinguishes them.

All of their names finish in: yne


Molecular formula





Homologous series of Alcohols


Alcohol is a group of chemical substances that are all the same. They all contain the functional group –OH, which gives alcohols their characteristics. Alcohols have names that finish in 'ol,' such as ethanol. Methanol, ethanol, and propanol are the first three alcohols in the homologous series. Because they are highly flammable, they can be used as fuel. They're also employed in marker pens, medications, and cosmetics as solvents (such as deodorants and perfumes).Ethanol is a kind of alcohol that can be found in alcoholic beverages like wine and beer. Ethanol is added to gasoline and used as a fuel.


Molecular formula





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