Homologous Series and Isomerism

 What Is a Homologous series?

Homologous series refers to a group of chemical compounds with the same functional group. Due to the presence of the same active group, the compounds exhibit similar chemical properties.

A typical example of Homologous series are alkanes, cycloalkanes and alkenes. If you pay precise attention to these groups, each group has a general chemical formula.

For example, all the alkanes are written as CnH2n+2.

Carbon compounds in a Homologous series have a different number of atoms. But a specific group has the same functional group substituting the Hydrogen atom. Moreover, two adjacent group members in the Homologous series differ by -CH2 group.

Homologous series Definition

Homologous series refers to the group or family of organic compounds with the same functional group, which contributes to similarities in the physical and chemical properties of the compounds of that specific group. Each Homologous series has a similar structure and can be represented by a standard chemical formula.

Examples of Homologous series of Alkanes:

Methane CH4

Ethane C2H6

Propane C3H8

As you can see, the hydrocarbons in the above Homologous series of alkane groups can be represented by one chemical formula called CnH2n+2.

Also, two consecutive organic compounds have a difference of CH2. Methane, Ethane and Propane have similar chemical properties as well.

The only difference is in the length of the carbon chain. The size of each compound exceeds CH2 from its immediate previous combination.

Examples Of Homologous series

Homologous series of Alkanes are as follows:

Homologous series of alkenes are:

Homologous series of alkynes are:

Homologous series of Alcohol groups are:

Properties Of Homologous series

•       Each group in a Homologous series has the same functional group.
•       The functional group decides the specific characteristic of the group.
•       The number of carbon atoms differs in each compound.
•       Each group of the Homologous series have one general chemical formula.
•       Compounds differ in the length of their carbon chain.
•       Except for the repeating unit in the group, all the compounds are similar. The repeating unit of alkanes is CH2.
•       In the Homologous series, the physical properties like boiling point melting point increases with an increase in the molecular mass.
•       Physical properties depend on the mass and composition of the organic compound. But the chemical properties of a specific group determined by the same functional group remains the same.
•       The individual compound or members of a group are called Homologues.
•       The phenomenon is often referred to as Homology.

Homologation Reaction

A homologation reaction is a chemical reaction in which the number of repeating groups of a molecule is increased. As a result, the chemical is no longer a member of its Homologous series.

Take the Arndt-Eistert homologation process as an example. In a carboxylic acid, this action increases the number of repeating units.

Multiple reactions are involved in this process. Furthermore, these reactions lead the atoms in the molecule to rearrange and reconfigure.

What Are Isomers?

Isomers are just the opposite of Homologues. Isomers have the same molecular formula but different chemical and physical properties.

For example, take butane and 2-methylpropane. Both compounds or isomers have the same molecular formula but different chemical properties.

Butane and 2-methylpropane are represented by the molecular formula C4H10.

In the case of the Homologous series, the homologues methane and ethane have different molecular formulas but similar chemical properties. However, it would help to remember that the general chemical formula is the same, not the molecular formula.

The general chemical formula of alkanes is CnH2n+2.

The molecular formula of methane is Methane CH4, and Ethane is C2H6. The molecular formula is different, so they cannot be called isomers. Instead, they are homologues.

Isomerism Definition

Isomerism is when organic compounds have the same molecular formula but different chemical structures.

In isomers, the number of carbon atoms is the same, but they differ in the orientation of the carbon atoms. In simple words, the number of carbon and hydrogen atoms are the same, but the arrangement of atoms is different.

There are two major types of isomers:

•       Structural isomers
•       Stereoisomers or Stereoisomerism

Let us understand each one of them in detail.

Structural Isomerism

Structural isomerism is also known as constitutional isomerism. In structural isomerism, the compounds or isomers have the same chemical formula, but the structure or arrangement of atoms is different.

Carbon can form only four bonds. So, there are many possibilities on how the carbon atom can bond with hydrogen or other carbon atoms. Commonly the carbon atoms can be connected in a chain-like formation like in butane. The particles can also form a branch-like structure, for example, in the case of 2-methylpropane.

Structural isomerism is further divided into the following groups:

•       Chain – n-pentane, 2-methylbutane or isopentane and 2,2-dimethylpropane or neopentane
•       Positional – 1,2-dibromobenzene, 1,3-dibromobenzene and 1,4-dibromobenzene
•       Functional – dimethyl ether and ethanol or ethyl alcohol
•       Metamerism – Diethyl ether and methyl propyl ether 
•       Tautomerism – Ketone-enol, enamine-imine,lactam-lactim.
•       Ring-chain – propene and cyclopropane

Chain isomerism is also called skeleton isomerism. The structure of chemical compounds differs in the bonding arrangement of carbon atoms.

Example of chain isomerism in pentane:

In position isomerism, the difference is in the position of the functional group. Example of position isomerism in compounds with chemical formula C3H7Br:

The compounds of chemical isomers have the same molecular formula, but a different functional group is attached to them. An example of functional isomerism is C3H6O.

Metamerism occurs due to different alkyl chains on each side of the functional group. It is a rare type of isomerism that happens in compounds that contain sulphur or oxygen.

Example: ethoxyethane (C2H5OC2H5) and methoxy-propane (CH3OC3H7). Both have the same molecular formula of C4H10O, but different alkyl groups are attached to the functional group.

Tautomerism occurs in compounds where the position of protons and electrons differs. The examples are keto-enol tautomerism.

Example of tautomerism: propene and cyclopropane

Stereoisomerism

In stereoisomerism, the compounds have the same molecular formula but a different arrangement of atoms. The orientation of groups in the mix is different.

Example of stereoisomerism:

Here you can see that the chemical formula C4H8 is the same in both compounds. You can see the structure is also somewhat identical. However, the orientation of the CH3 group is different in both compounds.

Conclusion

The phenomenon of Homologous series and Isomerism is opposite of one another. Both concepts form a significant part of organic chemistry.

That is why it becomes essential to understand the concepts with in-depth analysis and examples.