Structural Isomerism is the phenomenon of organic compound having same molecular formula but different structural arrangements. The compounds which shows the property of structural isomerism are called Structural Isomers. There are in total six types of structural isomerism.

In this article, we will learn in detail about structural isomers, structural isomerism, its types and examples in detail. Before getting into Structural Isomerism, let’s understand first What is an Isomer?

Isomer and Isomerism

An isomer is a molecule that has a identical molecular formula means the number of atoms of each type should be the same but differs in the arrangement of atoms in space.

While,

Isomerism is the phenomenon in which more than one compounds have the same chemical formula but have a difference in their chemical structures.

Example: n-Butane and isobutane are isomers of Butane. They have same molecular formula C₄H₁₀ while if see the arrangement, then in n-butane, there is a straight chain of 4 carbon atoms while in isobutane there is a straight chain of three carbon and the fourth carbon is linked to the middle carbon as a branch.

Structural Isomers Definition

Those isomers in which the atoms are completely arranged in a different order and bonding but the same molecular formulas. More simply, Compounds having the same molecular formula but different structures are known as Structural Isomers.

Structural isomers are also called as constitutional isomerism.

Example: butane and Isobutane

Structural Isomerism Definition

The isomers differing in the atomic arrangement of the molecules with the same molecular formula are known as structural isomers. The phenomenon of these structural isomers is called structural isomerism.

Structural isomerism is also called Constitutional Isomerism.

Types of Structural Isomerism

The six different types of structural isomers are mentioned below:

1. Chain Isomerism
2. Positional Isomerism
3. Functional Isomerism
4. Metamerism
5. Tautomerism
6. Ring-Chain Isomerism

Let’s learn them one by one in detail along with their examples are discussed below:

Chain Isomerism

The type of structural isomerism in which the two or more compounds have the same type of molecular formula with having difference in the main chains is called as Chain Isomerism. The two forms of chain isomerism are linear and branched isomerism.

Chain isomerism is also called as Skeletal Isomerism.

Example: C₅H₁₂: Pentane, Neo-Pentane, and Iso-Pentane. The structural image is shown below:

Positional Isomerism

The type of structural isomerism in which the position of the functional group is different with respect to the main chain is called as Positional Isomerism.

Example: C₃H₈O: 1-propyl alcohol, and 2-propanol. The structural image is shown below:

Functional Isomerism

The type of structural isomerism in which the compound have different functional groups of same number of atoms is called as Functional Isomerism. This is also called as Functional Group Isomerism.

Example: C3H6O: Propanone and Propanal

Metamerism

The type of structural isomerism in which different alkyl groups on either side of functional groups are present is called as Metamerism.

Example: C₄H₁₀O: Diethyl Ether and Methyl Propyl Ether

Tautomerism

The type of structural isomerism in which two isomers are in a stable dynamic equilibrium is called as Tautomerism. This is a special type of functional isomerism.

Example: C₃H₆O: Ketone and Enol

Ring-Chain Isomerism

The type of structural isomerism in which different chain structures appear to be connected in a ring pattern is called as Ring-Chain Isomerism.

They have two different structures as cyclic, and as open-chain.

Example: C₃H₆: Propene and Cyclopropene

Structural Isomers of Organic Compounds

The structural isomers of some common organic compounds are mentioned below:

Structural Isomers of Butane

Butane has a molecular formula of C₄H₁₀. There are two possible isomers of butane mentioned below:

• n-Butane, also called as unbranched butane (IUPAC: butane)
• Iso butane, also called as i-butane (IUPAC: 2-methylpropane)

Structural Isomers of Pentane

Pentane has a molecular formula of C₅H₁₂. There are three possible structural isomers of pentane mentioned below:

• n-Pentane, also called as unbranched pentane (IUPAC: pentane)
• Iso pentane (IUPAC: 2-methyl butane)
• Neo pentane (IUPAC: 2,2-dimethylpropane)

Structural Isomers of Hexane

Hexane has a molecular formula of C6H14. There are 5 isomers of hexane. Following are their IUPAC name:

• n-hexane
• 2-methyl pentane
• 3-methyl pentane
• 2,2-dimethyl butane
• 2,3-dimethyl butane

Structural Isomers of Heptane

Heptane has a molecular formula of C₇H₁₆. There are 9 isomers of heptane. Following are their IUPAC name:

• n-heptane
• 2-Methylhexane
• 3-Methylhexane
• 2,2-Dimethylpentane
• 2,3-Dimethylpentane
• 2,4-Dimethylpentane
• 3,3-Dimethylpentane
• 3-Ethylpentane
• 2,2,3-Trimethylbutane

Stereoisomers vs Structural isomers

Stereoisomer and Structural Isomers are two different kinds of isomerism possible in organic compounds. Here is the key differences between the stereoisomers and structural isomers: