Isomers and Isomerism: Structural Isomerism

When compounds have different arrangements of atoms or they spatially differ in their arrangement, and in the end they have the same molecular formula, then such compounds are isomers of each other. This difference in arrangement causes them to also have different chemical as well as physical properties. This whole phenomenon is referred to as Isomerism.

Let us understand structural isomerism, which is seen in compounds. Read on to learn how it leads to its structure and also know its types.

What are Isomers?

Molecules where the number of atoms each element has is the same, but there is a difference in their arrangement, then those molecules are isomers of each other.

E.g., C₄H₁₀ (Butane)→ n-butane and isobutane (structural isomers)

Types of Isomerism

Isomerism is broadly divided into two types:

Structural Isomerism → difference in connectivity of atoms.
Stereoisomerism → difference in spatial arrangement of atoms.

Breakdown of Structural Isomerism

Occur in molecules where, same molecular formula → atoms are connected differently

Here is a complete understanding of structural isomerism.

Subtypes of Structural Isomerism

Chain Isomerism – a carbon chain has different arrangement
E.g., in C₄H₁₀ → n-butane and isobutane

Chain Isomerism example

Position Isomerism – the carbon chain remains the same, but the position of the functional group or substituent changes
E.g., in C₃H₇OH → propan-1-ol and propan-2-ol

Position Isomerism example

Functional Group Isomerism – the molecular formula remains the same, but functional groups differ.
E.g., in C₂H₆O → ethanol (alcohol) and dimethyl ether (ether)

Functional Group Isomerism example

Metamerism – this is due to the unequal distribution of alkyl groups around a functional group
E.g., in C₄H₁₀O → diethyl ether and methyl propyl ether

Metamerism example

Tautomerism – Dynamic equilibrium between two structural forms differing in the position of a hydrogen atom and a double bond
E.g., keto-enol tautomerism in CH₃–CO–CH₂–H ⇌ CH₂= C(OH)–CH₃

Tautomerism example

Physical and Chemical Properties of Isomers

Property	Effect in Isomers	Example
Boiling Point	Straight chains > branched	n-butane > isobutane
Melting Point	More symmetry → higher MP	neopentane > n-pentane
Solubility	Depends on the type of functional group	alcohol (soluble) vs ether (less)
Reactivity	Different functional groups react differently	ethanol vs dimethyl ether

Representation of Structural Isomerism

We now know that in isomers:

Molecular Formula → remains the same
Structural Formula → changes based on connectivity

Example (Chain Isomerism):

n-Butane (straight chain) and isobutane (branched chain) both have the molecular formula C₄H₁₀, but their structural arrangements differ.

n-Butane vs isobutane

Details At A Glance

Property	Details
Definition	Compounds when they have same molecular formula but different connectivity
Broad Types	Structural Isomerism & Stereoisomerism
Structural Isomerism Subtypes	Chain, Position, Functional, Metamerism, Tautomerism
Example	C₄H₁₀ → n-butane & isobutane
Properties	Different chemical & physical properties

Summing Up

Structural isomerism occurs when compounds have the same molecular formula but differ in their connectivity of atoms. It gives rise to different types such as chain, position, functional, metamerism, and tautomerism, which end up leading to variations in physical and chemical properties.