Structural Isomerism

Structural isomerism, also known as constitutional isomerism, is a form of isomerism where molecules with the same molecular formula have different physical structures. This means that while the molecules contain the same number of each atom, these atoms are connected in a different sequence or arrangement. Let’s understand Structural isomerism and types of Structural isomerism in detail.

1.0Isomerism and Structural Isomerism

Isomerism is a phenomenon where compounds have the same molecular formula but differ in the arrangement of atoms or in their spatial configuration. This can lead to variations in physical and chemical properties, despite the sameness in molecular composition. 

Isomerism is divided into two main types: Structural Isomerism and Stereoisomerism.

In this article we will discuss Structural Isomerism in detail with its examples and types of Structural Isomerism.

2.0Structural Isomerism (Constitutional Isomerism)

Structural isomerism occurs when molecules have the same molecular formula but differ in the connectivity or sequence in which the atoms are bonded. This form of isomerism can be further classified into several subtypes:

3.0Types of Structural Isomerism

1. Chain Isomerism (Skeletal Isomerism)

This type of isomerism occurs when the carbon atoms are connected in different ways, leading to variations in the carbon skeleton. This can involve differences in branching or the overall shape of the molecule.

Example: Pentane (C₅H₁₂) has three isomers:

n-pentane: Straight chain of five carbon atoms.

Isopentane (methylbutane): A four-carbon chain with one methyl group on the second carbon.

Neopentane (dimethylpropane): A three-carbon chain with two methyl groups on the middle carbon.

Example-2 Structural isomers of C₇H₁₆ are 9.

2. Position Isomerism

This type arises when the functional groups are attached to different carbons in the molecule, despite the same carbon skeleton.

Example: The position of the hydroxyl group in alcohols like propanol:

1-Butanol: OH group on the first carbon.

2-Butanol: OH group on the second carbon.

3. Functional Group Isomerism

Isomers differ in the type of functional group they contain. This happens when the same atoms rearrange to form different functional groups, which changes the chemical properties of the compounds.

Example:

Alcohols and ethers: Both can have the same molecular formula but different structures. Ethanol (C₂H₅OH) and dimethyl ether (CH₃OCH₃) are both C₂H₆O but differ in their functional groups.

Ethanol (C₂H₅OH) - CH₃-CH₂-OH

Dimethyl ether (CH₃OCH₃)-  CH₃-O-CH₃

4. Tautomerism

A special case of functional group isomerism where the isomers exist in dynamic equilibrium and can convert into each other. This often involves the relocation of a hydrogen atom and a shift in a double bond.

Example:

Keto-enol tautomerism: Acetone (keto form, CH₃COCH₃) and its enol form (CH₂=C(OH)CH₃) can interconvert.

5. Ring-Chain Isomerism

Isomers differ in that one is a cyclic structure while the other is an open chain. This type is significant because the presence or absence of a ring can drastically change a molecule’s reactivity and physical properties.

Example:

Cyclobutane (C₄H₈): A four-membered ring.

1-butene (C₄H₈): An open chain with a double bond.

6. Metamerism

A type of isomerism that occurs when compounds have the same molecular formula but different alkyl groups on either side of a functional group like an ether or ester linkage.

Example: Ethyl methyl ether (CH₃OCH₂CH₃) and dimethyl ether (CH₃OCH₃).

4.0Degree of Unsaturation

The degree of unsaturation (DoU) is a helpful tool in studying isomerism in organic chemistry because it provides insights into the potential structural complexity of molecules. It indicates the number of rings and/or multiple bonds (like double or triple bonds) a molecule contains. This information aids in predicting:

1. Structural Isomerism: Different connectivity possibilities of atoms in molecules.
2. Geometric Isomerism: Possible different spatial arrangements due to double bonds.
3. Exclusion of Certain Isomers: Certain structures can be ruled out based on the DoU, such as fully saturated compounds if DoU is zero.

The degree of unsaturation, also known as the index of hydrogen deficiency (IHD), is a calculation used in organic chemistry to determine the number of rings and/or multiple bonds (double bonds or triple bonds) in a molecule relative to a fully saturated hydrocarbon. Here's how to calculate it:

Formula: The formula to calculate the degree of unsaturation is:

D.U. = (C+1) - $\frac{H+X-N}{2}$

Where:

• C = Number of carbon atoms
• N = Number of nitrogen atoms (each nitrogen adds one hydrogen's worth because of its trivalent nature)
• H = Number of hydrogen atoms
• X = Number of halogen atoms (halogens replace hydrogen atoms one-for-one)

5.0Solved Examples

Ex 1. How many Structural isomers are found in C₆H₁₄-

Ans. C₆H₁₄ has 5 structural isomers, Structures are:

(a–b), (a–c), (a–d), (a–e)  (b–d), (b–e), (c–d), (c–e)  are Chain isomers 

(b–c), (d–e) are Position isomers.

Ex 2. Determine number of Benzenoid isomers possible for the C₇H₈O

Solution:

Benzenoid isomers of C₇H₈O = 5

D.U. for C₇H₈O = 4 (1 ring + 3 pi bonds).

Since Alcoholic and phenolic groups are Functional isomers. Here are possible structures-

(a-b), (a-e) – Functional isomers           

b,c,d – Position isomers

6.0Sample Question on Structural Isomerism

There are several types of structural isomers, including: