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Functional Isomerism | Structural Isomerism

Learning Objective: To learn about functional isomerism, a subtype of structural isomerism. 

Skill Level – Intermediate

 

Prerequisites: 

Related – 

 

Chapter: Structural Isomerism

 

Author's Note: The functional groups in organic chemistry are key to any transformation. What if two molecules have the same molecular formula but different functional groups? 

For example, ethanol (an alcohol) and dimethyl ether (an ether) have the same common molecular formula C₂H₆O, but they both show different reactivity. Ethanol can undergo oxidation to form acetic acid, while dimethyl ether does not. Such a piece of knowledge is essential for predicting and manipulating chemical reactions. 

The importance of functional group isomerism extends beyond chemistry. Two different functional groups can interact differently with biological targets such as enzymes, receptors, and DNA. For instance, glucose and fructose are both C₆H₁₂O₆ isomers, but their different functional groups (aldehyde and ketone) lead to varied metabolic pathways and biological effects. 

Therefore, studying functional group isomerism is crucial to designing and utilizing various compounds, predicting reactivity, and finding wider applications. This section covers all the functional groups that show functional group isomerism, their DBE values, and examples with illustrations on how to draw functional group isomers.

Functional Isomerism

The compounds having the same molecular formula, but different functional groups are called functional isomers, and the phenomenon is called functional isomerism. 

For example, two compounds can have the molecular formula C2H6O. However, the functional group in one compound is alcohol (R-OH, where the OH atom is terminal), and the other is ether (R-O-R, O atom is central), both being functional isomers.

Since the structural change observed is in the functional group part of the molecule, these isomers also have different chemical properties.

The class of organic compounds showing functional isomerism are shown below-

 

a) alcohols and ethers (CnH2n+2O) (note- DBE*= zero, no unsaturation in the structure)

For example, the molecular formula C2H6O represents two functional isomers. The functional group can be hydroxy (-OH) or ether (R-O-R).

 

 

b) aldehydes, ketones, unsaturated alcohols, and unsaturated ethers (CnH2nO) (note- DBE*= one, one unsaturation in the structure, C=O or C=C)

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c) carboxylic acids and esters (CnH2nO2) (note- DBE*= one, one unsaturation in the structure)

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i) Amides and oximes (CnH2n+1NO) show functional isomerism

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Related reading- What is DBE? How to calculate DBE?)

Calculating DBE is essential to formulate the structures when compounds show functional group isomerism. DBE reveals the presence of unsaturation in the molecule and can rule out groups that don't have any. For example, two compounds can have molecular formulas, C2H6O and C3H6O; only C3H6O has one unsaturation (DBE of 1). So, the choice of functional groups for C2H6O is between alcohol or ether - oxygen-containing functional groups with no unsaturation. And in C3H6O, it is between keto (C=O) or alkene (C=C-C-OH) functional groups with one unsaturation.

 

Subscribe to learn all subtypes of structural isomerism.

 

Structural Isomerism
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