Functional group isomerism, also known as functional isomerism or functional group isomerization, is a type of structural isomerism in organic chemistry. It occurs when compounds with the same molecular formula have different functional groups, resulting in distinct chemical and physical properties.

In functional group isomerism, the arrangement of atoms remains the same, but the type of functional group present in the molecule changes. This leads to differences in the compound’s reactivity and behavior. Here are some key points about functional group isomerism:

1. Different Functional Groups: Functional group isomers have different types of functional groups attached to the same carbon skeleton.
2. Examples: A common example of functional group isomerism involves compounds with the molecular formula C3H6O:
   • Propenal (Acrolein): CH2=CH-CHO (aldehyde functional group)
   • Propenoic acid: CH2=CH-COOH (carboxylic acid functional group)
3. Chemical Properties: Functional group isomerism leads to differences in chemical reactivity. Different functional groups can participate in various types of chemical reactions.
4. Physical Properties: The type of functional group present in a molecule influences its physical properties, such as boiling points, melting points, solubility, and odor.
5. Naming: Functional group isomers are named based on the type of functional group present. The IUPAC nomenclature system is used to give systematic names to these compounds.
6. Biological Significance: Different functional groups can result in compounds with different biological activities. For example, changing the functional group in a drug molecule may lead to different therapeutic effects.
7. Functional Group Changes: Functional group isomerism can involve changes between functional groups like alcohols, aldehydes, ketones, carboxylic acids, and others.
8. Constitutional Isomerism: Functional group isomerism falls under the broader category of constitutional isomerism, where the connectivity of atoms in the molecule is different.

Functional group isomerism highlights how changing the type of functional group attached to a molecule can drastically alter its chemical behavior and properties. It is an important concept in organic chemistry and has implications in various fields, including pharmaceuticals, materials science, and chemical synthesis.