Positional isomerism is a type of structural isomerism where compounds with the same molecular formula have different arrangements of a functional group or substituent on the same carbon chain. This type of isomerism arises due to the different positions at which the functional group or substituent is attached to the main carbon backbone.

Here are some key points about positional isomerism:

1. Same Functional Group: Positional isomers have the same functional group but with different attachment positions on the carbon chain.
2. Examples: One common example of positional isomerism involves halogenated alkanes with the molecular formula C3H7Cl:
   • 1-Chloropropane: CH3-CH2-CH2-Cl (chlorine attached to the first carbon)
   • 2-Chloropropane: CH3-CH(CH3)-Cl (chlorine attached to the second carbon)
3. Effects on Properties: The position of a functional group or substituent on the carbon chain can significantly affect the physical and chemical properties of the compound. For example, boiling points, melting points, and reactivity may vary between positional isomers due to differences in molecular interactions.
4. Naming: Positional isomers are named by specifying the position of the functional group using numerical prefixes (e.g., 1- for the first carbon, 2- for the second carbon, etc.).
5. Biological Significance: In biochemistry, the position of a functional group on a molecule can influence its biological activity. For example, the position of a functional group in a drug molecule may determine its effectiveness or side effects.
6. Stereoisomerism and Positional Isomerism: Positional isomerism is distinct from stereoisomerism. While positional isomers differ in the position of functional groups on the same carbon chain, stereoisomers have the same connectivity but differ in the spatial arrangement of atoms.
7. Constitutional Isomerism: Positional isomerism is a subset of constitutional isomerism, where the connectivity of atoms in the molecule is different.

Positional isomerism underscores the importance of molecular structure in determining the properties and behavior of organic compounds. Even small changes in the position of functional groups can lead to significant differences in a compound’s behavior, reactivity, and effects.