Isomerism in Organic Chemistry

Basics

Structural Formula vs Molecular Formula

• Structural Formula: Shows the arrangement of atoms or molecules in a compound.
  • Example: The structural formula of methane (CH₄) shows a tetrahedral shape, with one carbon atom bonded to four hydrogen atoms.
• Molecular Formula: Indicates the total number of atoms of each element in a compound.
  • Example: The molecular formula of methane is CH₄, indicating one carbon atom and four hydrogen atoms.

Isomers

• Chemical compounds with the same chemical formula but different structural arrangements.
• Analogy: Gloves with the same number of fingers arranged differently.

Types of Isomerism

1. Structural (or Constitutional) Isomerism
2. Stereoisomerism

Structural Isomerism

• Chain Isomerism: Different carbon chain arrangements.
• Position Isomerism: Same functional group at different positions.
• Functional Group Isomerism: Different functional groups.
• Metamerism: Different alkyl groups attached to the same functional group.
• Tautomerism: Compounds that can interconvert, often involving carbonyl groups.
• Ring-Chain Isomerism: Cyclic vs open chain structures.

Stereoisomerism

• Geometrical (or Cis-Trans) Isomerism: Different spatial arrangements around a double bond.
• Optical Isomerism: Different behavior towards polarized light.

Chain Isomerism Examples

• Butane vs Isobutane:
  • Butane: Straight chain structure.
  • Isobutane: Branched structure.
• Pentane Isomers:
  • n-Pentane: Straight chain.
  • Isopentane: Branch on the second carbon.
  • Neopentane: Two branches on the second carbon.

Functional Group Isomerism Examples

• Alcohol and Ether:
  • Alcohol: C₂H₆O with -OH group.
  • Ether: C₂H₆O with -O- group.
• Aldehyde and Ketone:
  • Aldehyde: C₃H₆O with -CHO group.
  • Ketone: C₃H₆O with -C=O group.

Metamerism Examples

• Ether Functional Group
  • Diethyl ether: C₂H₅-O-C₂H₅.
  • Methylpropyl ether: C₃H₇-O-CH₃.
  • Same functional group but different alkyl groups.

Tautomerism Examples

• Keto-enol Tautomerism: Conversion often seen with carbonyl compounds.
  • Ketone: R-CO-R'
  • Enol: R-C(OH)=C

Ring-Chain Isomerism Examples

• Propene vs Cyclopropane:
  • Open-chain: C₃H₆ as propene.
  • Cyclic: C₃H₆ as cyclopropane.

Geometrical Isomerism

• Cis-Trans Isomerism: Different positions on the double bond.
  • Cis-2-butene: Both methyl groups on the same side of the double bond.
  • Trans-2-butene: Methyl groups on opposite sides.

Optical Isomerism

• Constructs of Optical Isomerism:
  • Optically Active Compounds: Rotate polarized light.
  • Asymmetrical (Chiral) Carbon: Carbon atom bonded to four different groups.
• Tests:
  • Nicool Prism and Polarimeter: Used to test optical activity.
• Examples:
  • Lactic Acid: Different forms mirror each other (enantiomers).

Summary

• Structural isomers differ in connectivity.
• Stereoisomers have the same connectivity but differ in spatial arrangement.
• Specific rules and examples help identify each type of isomerism.