5.1 Overview of Isomers | Constitutional Isomers and Stereoisomers | Organic Chemistry

Overview of Isomerism

• Isomerism in Stereochemistry: The primary topic includes various types of isomers.

• Vocabulary Highlights: Key terms to understand include:

  • Chiral

  • Achiral

  • Optically active

  • Enantiomers

  • Diastereomers

  • Racemic mixture

  • Meso compounds

• Emphasis on both definitions and practical recognition of these terms in molecular geometry.

Types of Isomers

• Conditions for Isomerism: Compounds must have the same chemical formula but different bond arrangements.

• Types of Isomers:

  • Constitutional Isomers (Structural Isomers): Different connectivity, but same molecular formula.

    • Example: C3H6

      • 1-Propene: a straight chain with a double bond.

      • Cyclopropane: a ring structure without double bonds.

    • Example: C3H7Cl

      • 1-Chloropropane: Chlorine on carbon-1.

      • 2-Chloropropane: Chlorine on carbon-2.

  • Stereoisomers: Same connectivity, different spatial arrangements.

    • Importance of understanding geometric dependence.

Stereochemistry

• Cis-Trans Isomers: A type of stereoisomerism observed in cycloalkanes and alkenes.

  • Cis Isomer: Substituents on the same side (e.g., both methyl groups on the same side of a cycle).

  • Trans Isomer: Substituents on opposite sides.

  • This concept can also apply to alkenes, noting the inability to rotate around a double bond due to pi bond restrictions.

Chiral and Achiral Molecules

• Chirality: Molecules are chiral if they are non-superimposable mirror images (like hands). Identical structures are achiral (e.g., a regular sheet of paper).

• Chiral Centers: An atom (often carbon) bonded to four different groups.

• Identifying Chiral Centers:

  • Look for sp3 hybridized atoms.

  • Ensure four different substituents are attached.

Enantiomers and Diastereomers

• Enantiomers: Chiral compounds that are mirror images but not superimposable.

  • Difficult to separate; share nearly identical physical properties.

• Diastereomers: Non-superimposable stereoisomers that are not mirror images.

  • Cis and Trans Isomers: Examples of diastereomers.

• When comparing chiral compounds, at least one chiral center must be the same and one different for them to be considered diastereomers.

Optical Activity

• Optically Active Compounds: Chiral compounds that rotate plane polarized light; achiral compounds do not.

• Racemization: Equal concentrations of enantiomers will cancel each other out, resulting in an optically inactive solution known as a racemic mixture.

Identifying Chiral Centers in Molecules

• Procedure for Counting Chiral Centers:

• Eliminate sp2 and sp hybridized atoms.

• Look for sp3 hybridized atoms with four different substituents.

• Common pitfalls include overlooking identical substituents.

• Example of Identifying Chiral Centers:

  • Determine atom configurations and ensure actual differences between substituents.

Conclusion

• Understanding isomerism and its classification is crucial for mastering stereochemistry in organic chemistry. Each concept builds on the last, providing a framework for deeper exploration in subsequent lessons.