Lecture 13: Cis-Trans Isomerism and Cycloalkanes Notes

Lecture 13: Cis-Trans Isomerism and Cycloalkanes

Overview

• Focus on cis-trans isomerism and cycloalkanes.
• Closeout of Chapter 4.
• Introduction to stereoisomers, especially distinguishing features between enantiomers and diastereomers.

Isomers

• Definition: Isomers are different compounds with the same molecular formula.
• Isomers can be broken down into two main categories:
  • Constitutional Isomers: Isomers with different connectivity of atoms.
  • Stereoisomers: Isomers that have the same connectivity but differ in the arrangement of their atoms in space.

Types of Stereoisomers

1. Enantiomers: Nonsuperimposable mirror images of each other.
2. Diastereomers: Stereoisomers that are not mirror images of one another.

Examples of Stereoisomers

• Using the example of alkenes:
  • Cis Alkenes: Substituents on the same side.
  • Trans Alkenes: Substituents on opposite sides.
• Study the examples of cis-trans in one, two dimethylcyclopropane:
  • Cis 1,2-dimethylcyclopropane: Both methyl groups positioned upwards (same face).
  • Trans 1,2-dimethylcyclopropane: One methyl group up, one down (opposite faces).

Stability of Stereoisomers

• The relative stability of stereoisomers can be determined by heats of combustion and heats of formation:
  • Enantiomers: Do not differ significantly in stability.
  • Diastereomers: May differ in stability, with measurable differences in heats of combustion and formation.
  • For example, cis 1,2-dimethylcyclopropane is less stable than trans due to van der Waals strain.

Cyclohexane Conformations

• Chair Conformations: Important for understanding the positioning of substituents in cyclohexane.
• Stereochemistry:
  • Cis-Trans Relationship: Recognized by whether substituents are on the same side or opposite sides in chair conformations.
  • Tests for Cis/Trans in Chains:
  1. Both substituents on the same side (both axial or both equatorial) indicate cis.
  2. When flattened, substituents are on the same side for cis.

Dimethylcyclohexane Example

• For instance, in cis-1,4-dimethylcyclohexane:
  • Both methyl groups can be placed in the equatorial position for maximum stability.
• The process for determining cis or trans involves drawing chair conformations, identifying positions (axial/equatorial), and performing ring flips to check configurations.

Difference Between Axial and Equatorial

• The axial substituents alternate up and down around the cyclohexane ring, while equatorial substituents remain more stable by avoiding steric hindrance.
• When visualizing or evaluating relationships between substituents:
  • Cis: Both substituents on the same face (both up or down).
  • Trans: Substituents on opposite faces.

Practical Applications in Chemistry

• Emphasizes understanding of how isomers impact chemical properties and behaviors in reactions.
• Stereochemistry plays critical roles in systems such as pharmaceuticals.
  • For example, Thalidomide: One isomer acts as a sedative, while the other can cause severe birth defects.

Enantiomers and Chirality

• Enantiomers: Non-superimposable mirror images; example using hands to demonstrate non-superimposable nature.
• Importance of chirality in chemistry:
  • Examples include naturally occurring compounds like carvone which delivers different tastes based on spatial arrangements.

Distinguishing Enantiomers from Diastereomers

• Enantiomers: Mirror images that cannot overlap; must have four different substituents on a stereocenter for chirality.
• Diastereomers: Not mirror images, same connectivity but different spatial arrangements. Example - dimethylcyclohexanes exhibit cis and trans forms.

Conclusion

• Recap of critical terms and relationships for understanding cis-trans isomerism and stereochemistry.
• Emphasis on thorough knowledge of stability, conformational analysis, and practical implications in organic chemistry. This understanding sets the foundation for more advanced topics in subsequent chapters, especially in stereochemistry.