February 12th

Energy Diagrams and Torsional Strain

• Energy Differences: When rotating molecules around a carbon-carbon bond, significant changes in energy profiles occur.

  • Example: The difference between two energy states can be highlighted, cited as 12 kilojoules per mole, referred to as torsional strain.

Torsional Strain Characteristics

• Eclipsing Interactions: Torsional strain arises from overlapping atoms (eclipsing interactions), increasing the overall energy of the compound.

  • Previous discussion emphasized this strain during eclipsing positions of molecules.

Molecular Conformations and Newman Projections

• Staggered vs Eclipsed: Observing conformations through Newman Projection views fosters understanding of molecule positions.

  • Staggered Conformation: Lowest energy configuration.

  • Eclipsed Conformation: Higher energy due to increased torsional strain.

• Ethane Example: No difference in energy between staggered and eclipsed, indicating minimal torsional strain.

• Energy Movement: Molecules at room temperature experience movement, overcoming barriers to achieve staggered conformations.

Propane and Carbocation Interaction

• Introduction of Groups: Adding larger groups (e.g., CH₃) instead of H atoms can noticeably affect energy states.

  • Propane (C₃H₈) shows increased torsional strain due to interactions between methyl groups and H atoms.

  • Propane’s torsional strain is measured at 14 kilojoules per mole, unveiling strain due to group size.

Butane's Complexity and Interaction Analysis

• Four Carbon Chain (Butane): Alters the torsion dynamics significantly.

  • Eclipsing Conformation with two methyl groups shows an additional energy increase:

    • 16 kilojoules per mole higher than at the lowest energy state.

  • Staggered methyl groups can lead to a further slight rise in energy:

    • Interaction of two bulky groups adds strain.

Gauche and Anti Conformations

• Types of Staggered Conformations:

  • Anti Conformation: Non-H atoms are 180° apart, generally more stable due to minimal interactions.

  • Gauche Conformation: Non-H atoms are 60° apart, slightly less stable and entails more strain.

  • Directionality terminology allows for clarity in molecular characterization.

Visual Analysis through Newman Projections

• Determining Relationships: Essential to analyze relationships in molecular structures (e.g., Cl and Br) through an accurate Newman Projection.

  • Important to consider visualization without solely depending on the structural formula.

Additional Practice Problems

• Inter-relational Properties: Focusing on angular relationships leads to improved understanding of steric interactions.

  • Students encouraged to draw projections to engage effectively in identifying gauche and anti relationships.

Cycloalkanes and Stability

• Stability Ranking: Cycloalkanes show variable stability, with cyclohexane being the most stable due to optimal bond angles (around 109.5°) and absence of eclipsed interactions.

  • Higher membered rings have better flexibility and stability:

  • Each carbons' hybridization contributes to their steric number and confirms their stability patterns.

Summary of Ring Strain Characteristics

• Cyclic Compounds Analysis: Variations in ring strain due to effective bond angles and torsional forces create a measurable energy profile.

  • Cyclopropane exhibits extreme strain due to its three-carbon configuration, whereas cyclohexane maintains a low energy state without eclipsed interactions.

• Conceptual Importance: Recognizing the significance of six-membered rings in the context of organic chemistry – their properties dictate relationships and subsequent reactions.

Conclusion

• Keep in mind the overarching ideas of increasing strain due to larger substituents and the depiction of molecular configurations using Newman projections to predict energy states.

• Focus on six-membered rings as a key area of study within molecular stability and interactions.