3. mm

Stability and Energy Levels

• Definition of Stability

  • Stability = Lower Energy

  • More stable compounds release less energy in reactions.

• Example: Cis- vs. Trans-Alkenes

  • Trans-alkenes are more stable than cis-alkenes.

  • Evidence:

    • Hydrogenation of trans-butene releases less energy.

    • Cis-butene releases ~2 kJ mol⁻¹ more energy.

  • Energy Profile Diagram:

    • Red lines = Energy levels of cis and trans forms.

    • Black arrows = Energy released during hydrogenation.

• C–N Bond Rotation in Amides

  • Why Rotation is Slow:

    • Nitrogen lone pair delocalizes with the C=O bond.

    • Partial double-bond character in the C–N bond resists rotation.

  • Energy Changes During Rotation:

    • Energy increases as the C–N bond rotates.

    • Maximum energy at 90° rotation (no conjugation).

    • Energy decreases as conjugation is regained.

  • Energy Profile Diagram:

    • Red lines = Energy of cis and trans forms.

    • Curved black line = Energy changes during rotation.

• Role of R Groups in Amides

  • Larger R Groups:

    • Cis form is much less stable than trans form.

    • Large equilibrium constant (KKK) favoring the trans form.

  • Small R Groups (e.g., H):

    • Cis and trans forms have equal energies.

    • Equilibrium constant K=1K = 1K=1.

• General Concept of Equilibrium Reactions

  • Amide rotation is an example of equilibrium.

    • Interconversion between cis and trans forms.

    • Reaction coordinate = Path of energy changes.

• Key Takeaways

  • Stability is about energy differences.

  • Energy profile diagrams visualize energy changes.

  • Equilibrium reflects the relative stabilities of interconverting forms.