3. Stability and energy levels

Q: What does "stability" mean in terms of energy?

A: Stability refers to the relative energy of a compound. A more stable compound has lower energy than a less stable compound.

Q: Why are trans-alkenes generally more stable than cis-alkenes?

A: Trans-alkenes are more stable because they release less energy during hydrogenation compared to cis-alkenes, indicating they have lower initial energy.

Q: How can we measure the stability difference between cis- and trans-alkenes?

A: By hydrogenating both and measuring the energy released. For example, cis-butene releases about 2 kJ mol⁻¹ more energy than trans-butene, showing it is less stable.

Q: What is an energy profile diagram?

A: An energy profile diagram illustrates the relative energy levels of molecules and the energy changes during a chemical reaction.

Q: Why is C–N bond rotation in an amide slow?

A: Because delocalization of the nitrogen lone pair gives the C–N bond partial double-bond character, which resists rotation.

Q: What happens to energy during amide C–N bond rotation?

A: Energy increases as the C–N bond rotates, reaching a maximum when conjugation is lost (90° rotation) and decreases as conjugation is regained.

Q: How does the size of R groups in amides affect stability?

A: Larger R groups make the cis arrangement less stable than the trans, resulting in a large equilibrium constant KKK favoring the trans form.

Q: What happens when both substituents on the nitrogen are hydrogen?

A: The cis and trans forms of the amide have equal energies, leading to an equilibrium constant K=1K = 1K=1.

Q: What does an equilibrium constant KKK indicate in amide rotation?

A: It represents the ratio of trans to cis forms, reflecting the relative stability of the two arrangements.

Q: How does amide rotation exemplify an equilibrium reaction?

A: The interconversion between cis and trans amide forms represents a reversible process, depicted as changes along a reaction coordinate in an energy profile.