The Final Chapter.

Question 1

Are conjugated or isolated bonds stronger and why?

Answer 1

Conjugated bonds- b/c they have resonance stabilization.

Question 2

Is a cumulated diene or an terminal alkyne stronger?

Answer 2

terminal alkyne - the cumulated diene has a lot of reactive pi bonding in once place.

Question 3

What is the expected product in a reaction of a conjugated diene and HBr.

Answer 3

two products - 1,2 bromoalkene, and 1,4 bromoalkene.

Question 4

Under normal conditions (40C) for addition to conjugated dienes, what is the predominant product?

Answer 4

the 1,4 - addition, internal alkene is more stable.

Question 5

Under cold conditions (-80C) for addition to conjugated dienes, what is the predominant product?

Answer 5

the 1,2 - addition, b/c the internal carbocation is more stable (forms easier.)

Question 6

In which reaction is NBS used as a source of Br2?

Answer 6

Free-radical allylic bromination. (reacts with HBr by-product to regenerate molecule of Br2.)

Question 7

Tell me about the REAL reason why 1,3 - butadiene is more stable than a similar compound with isolated double bonds.

Answer 7

1,3 - butadiene has four pi-bonding electrons that reside in two energy conformations. The lowest energy conformation contains 2 of these electrons, and evenly disperses them within the pi-orbitals of all four sp2 - hybridized carbons. The second pair of electrons reside in a slightly higher energy conformation, which allows them to only hangout between C1-C2 and C3-C4. This is why these carbons have more pi-bonding character. The partial pi-bonding character about the C3-C4 bond lowers the energy of the planar conformation, making it a more stable molecule.