$$1.46\ \text{Å}$$, because conjugation shortens the single bond between the double bonds.

Question 1

Which of the following statements best explains why conjugated dienes are more stable than non-conjugated dienes?

Accepted Answer

Conjugated dienes have delocalized π electrons, which lowers their overall energy.

Question 2

Which of the following compounds is aromatic according to Hückel’s rule?

Accepted Answer

Benzene

Question 3

What is the expected bond length (in Å) for the central C–C bond in 1,3-butadiene, and why?

Accepted Answer

$$1.46\ 	ext{Å}$$, because conjugation shortens the single bond between the double bonds.

Question 4

Which of the following best describes the molecular orbital (MO) picture of 1,3-butadiene?

Accepted Answer

The π electrons are delocalized over all four carbons, lowering the energy of the molecule.

Question 5

A compound absorbs UV light at $$\lambda_{max} = 268\ 	ext{nm}. $$What can you infer about its structure?

Accepted Answer

It likely contains a conjugated system with three double bonds.

Question 6

Which of the following statements about the Diels–Alder reaction is correct?

Accepted Answer

The reaction is a concerted cycloaddition that forms a six-membered ring.

Question 7

Which product is favored under kinetic control in the addition of HBr to 1,3-butadiene at low temperature?

Accepted Answer

The 1,2-addition product

Question 8

Which of the following best explains why benzene is more stable than expected for a molecule with three double bonds?

Accepted Answer

Benzene is stabilized by resonance energy due to delocalized π electrons.

Question 9

Which of the following is a requirement for a compound to be aromatic?

Accepted Answer

It must be a planar, cyclic molecule with a continuous ring of p orbitals and $$4n+2$$ π electrons.

Question 10

Which of the following best describes the effect of increasing conjugation on the UV-Vis absorption of a molecule?

Accepted Answer

Increasing conjugation decreases the energy gap ($$\Delta E) $$between HOMO and LUMO, resulting in a higher $$\lambda_{max}.$$

Question 11

Which of the following dienes is most reactive in a Diels–Alder reaction?

Accepted Answer

A diene locked in the s-cis conformation

Question 12

Which of the following is a correct statement about the stereochemistry of the Diels–Alder reaction?

Accepted Answer

The reaction is stereospecific: cis dienophiles give cis products, and trans dienophiles give trans products.

Question 13

Which of the following best describes the endo rule in the Diels–Alder reaction?

Accepted Answer

The endo product is generally formed faster due to secondary orbital interactions.

Question 14

Which of the following statements about resonance structures is correct?

Accepted Answer

The most important resonance structure is the one with the most complete octets and minimal charge separation.

Question 15

Which of the following best explains why cyclooctatetraene (COT) is not aromatic?

Accepted Answer

COT is not planar and does not have a continuous ring of p orbitals.

Question 16

Which of the following is a real-world application of the Diels–Alder reaction?

Accepted Answer

Synthesis of six-membered rings in pharmaceuticals

Question 17

Which of the following statements about the addition of hydrogen halides to conjugated dienes is correct?

Accepted Answer

The 1,4-addition product is favored at higher temperatures due to its greater stability.

Question 18

Which of the following best describes the effect of vulcanization on natural rubber?

Accepted Answer

It introduces disulfide crosslinks, increasing strength and rigidity.

Question 19

Which of the following noncovalent interactions is important for the stacking of aromatic rings in biological systems?

Accepted Answer

Offset stacking (π–π interactions)

Chapter 15: Dienes and Aromaticity – Structure, Stability, and Reactions

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