Question 1

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

Accepted Answer

Conjugated dienes have delocalized electrons that provide resonance stabilization.

Question 2

The heat of hydrogenation for trans-penta-1,3-diene is measured to be $$-225\ \mathrm{kJ}$$, while the predicted value based on isolated double bonds is $$-240\ \mathrm{kJ}. $$What does this difference indicate?

Accepted Answer

The compound is more stable due to resonance stabilization.

Question 3

Which molecular orbital in buta-1,3-diene is the lowest in energy and has all bonding interactions?

Accepted Answer

$$\pi_1$

Question 4

Why is the C2–C3 single bond in buta-1,3-diene shorter than a typical C–C single bond?

Accepted Answer

It has partial double bond character due to electron delocalization.

Question 5

Which of the following best describes the transition state of an $$S_N2$$ reaction involving an allylic halide?

Accepted Answer

It is stabilized by conjugation with the pi system.

Question 6

In the addition of HBr to buta-1,3-diene at $$-80\ ^\circ\mathrm{C}$$, which product predominates and why?

Accepted Answer

The 1,2-addition product, because it forms faster (kinetic control).

Question 7

Which of the following is true about the Diels-Alder reaction?

Accepted Answer

It is a concerted [4+2] cycloaddition forming a cyclohexene ring.

Question 8

What is the stereochemical requirement for a diene to participate in a Diels-Alder reaction?

Accepted Answer

The diene must be in the s-cis conformation.

Question 9

Which statement best explains why N-bromosuccinimide (NBS) is used for allylic bromination?

Accepted Answer

NBS provides a low, constant concentration of Br$$_2$$ and prevents HBr addition across the double bond.

Question 10

Which of the following correctly ranks the stability of carbocations?

Accepted Answer

1° \u003c 2° \u003c 3° \u003c substituted allylic

Question 11

Which molecular orbital in the allyl radical is singly occupied?

Accepted Answer

$$\pi_2$

Question 12

What is the effect of increasing conjugation on the wavelength of maximum absorption ($$\lambda_{max}) in UV-$$Vis spectroscopy?

Accepted Answer

$$\lambda_{max}$$ increases as conjugation increases.

Question 13

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

Accepted Answer

The product with the electron-withdrawing group syn to the diene is favored due to secondary orbital overlap.

Question 14

Which of the following is a characteristic of a thermodynamically controlled reaction?

Accepted Answer

The most stable product predominates after equilibrium is established.

Question 15

Which of the following best explains why cyclopentadiene undergoes the Diels-Alder reaction readily?

Accepted Answer

It is locked in the s-cis conformation.

Question 16

What is the main reason that the [2+2] cycloaddition of two ethylenes is thermally forbidden?

Accepted Answer

The symmetry of the molecular orbitals leads to antibonding overlap in the transition state.

Question 17

Which of the following best describes the effect of resonance on the stability of allylic radicals?

Accepted Answer

Resonance delocalizes the radical electron, increasing stability.

Question 18

Which of the following is true about the UV-Vis absorbance of a compound with a long chain of conjugated double bonds?

Accepted Answer

It absorbs at a longer wavelength than a compound with fewer conjugated bonds.

Question 19

Which of the following best describes the role of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in the Diels-Alder reaction?

Accepted Answer

The diene donates electrons from its HOMO to the dienophile's LUMO.

Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy (Chapter 15 Study Notes)

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