Question 1

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

Accepted Answer

Conjugated dienes allow for delocalization of pi electrons across adjacent p orbitals, lowering the overall energy.

Question 2

According to molecular orbital (MO) theory, how many molecular orbitals are formed when four atomic p orbitals combine in a conjugated system such as 1,3-butadiene?

Accepted Answer

$4$

Question 3

Which of the following best describes the difference between a bonding and an antibonding molecular orbital in a conjugated pi system?

Accepted Answer

Bonding orbitals have electron density between nuclei with no nodes, while antibonding orbitals have nodes and electron density outside the nuclei.

Question 4

Which of the following compounds would absorb light at the longest wavelength in UV/vis spectroscopy?

Accepted Answer

1,3,5-Hexatriene ($\mathrm{CH_2=CH-CH=CH-CH=CH_2}$)

Question 5

Which statement about the heat of hydrogenation of conjugated dienes is correct?

Accepted Answer

The heat of hydrogenation of conjugated dienes is less than the sum of the heats for two isolated double bonds, indicating extra stability.

Question 6

In the Diels-Alder reaction, which type of diene is required for the reaction to proceed efficiently?

Accepted Answer

A conjugated diene

Question 7

Which of the following best describes the electronic transition responsible for UV absorption in conjugated systems?

Accepted Answer

Promotion of an electron from a $\pi$ orbital to a $\pi^*$ orbital

Question 8

Which of the following is NOT a feature of a conjugated system?

Accepted Answer

Presence of sp3 hybridized carbons between double bonds

Question 9

A compound absorbs UV light at 350 nm. What does this suggest about its electronic structure compared to a compound that absorbs at 220 nm?

Accepted Answer

It has a more extended conjugated pi system, resulting in a smaller energy gap.

Question 10

Which of the following best explains why beta-carotene is colored and absorbs visible light?

Accepted Answer

It contains a large number of conjugated double bonds, lowering the energy gap for electronic transitions.

Question 11

Which of the following is the correct application of Beer's Law in UV/vis spectroscopy?

Accepted Answer

$A = \epsilon c l$

Question 12

Which of the following statements about the molecular orbitals in 1,3-butadiene is correct?

Accepted Answer

There are two bonding and two antibonding pi molecular orbitals.

Question 13

Which of the following best describes an allylic carbocation?

Accepted Answer

A carbocation adjacent to a double bond, allowing for resonance stabilization.

Question 14

Which of the following is a real-world application of conjugated systems in medicine?

Accepted Answer

Amphotericin, an antifungal drug, contains a conjugated system that is crucial for its biological activity.

Question 15

Which of the following is the correct order of increasing energy for molecular orbitals in a conjugated triene system?

Accepted Answer

$\pi_1 < \pi_2 < \pi_3 < \pi_4 < \pi_5 < \pi_6$

Question 16

Which of the following best describes the geometry around the double-bonded carbons in ethylene ($\mathrm{C_2H_4}$)?

Accepted Answer

Trigonal planar geometry with bond angles of approximately $120^\circ$

Question 17

Which of the following is true regarding the conservation of orbitals in MO theory?

Accepted Answer

The number of molecular orbitals formed equals the number of atomic orbitals combined.

Question 18

Which of the following is a correct statement about the effect of conjugation on the HOMO-LUMO gap?

Accepted Answer

Conjugation decreases the HOMO-LUMO gap, allowing absorption of lower energy (longer wavelength) light.

Conjugated Compounds and UV Spectroscopy: Structure, Bonding, and Reactivity

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