$$\frac{(3\ \text{bonding electrons} - 1\ \text{antibonding electron})}{2} = 1$$

Question 1

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

Accepted Answer

Conjugated dienes have alternating double and single bonds, allowing for electron delocalization and increased resonance stabilization.

Question 2

What is the bond order of the central C–C bond in 1,3-butadiene, given that it is a conjugated diene?

Accepted Answer

$$\frac{(3\ 	ext{bonding electrons} - 1\ 	ext{antibonding electron})}{2} = 1$$

Question 3

Which of the following is a correct rule for drawing resonance structures?

Accepted Answer

Electrons move, but atoms do not move.

Question 4

Why is the allylic carbocation more stable than a typical primary carbocation?

Accepted Answer

The allylic carbocation is stabilized by resonance, allowing the positive charge to be delocalized over multiple atoms.

Question 5

Which of the following best describes a cumulated diene?

Accepted Answer

A molecule with two double bonds on the same carbon atom.

Question 6

Which factor decreases the stability of a resonance structure?

Accepted Answer

Charge separation within the molecule.

Question 7

A compound absorbs light at 480 nm. According to the color wheel, what color will the compound appear to the human eye?

Accepted Answer

Orange

Question 8

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

Accepted Answer

The reaction forms a six-membered ring and is energetically favorable.

Question 9

In a Diels-Alder reaction, which product is generally favored when the dienophile contains electron-withdrawing groups and the diene is cyclic?

Accepted Answer

The endo product

Question 10

Which of the following best explains why cyclopentadiene dimerizes at room temperature?

Accepted Answer

Cyclopentadiene is always in the s-cis conformation, making it highly reactive in Diels-Alder reactions.

Question 11

Which of the following statements about UV-Vis absorption in conjugated systems is correct?

Accepted Answer

Increasing conjugation shifts the absorption maximum ($$\lambda_{max}) to $$longer wavelengths.

Question 12

A reaction of 1,3-butadiene with HBr at low temperature gives primarily which product?

Accepted Answer

The 1,2-addition product (kinetic product)

Question 13

Which of the following is NOT a valid resonance structure for the given molecule?

Accepted Answer

A structure in which a carbon atom has 10 electrons in its valence shell.

Question 14

Which of the following best describes the effect of electron-withdrawing groups (EWGs) on the dienophile in a Diels-Alder reaction?

Accepted Answer

EWGs make the dienophile more electrophilic, increasing the reaction rate.

Question 15

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

Accepted Answer

The stereochemistry of both the diene and the dienophile is preserved in the product.

Question 16

Which of the following best explains why the 1,4-addition product is favored at higher temperatures in the addition of HBr to 1,3-butadiene?

Accepted Answer

The 1,4-addition product is more stable (thermodynamic product) and is favored under reversible, high-temperature conditions.

Question 17

Which of the following statements about bond lengths in conjugated systems is correct?

Accepted Answer

Conjugation shortens the single bond between double bonds compared to a typical alkane single bond.

Question 18

Which of the following is a requirement for a group of atoms to participate in resonance?

Accepted Answer

The atoms must be coplanar to allow for effective p orbital overlap.

Question 19

Which of the following best describes the effect of increasing the number of resonance structures on the stability of a molecule?

Accepted Answer

Increasing the number of resonance structures increases the stability of the molecule.

Conjugated Unsaturated Systems: Structure, Stability, and Reactions

Study Guide - Practice Questions