$$\text{allyl} \u003e 3^\circ \u003e 2^\circ \u003e 1^\circ \u003e \text{vinyl}$$

Question 1

Which of the following statements best describes a conjugated system in organic molecules?

Accepted Answer

A system where two or more p orbitals on adjacent atoms overlap, allowing electrons to delocalize across the system.

Question 2

What is the primary reason for the increased stability of conjugated dienes compared to isolated dienes?

Accepted Answer

Delocalization of $$\pi$$ electrons over multiple adjacent p orbitals lowers the overall energy of the molecule.

Question 3

Which of the following correctly ranks the stability of carbon radicals from most to least stable?

Accepted Answer

$$	ext{allyl} \u003e 3^\circ \u003e 2^\circ \u003e 1^\circ \u003e 	ext{vinyl}$$

Question 4

In the free radical chlorination of propene at high temperature, which product is favored and why?

Accepted Answer

Allylic chlorination is favored because the allylic radical intermediate is stabilized by resonance.

Question 5

Which of the following best explains why the central C–C bond in 1,3-butadiene is shorter than a typical single bond?

Accepted Answer

There is partial double bond character due to delocalization of $$\pi$$ electrons across the four carbon atoms.

Question 6

Which of the following is a requirement for a diene to participate in a Diels-Alder reaction?

Accepted Answer

The diene must be in the s-cis conformation.

Question 7

A reaction between 1,3-butadiene and HBr at low temperature yields mostly which product, and why?

Accepted Answer

The 1,2-addition product, because the reaction is under kinetic control and forms faster.

Question 8

Which of the following statements about resonance structures is FALSE?

Accepted Answer

Atoms may be moved between resonance structures.

Question 9

Which of the following best describes the product of a Diels-Alder reaction between 1,3-cyclopentadiene and ethylene?

Accepted Answer

A bridged bicyclic ring system is formed.

Question 10

Why does N-bromosuccinimide (NBS) favor allylic bromination over addition to the double bond in alkenes?

Accepted Answer

NBS provides a low, steady concentration of Br$$_2$$, which favors radical substitution at the allylic position.

Question 11

Which of the following is NOT a correct statement about the Diels-Alder reaction?

Accepted Answer

The diene must be in the s-trans conformation to react.

Question 12

Which of the following best explains why the allyl carbocation is more stable than a typical primary carbocation?

Accepted Answer

The positive charge is delocalized over two carbons via resonance.

Question 13

Which of the following is a cumulated diene?

Accepted Answer

$$\ce{CH2=C=CH2} ($$allene)

Question 14

Which of the following best describes the difference between kinetic and thermodynamic control in the addition of HBr to 1,3-butadiene?

Accepted Answer

Kinetic control favors the product that forms fastest (1,2-addition), while thermodynamic control favors the more stable product (1,4-addition).

Question 15

Which of the following is a correct statement about the resonance hybrid of the allyl radical?

Accepted Answer

The unpaired electron is delocalized over the two terminal carbons.

Question 16

Which of the following best explains why conjugated dienes have a lower heat of hydrogenation than isolated dienes?

Accepted Answer

Conjugation stabilizes the molecule, so less energy is released upon hydrogenation.

Question 17

Which of the following is true regarding the stereochemistry of the Diels-Alder reaction?

Accepted Answer

The stereochemistry of the dienophile is retained in the product.

Question 18

Which of the following is a correct statement about the endo rule in the Diels-Alder reaction?

Accepted Answer

The endo product is favored when the dienophile has electron-withdrawing substituents oriented towards the diene's $$\pi$$ system.

Conjugated Unsaturated Systems: Conjugation, Resonance, and Allylic Substitution

Study Guide - Practice Questions