Question 1

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

Accepted Answer

The central C–C bond is part of a conjugated system, so the adjacent p orbitals overlap, giving it partial double bond character.

Question 2

What is the hybridization of each carbon atom in 1,3-butadiene?

Accepted Answer

All carbons are sp2 hybridized.

Question 3

Which of the following best describes the electron density in the π bonds of a conjugated diene such as 1,3-butadiene?

Accepted Answer

The electron density is delocalized over all four carbons.

Question 4

Which of the following is a requirement for resonance stabilization in an allylic carbocation?

Accepted Answer

There must be three adjacent sp2-hybridized carbons with overlapping p orbitals.

Question 5

Rank the following carbocations in order of increasing stability: methyl, primary, secondary, allylic, tertiary.

Accepted Answer

Methyl \u003c primary \u003c secondary = allylic \u003c tertiary

Question 6

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

Accepted Answer

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

Question 7

When 1,3-butadiene reacts with HBr at low temperature, which product is favored and why?

Accepted Answer

The 1,2-product is favored because it forms faster due to the proximity of Br- to the carbocation.

Question 8

Which product is the thermodynamic product in the electrophilic addition of HBr to 1,3-butadiene, and why?

Accepted Answer

The 1,4-product, because it is more substituted and thus more stable.

Question 9

Which conformation of a 1,3-diene is required for the Diels-Alder reaction to occur?

Accepted Answer

s-cis conformation

Question 10

Which of the following dienophiles would react fastest in a Diels-Alder reaction?

Accepted Answer

$$\mathrm{CH_2$=CHCO_2CH_3}$$

Question 11

In the Diels-Alder reaction, what is the major product when a cyclic diene reacts with a dienophile containing an electron-withdrawing group?

Accepted Answer

The endo product, because the electron-withdrawing group prefers to be under the ring and closer to the electron-rich diene.

Question 12

Which of the following best describes the mechanism of the Diels-Alder reaction?

Accepted Answer

A concerted mechanism involving three arrows and no intermediates.

Question 13

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

Accepted Answer

The reaction always forms a six-membered ring.

Question 14

Which of the following is NOT a requirement for a molecule to participate in resonance?

Accepted Answer

The molecule must have all atoms sp3-hybridized.

Question 15

Which of the following best explains why the 1,2-product is the kinetic product in the addition of HBr to 1,3-butadiene?

Accepted Answer

It forms faster because Br- is closer to the carbocation intermediate.

Question 16

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

Accepted Answer

They increase the reactivity of the dienophile by making it more electrophilic.

Question 17

Which of the following is true about the resonance hybrid of the allylic carbocation?

Accepted Answer

The positive charge is delocalized over two carbons.

Question 18

Which of the following best explains why the Diels-Alder reaction is considered a 'concerted' reaction?

Accepted Answer

All bond-making and bond-breaking events occur simultaneously in a single step.

Ch. 16 Part 2

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