Q: In the reaction of ethanol with $$n-$$butyllithium, which product is formed and why?

$$\text{CH}_3\text{CH}_2\text{O}^- + \text{C}_4\text{H}_{10}$$, because $$n-$$butyllithium deprotonates ethanol.

Question 1

Which of the following best explains why the $$	heta_{1-	ext{C}\equiv	ext{C}-\alpha-	ext{C}}$$ dihedral angle in conformer A of 1-cyano-3-butyne is 65.2° instead of the expected 60° as in ethane?

Accepted Answer

The triple bond in the alkyne increases electron repulsion, resulting in a larger dihedral angle.

Question 2

In the reaction of cyclohexanol with $$	ext{H}_2	ext{SO}_4 ($$dilute), which intermediate is most likely formed and why?

Accepted Answer

A tertiary carbocation, stabilized by both hyperconjugation and possible resonance.

Question 3

Which of the following statements about the resonance stabilization of the benzylic carbocation is correct?

Accepted Answer

Resonance stabilization occurs because the positive charge is delocalized over the aromatic ring.

Question 4

Given the following acid-base reaction: $$	ext{C}_6	ext{H}_5	ext{OH} + 	ext{OH}^- ightleftharpoons 	ext{C}_6	ext{H}_5	ext{O}^- + 	ext{H}_2	ext{O}$$, which product is favored at equilibrium and why?

Accepted Answer

Phenoxide ion, because the negative charge is stabilized by resonance in the aromatic ring.

Question 5

Which of the following best describes the role of $$\pi$$ conjugation in stabilizing carbocation intermediates during electrophilic addition to alkenes?

Accepted Answer

$$\pi$$ conjugation allows delocalization of the positive charge, lowering the energy of the intermediate.

Question 6

In the addition of HBr to 1,3-butadiene, which product is favored and why?

Accepted Answer

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

Question 7

Which of the following is the correct electron-pushing mechanism for the reaction of cyclohexanol with $$	ext{H}_2	ext{SO}_4 ($$dilute)?

Accepted Answer

Protonation of the hydroxyl group, followed by loss of water to form a carbocation, then nucleophilic attack by water.

Question 8

Which of the following statements about the conformational potential energy surface (PES) of 1-cyano-3-butyne is correct?

Accepted Answer

The lowest energy conformer has the CN and CCH groups anti to each other.

Question 9

In the reaction of ethanol with $$n-$$butyllithium, which product is formed and why?

Accepted Answer

$$	ext{CH}_3	ext{CH}_2	ext{O}^- + 	ext{C}_4	ext{H}_{10}$$, because $$n-$$butyllithium deprotonates ethanol.

Question 10

Which of the following best explains the regioselectivity observed in the addition of HBr to an alkene?

Accepted Answer

The more stable carbocation intermediate forms, leading to Markovnikov addition.

Question 11

Which of the following is true regarding the stability of the two carbocation intermediates shown in the reaction with $$	ext{H}_3	ext{C}-	ext{Cl}$$?

Accepted Answer

The intermediate with the lower energy is more stabilized by hyperconjugation.

Question 12

In the acid-catalyzed dehydration of cyclohexanol, which step is rate-determining?

Accepted Answer

Loss of water to form the carbocation intermediate.

Question 13

Which of the following best describes the effect of resonance on the acidity of phenol?

Accepted Answer

Resonance stabilizes the phenoxide ion, making phenol more acidic than cyclohexanol.

Question 14

Which of the following is the correct order of $$pK_a$$ values for the following compounds: phenol, cyclohexanol, ethanol?

Accepted Answer

$$pK_a($$phenol) \u003c $$pK_a($$cyclohexanol) \u003c $$pK_a($$ethanol)

Question 15

In the reaction coordinate diagram for the addition of HCl to 2-methyl-2-butene, what does the highest peak represent?

Accepted Answer

The transition state for carbocation formation.

Question 16

Which of the following is a correct statement about the mechanism of electrophilic addition to conjugated dienes?

Accepted Answer

The reaction can yield both 1,2- and 1,4-addition products due to resonance stabilization of the allylic carbocation.

Question 17

Which of the following best explains why the product distribution in the addition of HBr to 1,3-butadiene changes with temperature?

Accepted Answer

At low temperature, the kinetic product dominates; at high temperature, the thermodynamic product dominates.

Question 18

Which of the following is the most stable carbocation intermediate in the acid-catalyzed dehydration of cyclohexanol?

Accepted Answer

A tertiary cyclohexyl carbocation.

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