Question 1

Which of the following statements best explains why rearrangement occurs during the acid-catalyzed hydration of but-3-en-2-ylbenzene (reaction A) but not during oxymercuration-reduction (reaction B)?

Accepted Answer

Acid-catalyzed hydration proceeds via a carbocation intermediate, which can rearrange to form a more stable carbocation, while oxymercuration-reduction proceeds via a mercurinium ion intermediate that does not rearrange.

Question 2

In the oxymercuration-reduction of an alkene, which of the following best describes the role of NaBH$$_4 in $$the reaction?

Accepted Answer

NaBH$$_4$$ reduces the organomercury intermediate to form the alcohol, replacing the mercury with hydrogen.

Question 3

Which of the following is the major product when but-3-en-2-ylbenzene undergoes acid-catalyzed hydration?

Accepted Answer

The alcohol forms at the more substituted carbon after carbocation rearrangement, following Markovnikov's rule.

Question 4

Why does the hydroboration-oxidation of an alkene (e.g., reaction C) result in anti-Markovnikov addition of water?

Accepted Answer

The boron atom adds to the less substituted carbon, and subsequent oxidation replaces boron with OH, resulting in anti-Markovnikov alcohol formation.

Question 5

In the reaction of (E)-stilbene with N-bromosuccinimide (NBS) in DMSO/H$$_2O$$, what is the relationship between the addition of Br and OH in the observed halohydrin products?

Accepted Answer

Only anti addition occurs.

Question 6

Which intermediate is formed during the reaction of an alkene with Br$$_2 in a $$polar protic solvent such as methanol (HOCH$$_3$$)?

Accepted Answer

A bromonium ion intermediate.

Question 7

How does the solvent methanol (HOCH$$_3) $$stabilize the bromonium ion intermediate during halogen addition to an alkene?

Accepted Answer

Methanol acts as a nucleophile, attacking the more substituted carbon of the bromonium ion and stabilizing the positive charge.

Question 8

When the solvent is changed from methanol (HOCH$$_3) to $$carbon tetrachloride (CCl$$_4) in $$the bromination of an alkene, what effect does this have on the stabilization of the bromonium ion intermediate?

Accepted Answer

CCl$$_4$$ does not significantly stabilize the bromonium ion, leading to a less nucleophilic environment.

Question 9

Which of the following best describes the transition state for the addition of Br$$_2 to an $$alkene in CCl$$_4$$?

Accepted Answer

The transition state involves a cyclic bromonium ion intermediate.

Question 10

In the chair conformations of cyclohexane derivatives, which factor determines the equilibrium position between two isomers (X and Y) with substituents at different positions?

Accepted Answer

The isomer with more substituents in the equatorial position is favored due to reduced steric strain.

Question 11

Which of the following statements is true regarding the stereochemical outcome of catalytic hydrogenation of an alkene using Pd/C in pyridine?

Accepted Answer

Hydrogenation occurs with syn addition, resulting in both hydrogens adding to the same face of the alkene.

Question 12

How can you tell, based solely on the products, that a reaction does not proceed via a carbocation intermediate?

Accepted Answer

The products retain the original carbon skeleton without rearrangement, indicating no carbocation intermediate.

Question 13

In the halohydrin formation from (E)-stilbene and NBS in DMSO/H$$_2O$$, four stereoisomers are observed. What does this indicate about the mechanism?

Accepted Answer

The reaction allows for attack from both faces of the intermediate, leading to multiple stereoisomers.

Question 14

Which of the following best explains why the relative amounts of halohydrin products formed from (E)-stilbene and (Z)-stilbene differ?

Accepted Answer

The stereochemistry of the starting alkene affects the accessibility of the intermediate to nucleophilic attack, leading to different product ratios.

Question 15

During the oxymercuration of an alkene, which atom is attacked by water in the key step of the mechanism?

Accepted Answer

The more substituted carbon of the mercurinium ion, due to greater carbocation character.

Question 16

Which of the following is a reason why mercury acetate (Hg(OAc)$$_2) is $$considered toxic and less desirable for use in organic synthesis?

Accepted Answer

It is a heavy metal compound that can cause environmental and health hazards.

Question 17

In the hydroboration-oxidation of an alkene, which reagent is responsible for the oxidation step that converts the organoborane intermediate to an alcohol?

Accepted Answer

H$$_2O$_2 / OH$^-$$

Question 18

Which of the following best describes the regioselectivity of nucleophilic attack in the bromonium ion intermediate formed during the addition of Br$$_2 to an $$alkene?

Accepted Answer

Nucleophilic attack occurs at the less substituted carbon due to less steric hindrance and greater positive character.

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