Question 1

Which statement best describes the general mechanism of an electrophilic addition reaction to an alkene?

Accepted Answer

The $$\pi$$ bond of the alkene acts as a nucleophile and attacks an electrophile, forming a carbocation intermediate, which is then attacked by a nucleophile to form two new $$\sigma$$ bonds.

Question 2

In the addition of HBr to 2-methylpropene, which carbon does the Br atom attach to, and why?

Accepted Answer

Br attaches to the more substituted carbon because the more stable carbocation intermediate forms at that position.

Question 3

Which of the following carbocations is the most stable?

Accepted Answer

Tertiary carbocation ($$R_3C^+)$$

Question 4

What is the major product when 2-methyl-2-butene reacts with HCl?

Accepted Answer

2-chloro-2-methylbutane

Question 5

Which principle explains why the transition state of an exergonic electrophilic addition reaction resembles the reactants more than the products?

Accepted Answer

Hammond Postulate

Question 6

Why does hydroboration-oxidation of an alkene result in the OH group attaching to the less substituted carbon?

Accepted Answer

The boron atom adds to the less substituted carbon during hydroboration, and is replaced by OH during oxidation.

Question 7

Which of the following reactions does NOT involve carbocation rearrangement?

Accepted Answer

Hydroboration-oxidation

Question 8

What is the stereochemical outcome of the addition of Br$$_2 to a $$trans-2-butene?

Accepted Answer

Anti addition, forming enantiomers with substituents on opposite sides of the ring

Question 9

A student performs the acid-catalyzed hydration of 3,3-dimethyl-1-butene and observes an unexpected major product. What is the most likely explanation?

Accepted Answer

A carbocation rearrangement (1,2-hydride or methyl shift) occurred, leading to a more stable carbocation and a different product.

Question 10

Which reagent is commonly used for the epoxidation of alkenes?

Accepted Answer

m-CPBA (meta-chloroperoxybenzoic acid)

Question 11

What is the product when cyclohexene undergoes ozonolysis followed by reductive workup?

Accepted Answer

Adipaldehyde (hexane-1,6-dial)

Question 12

Which of the following best describes a regioselective reaction?

Accepted Answer

A reaction that forms more of one constitutional isomer than another.

Question 13

During the acid-catalyzed addition of water to an alkene, which step is rate-limiting?

Accepted Answer

Formation of the carbocation intermediate

Question 14

Which of the following is true about the addition of Br$$_2 to an $$alkene?

Accepted Answer

It proceeds via a cyclic bromonium ion intermediate and results in anti addition.

Question 15

Which reaction is used in biological systems to hydrate fumarate to malate?

Accepted Answer

Enzyme-catalyzed hydration

Question 16

What is the stereochemical outcome when a chiral center is formed in the product of an addition reaction to an achiral alkene?

Accepted Answer

A racemic mixture of enantiomers is formed.

Question 17

Which of the following is a syn addition reaction?

Accepted Answer

Hydroboration-oxidation of alkenes

Question 18

What is the major product when 1-methylcyclohexene reacts with BH$$_3/$$THF followed by H$$_2O$_2/$$NaOH?

Accepted Answer

Trans-2-methylcyclohexanol

Question 19

Which statement is true about enzyme-catalyzed addition reactions in biological systems?

Accepted Answer

They are completely stereoselective and often stereospecific, forming only one stereoisomer.

Chapter 6: The Reactions of Alkenes – Mechanisms and Stereochemistry of Addition Reactions

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