Question 1

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

Accepted Answer

The alkene acts as a nucleophile, the electrophile attacks the alkene, and two new $$\sigma$$ bonds are formed as the $$\pi$$ bond breaks.

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 there.

Question 3

Which of the following carbocations is the most stable?

Accepted Answer

Tertiary carbocation ($$R_3C^+)$$

Question 4

What is the Hammond Postulate and how does it apply to electrophilic addition reactions?

Accepted Answer

The transition state of a reaction resembles the species (reactants or products) to which it is closer in energy; in electrophilic addition, the transition state resembles the carbocation intermediate.

Question 5

Which rule determines the major product in the addition of a hydrogen halide to an unsymmetrical alkene?

Accepted Answer

Markovnikov's rule: the electrophile adds to the less substituted sp$$^2$$ carbon.

Question 6

Why does the acid-catalyzed addition of water to an alkene sometimes produce unexpected products?

Accepted Answer

Carbocation rearrangements, such as 1,2-hydride or 1,2-methyl shifts, can occur, leading to more stable carbocation intermediates and unexpected products.

Question 7

Which of the following reactions does NOT involve the formation of a carbocation intermediate?

Accepted Answer

Hydroboration-oxidation of an alkene

Question 8

In the hydroboration-oxidation of an alkene, where does the OH group end up?

Accepted Answer

On the less substituted sp$$^2$$ carbon, replacing the boron atom

Question 9

Which statement best describes the stereochemistry of the addition of Br$$_2 to a $$trans-alkene?

Accepted Answer

Anti addition occurs, resulting in enantiomers with substituents pointing in opposite directions.

Question 10

Which of the following is a consequence of the formation of a chiral center in the product of an electrophilic addition to an achiral alkene?

Accepted Answer

A racemic mixture of enantiomers is formed.

Question 11

Which reaction is used in biological systems to hydrate alkenes, and what is its stereochemical outcome?

Accepted Answer

Enzyme-catalyzed hydration, which is completely stereoselective and forms only one stereoisomer.

Question 12

What is the product of ozonolysis of an alkene?

Accepted Answer

Two carbonyl compounds (ketones and/or aldehydes) where the double bond was located.

Question 13

Which of the following statements about carbocation rearrangement is TRUE?

Accepted Answer

A carbocation will rearrange only if the rearrangement leads to a more stable carbocation.

Question 14

Which reaction mechanism involves the formation of a three-membered ring intermediate?

Accepted Answer

Addition of Br$$_2 or Cl$_2 to an $$alkene

Question 15

What is the stereochemical outcome of the hydroboration-oxidation reaction of a cis-alkene?

Accepted Answer

Syn addition occurs, with H and OH adding to the same side of the double bond.

Question 16

Which of the following is NOT a product that can be synthesized from an alkene?

Accepted Answer

A vicinal diiodide (by direct addition of I$$_2)$$

Question 17

Which statement best describes the difference between regioselective and stereoselective reactions?

Accepted Answer

Regioselective reactions favor the formation of one constitutional isomer, while stereoselective reactions favor the formation of one stereoisomer.

Question 18

Which of the following statements about the addition of a carbene to an alkene is correct?

Accepted Answer

Syn addition occurs, forming a cyclopropane ring.

Question 19

Why is hydroboration-oxidation preferred over acid-catalyzed hydration for converting alkenes to alcohols in some cases?

Accepted Answer

Hydroboration-oxidation avoids carbocation rearrangements and harsh acidic conditions.

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

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