Question 1

Which of the following statements best explains why tertiary alkyl halides do not undergo $$S_N2$$ reactions efficiently?

Accepted Answer

Steric hindrance around the carbon prevents backside attack by the nucleophile in $$S_N2.$$

Question 2

Given the following rate law for the reaction of methyl bromide with acetate: $$rate = k[	ext{CH}_3	ext{Br}][	ext{OAc}^-]$$, what does this indicate about the reaction mechanism?

Accepted Answer

The reaction is bimolecular and proceeds via a concerted mechanism ($$S_N2).$$

Question 3

Which of the following best describes the stereochemical outcome of an $$S_N2$$ reaction at a stereogenic center?

Accepted Answer

Inversion of configuration

Question 4

A chemist wants to convert a primary alcohol to an alkyl bromide with inversion of configuration. Which reagent and mechanism should be used?

Accepted Answer

PBr$$_3 in $$pyridine, $$S_N2$$ mechanism

Question 5

Which factor most strongly favors an $$S_N1$$ mechanism over $$S_N2$$ for alkyl halide substitution?

Accepted Answer

Highly substituted (tertiary) alkyl halide

Question 6

According to the Hammond postulate, which statement is true for the rate-determining step of an endothermic $$S_N1$$ reaction?

Accepted Answer

The transition state resembles the products, so anything that stabilizes the product also stabilizes the transition state.

Question 7

Which of the following alkyl halides would react fastest in an $$S_N1$$ reaction?

Accepted Answer

Tertiary alkyl bromide

Question 8

Why do polar protic solvents favor $$S_N1$$ reactions?

Accepted Answer

They stabilize the carbocation intermediate and the leaving group via solvation.

Question 9

Which of the following is a correct statement about the effect of nucleophile strength on substitution mechanisms?

Accepted Answer

Strong nucleophiles favor $$S_N2$$ reactions.

Question 10

A chiral secondary alkyl bromide is treated with NaOH in DMSO. What is the expected stereochemical outcome?

Accepted Answer

Inversion of configuration

Question 11

Which of the following best explains why allylic and benzylic halides can undergo both $$S_N1$$ and $$S_N2$$ reactions efficiently?

Accepted Answer

Their carbocations and transition states are stabilized by resonance.

Question 12

Which of the following is the correct rate law for an $$S_N1$$ reaction?

Accepted Answer

$$rate = k[	ext{RX}]$$

Question 13

A student treats 1-bromo-2-phenylethane with methanol under $$S_N1$$ conditions. What is the expected product mixture?

Accepted Answer

A racemic mixture of both enantiomers

Question 14

Which of the following statements about leaving groups is correct?

Accepted Answer

A better leaving group increases the rate of both $$S_N1$$ and $$S_N2$$ reactions.

Question 15

Which solvent would most likely favor an $$S_N2$$ reaction mechanism?

Accepted Answer

Dimethyl sulfoxide (DMSO, polar aprotic)

Question 16

Which of the following best describes the transition state of an $$S_N2$$ reaction?

Accepted Answer

Trigonally bipyramidal with partial bonds to both nucleophile and leaving group

Question 17

A chemist observes that the rate of substitution of neopentyl bromide is extremely slow, even with a strong nucleophile. What is the most likely explanation?

Accepted Answer

Steric hindrance prevents backside attack required for $$S_N2.$$

Question 18

Which of the following is NOT a product type formed by nucleophilic substitution of alkyl halides?

Accepted Answer

Alkenes

Question 19

A chiral secondary alkyl chloride is treated with water in ethanol. What is the expected stereochemical outcome?

Accepted Answer

Racemization

Question 20

Which of the following statements about the effect of degree of substitution on $$S_N1$$ and $$S_N2$$ reactions is correct?

Accepted Answer

Increasing substitution increases the rate of $$S_N1$$ reactions and decreases the rate of $$S_N2$$ reactions.

Alkyl Halides & Nucleophilic Substitution: SN2 and SN1 Mechanisms

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