Deck 7: Substitution Reactions: the Sn2 and Sn1 Reactions

Question 1

Which of the following expressions correctly states the rate law for the S_N2 reaction shown here? (NaN₃= sodium azide)

A) rate =k[2 -bromopropane ]
B) rate =k [sodium azide]
C) rate =k[2 -bromopropane ] [sodium azide]
D) rate =k[2 -bromopropane }2[ sodium azide ]
E) It is impossible to tell from the information given.

Answer 1

rate =k[2 -bromopropane ] [sodium azide]

Question 2

Which of the following is a likely product of the reaction sequence shown?

A)
B)
C)
D)
E)

Answer 2

Question 3

Chemicals such as dimethyl sulfate and iodomethane are considered biologically hazardous because:

A)They can disrupt the body's endocrine system.
B)They are acute respiratory hazards, leading to pulmonary edema.
C)They can alkylate nucleotide bases, leading to mutagenic and carcinogenic effects.
D)They can cause damage to the liver and kidneys.
E)They are corrosive to the eyes and upper respiratory tract.

Answer 3

They can alkylate nucleotide bases, leading to mutagenic and carcinogenic effects.

Question 4

In which of the following instances would it be expected that the reaction would always have an equilibrium constant that is greater than 1 ?

A) ΔH and ΔS for the reaction are both positive.
B) Δ H for the reaction is positive, while Δ S for the reaction is negative.
C) ΔH and ΔS for the reaction are both negative.
D) Δ H and ΔS for the reaction are both zero.
E) ΔH for the reaction is negative, while Δ S for the reaction is positive.

Question 5

Which of the following is a Lewis acid but not a Brønsted acid?

A) NH₃
B) H₂O
C) AlCl₃
D) CH₃OH
E) CH₃CH₂SH

Question 6

Which of these carbocations is most stable?

A) A
B) B
C) C
D) D
E) E

Question 7

Identify the product of the following reaction.

A)
B)
C)
D)
E)

Question 8

Which of the following is the product of this reaction?

A)
B)
C)
D)
E)

Question 9

Which of the following would react most rapidly in an S_N1 reaction?

A)
B)
C)
D)
E)

Question 10

Which of these choices is the best leaving group?

A) OH-
B) NH₃
C) Cl-
D) CH₃O-
E) H₂O

Question 11

Which of the following would not be a valid reason for a substitution reaction being irreversible?

A)The reaction is highly exothermic.
B)The reaction requires a catalyst to proceed.
C)One product is a gas that bubbles out of the reaction medium.
D)One product crystallizes out of the reaction medium.
E)One reactant is present in a very large quantity relative to the other.

Question 12

For the reaction profile shown below, which statement(s) could be made?
I.The transition state will look more like the starting materials than the products.
II.The reaction is endothermic.
III.The equilibrium constant for the forward reaction will be larger than 1.
IV.The activation barriers for the forward and reverse reactions are equal.

A)I only
B)I and II
C)I and III
D)I, II and III
E)I, III and IV

Question 13

For the following reaction, what will be the effect on the rate if [NaN₃] is tripled?

A) The rate doubles.
B) The rate triples.
C) The rate quadruples.
D) The rate stays the same.
E) There is not enough information to determine the effect.

Question 14

Which of the following factors would not be expected to affect the entropy of a reaction?

A)Acyclic reactants are transformed into cyclic products.
B)A weak bond in the reactant is replaced with a strong bond in the product.
C)Two molecules of reactants are transformed into three molecules of products.
D)A double bond in the reactant is converted into a single bond in the product.
E)Gaseous reactants are transformed into solid products.

Question 15

Identify the HOMO and the LUMO in the following S_N2 reaction.

A) The HOMO is a bonding C-Br σ and the LUMO is an antibonding C-Br σ* .
B) The HOMO is a bonding C-Br σ* and the LUMO is an antibonding C-Br σ .
C) The HOMO is a nonbonding (n) on hydroxide ion and the LUMO is an antibonding OH σ*.
D) The HOMO is a nonbonding (n) on hydroxide ion and the LUMO is an antibonding C-Br σ* .
E) None of these choices correctly identify the HOMO and LUMO in this reaction.

Question 16

Which of the following electrophiles would undergo an S_N2 reaction with thiolate ion (HS^-) at the fastest rate?

A) A
B) B
C) C
D) D
E) E

Question 17

Which of the following ideas best explains why S_N2 reactions proceed with inversion of configuration?

A) steric hindrance
B) orbital overlap
C) nucleophile strength
D) solvent effects
E) leaving group ability

Question 18

Which of the following will react most slowly with cyanide nucleophile (NC^-) in an S_N2 reaction?

A)
B)
C)
D)
E) CH₃I

Question 19

Which of the following statements about Lewis acids and bases is false?

A)Lewis acids are also known as electrophiles.
B)A vacant orbital of a Lewis acid will interact with a filled orbital of a Lewis base to form a sigma bond.
C)All Brønsted bases are also Lewis bases.
D)Typically the LUMO of the Lewis base interacts with the HOMO of the Lewis acid.
E)The reaction of hydride anion with a methyl cation would be an example of a Lewis base/Lewis acid reaction.

Question 20

Which of the following is the rate limiting step in an S_N1 reaction?

A)
B)
C)
D)

E)

Question 21

Identify the Lewis acid and the Lewis base in the reaction shown here.

Question 22

Hydroxide is a poor leaving group. Describe three ways to convert the hydroxyl group to a better leaving group.

Question 23

Draw the product of the following sequence of reactions. Show stereochemistry at any stereogenic carbon.

Question 24

Which of these is the most efficient method for making the ether shown in the highest possible yield using a Williamson ether synthesis?

A)
B)
C)
D)
E) Methods b and d are equally efficient.

Question 25

Identify the HOMO and the LUMO in the reaction shown here.

Question 26

Draw an energy diagram for the SN2 reaction shown here. Show the correct relative energies of the starting materials and products.

Question 27

Draw a mechanism for the following transformation. Show all curved arrows, necessary lone pairs, and nonzero formal charges.

Question 28

Explain why SN2 reactions proceed with inversion of configuration at a stereogenic carbon, but product mixtures in SN1 reactions show both inversion and retention of configuration.

Question 29

What is the most likely stereochemical outcome of the S_N1 reaction shown here?

A) complete inversion of configuration
B) complete retention of configuration
C) racemization
D) more retention than inversion
E) more inversion than retention

Question 30

S_N1 reactions run in polar solvents have faster rates than S_N1 reactions run in nonpolar solvents.
Which statement provides the best explanation for the difference in rates?

A) Polar solvents stabilize the transition state in the rate-determining step.
B) Nonpolar solvents stabilize the transition state in the rate-determining step.
C) Polar solvents stabilize the carbocation intermediate.
D) Nonpolar solvents stabilize the carbocation intermediate.
E) The reaction mechanism is different in polar solvents than in nonpolar solvents.

Question 31

What sequence of reactions could be used for the following synthesis?

A)
B)
C)
D)
E)

Question 32

Use molecular orbital interactions to illustrate why SN2 reactions proceed with inversion of configuration at the substrate.

Question 33

Which of the following compounds is the product of the reaction shown that displays retention of configuration compared to the starting material?

A)
B)
C)
D)
E)

Question 34

Draw the SN2 product of the following reaction. Show stereochemistry at any stereogenic carbon.

Question 35

Which of the following is the weakest nucleophile in protic solvent in an SN2 reaction, and why?

Question 36

Draw the expected transition state geometry for each of these SN2 reactions based on the reaction enthalpy values. (Hint: Will the TS look more like the reactant or the product?):

Question 37

Which structure is the most likely S_N1 product under the following conditions?

A)
B)
C)
D)
E) None of these products will form.

Question 38

Which of these alkyl bromides will react faster in an SN2 reaction? Explain.

Question 39

What is the likely product of reaction of dimethylamine with excess methyl iodide?

A) HNI₂
B)HNMe₃⁺I^-
C)NMe₄₊I^-
D) HNMeI
E) Me₃N

Question 40

In each pair shown, which S_N2 reaction would proceed faster?

Question 41

Explain why S_N1 reactions are favored by the use of a polar solvent.

Question 42

The final step in the SN₁ reaction of water with tert-butyl bromide to form tert-butyl alcohol is the deprotonation of the oxonium ion to give the alcohol plus hydronium:

The relative energies of the starting materials and products in this step are expected to be very similar. Explain why in practice this reaction proceeds essentially to completion.

Question 43

Outline a multistep synthesis of the target molecule from the starting material shown.You may
use any organic or inorganic reagents.Show the reagents necessary for each step and the product
of each step.

Question 44

Predict the product of the following reaction.

Question 45

Outline a multistep synthesis of the target molecule from the starting material shown.You may
use any organic or inorganic reagents.Show the reagents necessary for each step and the product
of each step.

Question 46

Outline a multistep synthesis of the target molecule from the starting material shown.You may
use any organic or inorganic reagents.Show the reagents necessary for each step and the product
of each step.

Question 47

Which of these molecules will react faster in an SN1 reaction with methanol? Explain.

Question 48

Show how you would perform the following synthesis.Show the reagents you would use for each step and the organic intermediate formed in each step.

Question 49

An intramolecular Williamson ether synthesis may be used to produce epoxides.Predict the
product of the following reaction conditions and provide a mechanism for the transformation.

Question 50

Draw the structure of the alkyl bromide starting material necessary to react with a cyanide ion to
produce the molecule shown.

Question 51

Which of these carbocations is more stable? Explain.

Question 52

Provide the missing reagents in the following reaction sequence.

Question 53

Based on nucleophile, substrate, and reaction conditions, state the most likely reaction mechanism (SN₁ or SN₂) in each example. Draw the products, including stereochemistry as needed.

Question 54

Draw an energy diagram for the S_N1 reaction between tert-butyl bromide and water; assume that the reaction is overall exothermic.

Question 55

Supply the reagents/intermediates in the multistep synthesis shown below.