Chapter 11—Homework and practice questions Reactions of Alkyl

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Chapter 11—Homework and practice questions Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

SHORT ANSWER Exhibit 11-1

Circle

your response in each set below.

1. Circle the

least

reactive compound in an S N 2 reaction. ANS: 2. Circle the

best

leaving group in an elimination reaction. ANS: 3. Circle the

best

nucleophile in a substitution reaction at a primary carbon. ANS: 1

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

4. Circle the

least

reactive compound in an S N 1 reaction. ANS: 5. ANS: Circle the

best

solvent for an S N 2 reaction.

Exhibit 11-2

Consider the pair of reactions below to answer the following question(s). a. b.

or

6. Refer to Exhibit 11-2. a. Which reaction would be predicted to be faster? b. Explain your answer to the question in part

a

. ANS: a. b b. − SH is a better nucleophile than − OH because nucleophilicity usually increases in going down a column of the periodic table and sulfur is below oxygen in group 6A. 7. How do you define a chiral carbon? 2

Chapter 11 8. Refer to Exhibit 11-2. The mechanism for these reactions is: a. S N 1 b. S N 2 c. E1 d. E2 ANS: b

Exhibit 11-3

Consider the pair of reactions below to answer the following question(s). a. b.

or

9. Refer to Exhibit 11-3. a. Which reaction above is faster? b. Explain your answer to the question in part

a

. ANS: a. a b. The conversion of

tert

-butyl chloride to

tert

-butyl alcohol proceeds faster in 70% H 2 O/30% CH 3 OH than in 50% H 2 O/50% acetone because S N 1 reactions are faster in more polar solvents. Acetone (dielectric polarization,

P

= 20.7) is less polar than CH 3 OH (

P

= 33.6), and the percentage of H 2 O is higher in the water/methanol system. 10. The original question has been combined with question #9 as part b. This placeholder question is here to maintain the integrity of the numbering system between the printed copy and ExamView. Therefore, it has been marked "do not use on test" in ExamView's question information dialog. As a result, this placeholder question is automatically prevented from being chosen as a test question.

ANS: Answer not provided.

11. Refer to Exhibit 11-3. The kinetics of these reactions are: a. second-order b. first-order in nucleophile 3

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

c. not measurable d. first-order in alkyl halide ANS: d

Exhibit 11-4

Consider the pair of reactions below to answer the following question(s). a.

or

b. 12. Refer to Exhibit 11-4. The alkyl bromide starting materials in these reactions are classified as: a. 3 ° b. 2 ° c. 1 ° d. 4 ° ANS: c 13. Refer to Exhibit 11-4. The solvent in these reactions is: a. nonpolar aprotic b. polar aprotic c. polar protic d. nonpolar protic ANS: b 14. Refer to Exhibit 11-4. The nucleophile in these reactions is: a. K c. Br + b. alkyl group − d. I − ANS: d 4

Chapter 11 15. Refer to Exhibit 11-4. Which reaction is

faster

?

ANS: b 16. Refer to Exhibit 11-4. The mechanism for these reactions is: a. S c. S N N 2 b. E2 1 d. E1 ANS: a

Exhibit 11-5

Consider the pair of reactions below to answer the following question(s). a. b.

or

17. Refer to Exhibit 11-5. a. Which reaction above is faster? b. Explain your answer to the question in part

a

. ANS: a. b b. Tosylate anion is a better leaving group than amide ion; tosylate is the weak conjugate base of a strong acid, while amide is the strong conjugate base of a weak acid. 18. The original question has been combined with question #17 as part b. This placeholder question is here to maintain the integrity of the numbering system between the printed copy and ExamView. Therefore, it has been marked "do not use on test" in ExamView's question information dialog. As a result, this placeholder question is automatically prevented from being chosen as a test question.

ANS: Answer not provided.

5

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

19. Refer to Exhibit 11-5. Doubling the concentration of potassium hydroxide in these reactions: a. causes the reaction mechanism to change b. halves the rate of reaction c. has no effect on the rate of reaction d. doubles the rate of reaction ANS: d 20. Refer to Exhibit 11-5. The mechanism for these reactions is: a. S c. S N N 2 b. E2 1 d. E1 ANS: b

Exhibit 11-6

Consider the reaction below to answer the following question(s). ANS: A 21. Refer to Exhibit 11-6. The substrate in the reaction is: 22. Refer to Exhibit 11-6. Compound B is the: a. S N 2 product b. S N 1 product c. E2 product d. E1 product ANS: b 23. Refer to Exhibit 11-6. Compound C is the: 6

a. S N 2 product b. S N 1 product c. E2 product d. E1 product ANS: d 24. Refer to Exhibit 11-6. Write the complete reaction mechanism for the formation of Compound C in this reaction.

ANS: Chapter 11

Exhibit 11-7

To answer the following question(s) consider the data below: Reaction of bromomethane with sodium hydroxide in water forms methanol. If sodium iodide is added to the reaction mixture, the rate of methanol formation is dramatically increased (i.e. sodium iodide is a catalyst).

25. Refer to Exhibit 11-7. The mechanism involved in the reaction of CH 3 Br and NaOH is: a. S N 1 b. S N 2 c. E1 d. E2 ANS: b 26. Refer to Exhibit 11-7. Write a reaction pathway that accounts for the effect of added NaI.

ANS: 27. Refer to Exhibit 11-7. Draw a reaction energy diagram showing the two different reaction pathways (i.e. catalyzed and uncatalyzed). Indicate structures for all energy minima in the diagram.

7

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

ANS: 28. Refer to Exhibit 11-7. Explain why adding NaI increases the reaction rate.

ANS: Iodide is a better nucleophile than hydroxide so we expect reaction between CH 3 Br and iodide to proceed faster than the reaction between CH 3 Br and hydroxide. CH 3 I is a more reactive alkyl halide than CH 3 Br because iodide is a better leaving group than bromide so when CH 3 I forms it reacts with hydroxide faster than CH 3 Br to form CH 3 OH. The overall result is faster formation of CH 3 OH from CH 3 Br when NaI is added.

29. Refer to Exhibit 11-7. Would you expect the same catalytic effect on this reaction if you added NaCl instead? Explain your answer.

ANS: No. Chloride ion is a poorer nucleophile as well as a poorer leaving group than bromide ion.

Exhibit 11-8

Consider the reaction below to answer the following question(s): 8

30. Refer to Exhibit 11-8. Write the product that results from the electron flow in the reaction, clearly indicating any stereochemistry.

ANS: Chapter 11 31. Refer to Exhibit 11-8. Draw a Newman projection of the reactive conformation of the starting material.

ANS: 32. Refer to Exhibit 11-8. The mechanism of this reaction is: a. S N 1 b. S N 2 c. E1 d. E2 ANS: d

Exhibit 11-9

Consider the reaction below to answer the following question(s). 9

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

33. Refer to Exhibit 11-9. Write the complete stepwise mechanism for this reaction. Clearly show the formation of both products. Show all electron flow with arrows and draw all intermediate structures.

ANS: 34. Refer to Exhibit 11-9. This reaction obeys a rate law of the form: a. rate =

k

[RCl][CH 3 CH 2 OH] b. rate =

k

[RCl][H 2 O][CH 3 CH 2 OH] c. rate =

k

[RCl] d. rate =

k

[RCl][H 2 O] ANS: c

Exhibit 11-10

Draw the structure of the major organic product(s) for each of the following reactions. Indicate the stereochemistry for each reaction when appropriate.

ANS: 35. 10

36. ANS: 37. ANS: 38. ANS: 39. ANS: Chapter 11 11

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

40. ANS: 41. ANS: 42. ANS: 43. ANS: 44. ANS: 12

Chapter 11

PRACTICE QUESTIONS

Exhibit 11-11

Consider the reaction below to answer the following question(s). 45. Refer to Exhibit 11-11. List all chirality centers in the starting material by number.

ANS: Carbon 2 is a chirality center.

46. Refer to Exhibit 11-11. As indicated by the carbon numbers, during the course of this reaction the bond between the nitrogen atom and carbon 3 is broken and a bond between the nitrogen atom and carbon 2 is formed. On the structures provided below, draw arrows showing electron flow for the mechanism that accounts for these bonding changes. ANS: 47. Refer to Exhibit 11-11. If the absolute configuration at carbon 2 in the starting material is

R

, what is the absolute configuration at carbon 2 in the product?

ANS: The absolute configuration at carbon 2 in the product is

S

because an intramolecular S N 2 reaction occurred in the first step of the reaction to invert the configuration at that carbon.

13

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

Exhibit 11-12

For each substrate below, choose which reaction type is favored. Place the letter of the reaction type in the blank to the left of the substrate. a. S N 1 b. S N 2 c. E1 d. E2 48. ______ ANS: b 49. ______ ANS: d 50. For the substitution reaction: diagram. Identify the reactant as

R

or

S

or neither. Draw the structure present at the point indicated by the asterisk (*) on the following energy 14

Chapter 11 ANS: The reactant is the

R

enantiomer.

51. For the substitution reaction: diagram. Draw the structure present at the point indicated by the asterisk (*) on the following energy 15

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

ANS: 16 52. For the elimination reaction: Draw the structure present at the point indicated by the asterisk (*) on the following energy diagram.

ANS: ANS: 53. For the elimination reaction: Draw the structure present at the point indicated by the asterisk (*) on the following energy diagram Chapter 11 17

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

MULTIPLE CHOICE

1. Consider the two lines shown on the energy diagram below. In an S N 2 reaction, these compare the effect of the: a. substrate. b. nucleophile. c. leaving group. d. solvent. e. nucleophile or solvent f. substrate or leaving group ANS: F 2. Consider the two lines shown on the energy diagram below. In an S N 2 reaction, a. the upper left line could represent Cl – and the lower left line I – . b. the upper left line could represent OH – and the lower left line CH 3 COO – . c. the upper left line could represent H 2 O and the lower left line H 2 S. d. the upper left line could represent (CH 3 ) 2 NH and the lower left line (CH 3 ) 2 N – . ANS: B 18

3. Consider the two energy diagrams below for an S N 2 reaction.

A B

Chapter 11 Which of the following is correct?: a. The change in the variable shown in A affects the transition state while that of B affects the reactant. b. The change in the variable shown in A affects the reactant while that of B affects the transition state. c. The changes in the variables shown in both A and B affect the transition state. d. The changes in the variables shown in both A and B affect the reactants. ANS: A 4. Consider the two lines shown on the energy diagram below. In an S N 2 reaction, these lines could compare the effect of which of the following solvents? a. CH 3 OH and CH 3 CH 2 OH b. CH 3 OH and H 2 O c. CH 3 OH and CH d. DMSO and CH 3 3 CN CN ANS: C 19

20

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

5. Consider the two energy diagrams labeled A and B given below.

A B

Which of the following is correct with respect to these diagrams? a. A represents an S N 2 reaction. b. B represents an S N 1 reaction. c. B represents an S N 2 reaction. d. A represents an S N 1 reaction. e. both a and b f. both c and d ANS: F 6. Consider the substitution reaction:

The product(s) of the reaction is(are): a.

R

enantiomer e.

S

enantiomer Chapter 11 b.

S

enantiomer f.

R

enantiomer c. An equimolar mixture of a and b. d. A mixture of the major product a with the minor product b. ANS: C 7. Consider the substitution reaction: g. An equimolar mixture of e and f h. A mixture of the major product f with the minor product e. The product(s) of the reaction is(are): a.

S

enantiomer e.

R

enantiomer 21

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

b.

R

enantiomer f.

S

enantiomer c. A equimolar mixture of a and b. d. A mixture of the major product a with the minor product b. g. A equimolar mixture of e and f. h. A mixture of the major product f with the minor product e. ANS: B 8. Consider the two energy diagrams given below.

A B

Which of the following is correct with respect to these diagrams? a. A could represent an E2 reaction. b. B could represent an E1 reaction. c. A could represent an E1cb reaction. ANS: C 9. Consider the elimination reaction: The product(s) of the reaction is(are): a. 22

b. c. An equimolar mixture of a and b. d. A mixture of the major product a with the minor product b. e. A mixture of the major product b with the minor product a. ANS: D 10. Consider the elimination reaction: The product(s) of the reaction is(are): a. b. c. An equimolar mixture of a and b. d. A mixture of the major product a with the minor product b. e. A mixture of the major product b with the minor product a. ANS: D 11. Nucleophilic substitution and elimination reactions often complete with one another. In general, which of the following would enhance the rate of the E2 reaction over the S N 2 reaction? a. use of a weak nucleophile b. branching at the α or β carbon c. use of a weak base d. All of the above would enhance the E2 rate. ANS: B Chapter 11 23

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