Chapter 11 homework and practice questions reactions of alkyl halides nucleophilic substitutions and eliminations PDF

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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 SN2 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:

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Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

4.

Circle the least reactive compound in an SN1 reaction.

ANS:

5.

Circle the best solvent for an SN2 reaction.

ANS:

Exhibit 11-2 Consider the pair of reactions below to answer the following question(s). a.

or b.

6. a. b.

Refer to Exhibit 11-2. Which reaction would be predicted to be faster? 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. a. b. c. d.

Refer to Exhibit 11-2. The mechanism for these reactions is: S N1 S N2 E1 E2

ANS: b

Exhibit 11-3 Consider the pair of reactions below to answer the following question(s). a.

or b.

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% H2O/30% CH3OH than in 50% H2O/50% acetone because SN1 reactions are faster in more polar solvents. Acetone (dielectric polarization, P = 20.7) is less polar than CH3OH (P = 33.6), and the percentage of H2O 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

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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.

a. b. c. d.

Refer to Exhibit 11-4. The alkyl bromide starting materials in these reactions are classified as: 3° 2° 1° 4°

ANS: c 13. a. b. c. d.

Refer to Exhibit 11-4. The solvent in these reactions is: nonpolar aprotic polar aprotic polar protic nonpolar protic

ANS: b 14. a. b. c. d.

Refer to Exhibit 11-4. The nucleophile in these reactions is: K+ alkyl group Br I −

−

ANS: d

4

Chapter 11

15.

Refer to Exhibit 11-4. Which reaction is faster?

ANS: b

16. a. b. c. d.

Refer to Exhibit 11-4. The mechanism for these reactions is: S N2 E2 S N1 E1

ANS: a Exhibit 11-5 Consider the pair of reactions below to answer the following question(s). a.

or b.

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.

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Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

19. a. b. c. d.

Refer to Exhibit 11-5. Doubling the concentration of potassium hydroxide in these reactions: causes the reaction mechanism to change halves the rate of reaction has no effect on the rate of reaction doubles the rate of reaction

ANS: d 20. a. b. c. d.

Refer to Exhibit 11-5. The mechanism for these reactions is: S N2 E2 S N1 E1

ANS: b

Exhibit 11-6 Consider the reaction below to answer the following question(s).

21.

Refer to Exhibit 11-6. The substrate in the reaction is:

ANS: A

22. a. b. c. d.

Refer to Exhibit 11-6. Compound B is the: SN2 product SN1 product E2 product E1 product

ANS: b

23.

6

Refer to Exhibit 11-6. Compound C is the:

Chapter 11

a. b. c. d.

SN2 product SN1 product E2 product 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:

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. a. b. c. d.

Refer to Exhibit 11-7. The mechanism involved in the reaction of CH3Br and NaOH is: S N1 S N2 E1 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.

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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 CH3Br and iodide to proceed faster than the reaction between CH3Br and hydroxide. CH3I is a more reactive alkyl halide than CH3Br because iodide is a better leaving group than bromide so when CH3I forms it reacts with hydroxide faster than CH3Br to form CH3OH. The overall result is faster formation of CH3OH from CH3Br 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

Chapter 11

30.

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

ANS:

31.

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

ANS:

32. a. b. c. d.

Refer to Exhibit 11-8. The mechanism of this reaction is: S N1 S N2 E1 E2

ANS: d

Exhibit 11-9 Consider the reaction below to answer the following question(s).

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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. a. b. c. d.

Refer to Exhibit 11-9. This reaction obeys a rate law of the form: rate = k[RCl][CH3CH2OH] rate = k[RCl][H2O][CH3CH2OH] rate = k[RCl] rate = k[RCl][H2O]

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.

35. ANS:

10

Chapter 11

36. ANS:

37. ANS:

38. ANS:

39. ANS:

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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 SN2 reaction occurred in the first step of the reaction to invert the configuration at that carbon.

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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. b. c. d.

S N1 S N2 E1 E2

48.

______

ANS: b

49.

______

ANS: d

50.

For the substitution reaction:

Draw the structure present at the point indicated by the asterisk (*) on the following energy diagram. Identify the reactant as R or S or neither.

14

Chapter 11

ANS:

The reactant is the R enantiomer.

51.

For the substitution reaction:

Draw the structure present at the point indicated by the asterisk (*) on the following energy diagram.

15

Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

ANS:

52.

For the elimination reaction:

Draw the structure present at the point indicated by the asterisk (*) on the following energy diagram.

16

Chapter 11

ANS:

53.

For the elimination reaction:

Draw the structure present at the point indicated by the asterisk (*) on the following energy diagram

ANS:

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Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

MULTIPLE CHOICE 1.

Consider the two lines shown on the energy diagram below.

In an SN2 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 SN2 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 CH3COO– . c. the upper left line could represent H2O and the lower left line H2S. – d. the upper left line could represent (CH3)2NH and the lower left line (CH3)2N . ANS: B

18

Chapter 11

3.

Consider the two energy diagrams below for an SN2 reaction. A

B

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 SN2 reaction, these lines could compare the effect of which of the following solvents? a. CH3OH and CH3CH2OH c. CH3OH and CH3CN b. CH3OH and H2O d. DMSO and CH3CN ANS: C

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Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

5.

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

Which of the following is correct with respect to these diagrams? a. A represents an SN2 reaction. b. B represents an SN1 reaction. c. B represents an SN2 reaction. d. A represents an SN1 reaction. e. both a and b f. both c and d ANS: F 6.

20

Consider the substitution reaction:

B

Chapter 11

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

e. S enantiomer

b. S enantiomer

f.

R enantiomer

c. An equimolar mixture of a and b. g. An equimolar mixture of e and f d. A mixture of the major product a with the h. A mixture of the major product f with the minor product b. minor product e. ANS: C 7.

Consider the substitution reaction:

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

e. R enantiomer

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Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

b. R enantiomer

f.

c.

g. A equimolar mixture of a and b.

d.

S enantiomer

A equimolar mixture of e and f. h.

A mixture of the major product a with the minor product b.

A mixture of the major product f with the minor product e.

ANS: B 8.

Consider the two energy diagrams given below. A

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

Chapter 11

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.

a. b. c. d.

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 SN2 reaction? use of a weak nucleophile branching at the α or β carbon use of a weak base All of the above would enhance the E2 rate.

ANS: B

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