54968550 Practice Problems for Chapter 6 With Answers 1[1]

Chapter 6

MULTIPLE CHOICE QUESTIONS

Topic: Reaction kinetics

1. Consider the SN2 reaction of butyl bromide with OH- ion.

CH3CH2CH2CH2Br + OH- → CH3CH2CH2CH2OH + Br-

Assuming no other changes, what effect on the rate would result from simultaneously doubling the concentrations of both butyl bromide and OH- ion?

A) No effect.B) It would double the rate.C) It would triple the rate.D) It would increase the rate four times.E) It would increase the rate six times.

Ans: D


2. Consider the SN2 reaction of 2-iodopentane with CH3CO2− ion.

I

ONa

O OO

NaI

Assuming no other changes, what effect on the rate would result from simultaneously

doubling the concentrations of both 2-iodopentane and CH3CO2− ion?


Ans: D

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3. Consider the SN2 reaction of 1-chloro-5-methylhexane with CN− ion.

Cl NaCN CN NaCl

Assuming no other changes, what effect on the rate would result from simultaneously

doubling the concentrations of both 1-chloro-5-methylhexane and CN− ion?


Ans: D


4. Select the rate law for the following reaction, e.g.,CH3CH2CH2CHBrCH3 + OH− → CH3CH2CH2CHOHCH3 + X−

( RBr )A) Rate = k [RBr]B) Rate = k [RBr] [OH−]C) Rate = k [RBr]2 [OH−]D) Rate = k [RBr] [OH−]2

E) Rate = k [RBr]2 [OH−]2

Ans: B

Topic: Activation Energy

5. Increasing the temperature of a chemical reaction usually increases greatly the rate of the reaction. The most important reason for this is that increasing the temperature:

A) Increases the collision frequency.B) Decreases the probability factor.C) Increases the fraction of collisions with energy greater than Eact.D) Decreases the energy of activation.E) Makes the reaction more exothermic

Ans: C

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Chapter 6

Topic: Activation Energy

6. Increasing the temperature of a chemical reaction usually increases greatly the rate of the reaction. The primary reason for this is that increasing the temperature:

A) Decreases the energy of activation.B) Increases the total number of collisions between reactantsC) Decreases the rate of the reverse reaction.D) Favors endothermic processes.E) None of the above properly explains the observed increase in reaction rates with

increase in temperature.Ans: E

Topic: Reaction Kinetics

7. The rate equation for a nucleophilic substitution reaction of a tertiary alkyl bromide (R-Br) with I− ion would be:

A) Rate = k [RBr]B) Rate = k [I−]C) Rate = k [RBr][I−]D) Rate = k [RBr]2[I−]E) Rate = k [RBr][I−]2

Ans: A


8. The rate equation for a nucleophilic substitution reaction of a secondary alkyl chloride (R-Cl) with I− ion would be:

A) Rate = k [RCl]B) Rate = k [I−]C) Rate = k [RCl][I−]D) Rate = k [RCl]2[I−]E) Rate = k [RCl][I−]2

Ans: C

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Chapter 6


9. Consider the reaction of 2-chloro-2-methylpentane with sodium iodide.

Cl

NaI

I

NaCl

Assuming no other changes, how would it affect the rate if one simultaneously doubled the concentration of 2-chloro-2-methylpentane and sodium iodide?

A) No effectB) It would double the rate.C) It would triple the rate.D) It would quadruple the rate.E) It would increase the rate five times.

Ans: B

Topic: Predicting mechanisms, products

10. What would you expect to be the chief organic product(s) when tert-butyl bromide reacts with sodium acetylide, i.e.,

BrHC C: Na

I

HC CH

IV

II III

?

A) IB) IIC) IIID) IVE) None of these

Ans: D

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Chapter 6


11. What would you expect to be the chief organic product(s) when 2-bromo-2-methylpentane reacts with sodium propynide, i.e.,

BrC: Na

I

HC CH

IV

II

V

HC CH

III

?

A) IB) IIC) IIID) IVE) V

Ans: D

Topic: Predicting products

12. What product(s) would you expect to obtain from the following SN2 reaction?

Br

HH

CH3

CH3ONa

CH3OH

OCH3

HH

CH3 H

OCH3H

CH3

I II

CH3O

III

Br

H

H

?

A) IB) IIC) An equimolar mixture of I and II.D) IIIE) None of these

Ans: B

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Chapter 6


13. What major product(s) are likely to be obtained from the following reaction?

Cl

H

H

CH3CH2ONa

CH3CH2OH, 65oC

O

H

H H

O

H

I II

Cl

HO

H

III

?

A) I, by predominantly SN2 B) II, by predominantly SN2C) An equimolar mixture of I and II, by predominantly SN1.D) III, by substitution of the alkyl group, rather than substitution of the chloro groupE) Actually, none of these products are likely to be obtained as major products, because

elimination will probably predominate, leading to the formation of an alkene.Ans: E


14. Treating (CH3)3C-Cl with a mixture of H2O and CH3OH at room temperature would yield:

A) CH2=C(CH3)2

B) (CH3)3COHC) (CH3)3COCH3

D) All of theseE) None of these

Ans: D

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Chapter 6


15. Treating 1-bromo-1-methylcyclohexane with CH3OH at room temperature would yield:A)

OCH3

B)

C)

D) All of theseE) None of these

Ans: D


16. The major product of the following reaction would be:CH2Cl

H OCH3

CH3

OH−

CH2OH

H3CO H

CH3

CH2Cl

HO H

CH3

I

III IV

CH2Cl

H OH

CH3

II

CH2OH

H OCH3

CH3

SN2?

A) IB) IIC) IIID) IVE) An equimolar mixture of I and II.

Ans: B

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Chapter 6


17. Which would be formed in the following reaction?CH3

BrH3C

H

CH3OH

OCH3

CH3H3C

H

CH3

CH3

CH3

CH2

I

IVIII

II

CH3

OCH3H3C

H

55oC?

A) IB) IIC) IIID) IVE) All of the above

Ans: E

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Chapter 6


18. Which would be formed in the following reaction?CH3

Br

H

H3COH

O

CH3

O

H

H3C

O

I

III

O

CH3

H

H3C

O

II

IV

50oC?

A) IB) IIC) IIID) IVE) All of the above

Ans: E

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Chapter 6


19. Which would be the major product of the following reaction?

H

ClH

CH3

OC(CH3)3

I

CH3

H

H

OC(CH3)3

CH3

H

I II

H3C

III

H3C

IV

(CH3)3COH

55oC

?(CH3)3CO−

A) IB) IIC) IIID) IVE) None of the above

Ans: C

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Chapter 6


20. When 1,4-diiodo-2,2-dimethylbutane (0.10 mol) is treated with 0.10 mol of NaCN in dimethyl sulfoxide at 30°C, the product formed is:

A)NC

I

B)I

CN

C) both A) and B).

D)NC

CN

E) I

Ans: B


21. The major product(s) of the following reaction is(are):

H

CH3

H

Cl

H

CH3

H

I

H

CH3

I

H

I II

CH3

III IV

H

CH3

H

OCH2CH3

NaI, CH3CH2OH

25oC

A) IB) IIC) IIID) IVE) Equal amounts of I and II

Ans: A

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Chapter 6


22. What would be the major product of the following reaction?

CH2Cl

H CH3

OH−

CH2OH

H3C H

CH2Cl

HO CH3

I

III

II

IV

CH2Cl

H OH

CH2OH

H CH3

SN2

A) IB) IIC) IIID) IVE) An equimolar mixture of I and II

Ans: B

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Chapter 6


23. What would be the major product(s) of the following reaction?

CH3H3C

H Br

CH3H3C

H OCH3

OCH3H3C

H CH3

I II

H3C CH3 H3C

III IV

CH3O− / CH3OH

55oC

A) IB) IIC) III and IVD) I and IIE) All of the above

Ans: C

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Chapter 6


24. What would be the major product of the following reaction?

HH3C

H Br

HH3C

H CN

CNH3C

H Br

I II

III IV

CNH3C

H H

HH3C

NC Br

CΝ −

25oC

A) IB) IIC) IIID) IVE) Equal amounts of I and III

Ans: C


25. What would be the major product obtained when trans-1-bromo-3-methylcyclopentane is allowed to react with NaSH at 25oC?

HH3C

H SH

SHH3C

H H

SHH3C

H Br

HH3C

HS Br

I II III IV

A) IB) IIC) IIID) IVE) Equal amounts of I and II

Ans: B

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Chapter 6

Topic: Synthesis, reagent selection

26. You want to synthesize 3-methyl-2-pentene from 2-chloro-3-methylpentane. Which reagent would you use?

A) HCl, heatB) NH3(aq), 25oCC) CH3CO2Na, CH3CO2H, heatD) CH3CH2ONa, CH3CH2OH, heatE) CH3CH2OH, heat

Ans: D


27. When tert-pentyl chloride undergoes solvolysis in aqueous ethanol at room temperature, there is/are formed:

A)

OHB)

OC2H5C)

D)

E) All of theseAns: E


28. Reaction of sodium ethoxide with 1-bromopentane at 30°C yields primarily:A) CH3CH2CH2CH=CH2

B) CH3CH2CH=CHCH3

C) CH3CH2CH2CH2CH3

D) CH3CH2CH2CH2CH2OHE) CH3CH2CH2CH2CH2OCH2CH3

Ans: E

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Chapter 6


29. Heating an alcoholic solution of sodium hydroxide and 1-bromopentane at 60°C yields primarily:

A)

B)

C)D) OH

E) O

Ans: D


30. If 0.10 mol of HSCH2CH2OH reacts at 25°C, sequentially, with 0.20 mol of NaH, 0.10 mol of CH3CH2Br and H2O, which is the major product?

A) HSCH2CH2OCH2CH3

B) CH3CH2SCH2CH2OHC) CH3CH2SCH2CH2OCH2CH3

D) CH2=CH2

E) CH3CH3

Ans: B


31. When 0.10 mol of ICH2CH2CH2CH2Cl reacts with 0.10 mol of NaOCH3 in CH3OH at 40°C, the major product is:

A) CH3OCH2CH2CH2CH2ClB) CH3OCH2CH2CH2CH2IC) CH3OCH2CH2CH2CH2OCH3

D) CH2=CHCH2CH2ClE) CH2=CHCH2CH2I

Ans: A

Topic: Generalities

32. SN2 reactions of the type, Nu- + RL → Nu-R + L-, are favored:A) when tertiary substrates are used.B) by using a high concentration of the nucleophile.C) by using a solvent of high polarity.D) by the use of weak nucleophiles.E) by none of the above.

Ans: B

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Chapter 6

Topic: Generalities

33. Which of the following statements is (are) true of SN1 reactions of alkyl halides in general?

A) The rate of an SN1 reaction depends on the concentration of the alkyl halide.B) The rate of an SN1 reaction depends on the concentration of the nucleophile.C) SN1 reactions of alkyl halides are favored by polar solvents.D) Answers A) and C) only are true.E) Answers A), B) and C) are true.

Ans: D

Topic: Generalities

34. Which of the following statements is (are) true of SN1 reactions of alkyl halides in general?

A) The rate of an SN1 reaction depends on the concentration of the alkyl halide.B) The rate of an SN1 reaction depends on the concentration of the nucleophile.C) SN1 reactions of alkyl halides occur faster in polar aprotic solvents (compared to protic

solvents) D) Answers A) and C) only are true.E) Answers A), B) and C) are true.

Ans: A

Topic: Generalities

35. SN1 reactions of the type, Nu- + RL → Nu–R + L-, are favored:A) when tertiary substrates are used.B) by using a high concentration of the nucleophile.C) when L- is a strong base.D) by use of a non-polar solvent.E) by none of the above.

Ans: A

Topic: Generalities

36. Which of the following statements is (are) true of an SN2 reaction of (R)-2-bromobutane with hydroxide ion?

A) Doubling the hydroxide ion concentration would double the rate of the reaction. (Assume that all other experimental conditions are unchanged.)

B) The major product would be (S)-2-butanol.C) Doubling the concentration of (R)-2-bromobutane would double the rate of the reaction.

(Assume that all other experimental conditions are unchanged.)D) All of the aboveE) Two of the above

Ans: D

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Chapter 6

Topic: Generalities

37. SN1 reactions of the following type,

Nu:- + R-X → R-Nu + :X-

are favored:A) by the use of tertiary substrates (as opposed to primary or secondary substrates).B) by increasing the concentration of the nucleophile.C) by increasing the polarity of the solvent.D) by use of a weak nucleophile.E) by more than one of the above.

Ans: E

Topic: Generalities

38. SN1 reactions of the following type,

Nu:- + R-X → R-Nu + :X-

are favored:A) by the use of tertiary substrates (as opposed to primary or secondary substrates).B) by increasing the concentration of the nucleophile.C) by increasing the polarity of the solvent.D) by use of a strong base.E) by more than one of the above.

Ans: A

Topic: Generalities

39. A true statement about the transition state(s) of an SN2 reaction is:A) the two transition states are of unequal energy.B) the transition states precede and follow an unstable reaction intermediate.C) the single transition state represents the point of maximum free energy of the reaction.D) existence of this transition state implies an exothermic reaction.E) the transition state will always have a net charge of -1.

Ans: C

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Chapter 6

Topic: Generalities

40. Elimination reactions are favored over nucleophilic substitution reactions:A) at high temperatures.B) when tert-butoxide ion is used.C) when 3° alkyl halides are used as substrates.D) when nucleophiles are used which are strong bases and the substrate is a 2° alkyl halide.E) in all of these cases.

Ans: E

Topic: Carbocations

41. The hybridization state of the charged carbon in a carbocation isA) sp4

B) sp3

C) sp2

D) spE) s

Ans: C

Topic: Carbocations

42. The p orbital of a methyl cation, CH3+, contains how many electrons?

A) 1B) 2C) 3D) 4E) 0

Ans: E

Topic: Carbocations

43. The p orbital of the charged carbon in the isopropyl cation, (CH3)2CH+, contains how many electrons?

A) 1B) 2C) 3D) 4E) 0

Ans: E

179

Chapter 6

Topic: Generalities, solvent effects

44. Considering the relative solvation of reactants and the transition states of SN reactions of these reactants, predict which general type of reaction would be most favored by the use of a polar solvent.

A) Y: + RX → RY+ + X:-

B) Y:- + RX → RY + X:-

C) Y: + RX+ → RY+ + X:D) Y:- + RX+ → RY + X:E) RX+ → R+ + X:

Ans: A


45. Which is not a polar aprotic solvent?A)

H3CC

CH3

O

B) H3C C N

C)

H3CS

CH3

O

D)

HC

N

O

CH3

CH3E)

H3CCH

OH

CH3

Ans: E


46. Which is a polar aprotic solvent?A) 2-methylhexaneB) CH3CNC) NH3(l)D) CH3CH2CH2OCH2CH2CH3

E) 2-methyl-2-propanol

Ans: E

180

Chapter 6

Topic: Generalities, nucleophiles

47. Identify the nucleophile in the following reaction.

2 H2O + RX ROH + H3O+ + X−

A) X-B) H3O+

C) ROHD) H2OE) RX

Ans: D


48. Identify the nucleophile in the following reaction.

2 R’OH + RX ROR’ + [ROH2]+ + X−

A) X−

B) [ROH2]+

C) ROR’D) R’OHE) RX

Ans: D

Topic: Transition State theory

49. An increase in the kinetic energy of reacting molecules results in:A) a decrease in reaction rate.B) an increase in the probability factor.C) a decrease in the probability factor.D) an increase in the reaction rate.E) no changes.

Ans: D

181

Chapter 6

Topic: Generalities, leaving group

50. Identify the leaving group in the following reaction.S−Na+

I

S

Na+ + I −

A) C6H5S-

B) Na+

C) CH3CH2ID) C6H5SCH2CH3

E) I-

Ans: E


51. Which is the weakest nucleophile in polar aprotic solvents?A) I-

B) Br-

C) Cl-

D) F-

E) All of the above are equally strong nucleophiles, regardless of the type of solvent usedAns: A


52. Which is the weakest nucleophile in polar protic solvents?A) I-

B) Br-

C) Cl-

D) F-

E) All of the above are equally strong nucleophiles, regardless of the type of solvent usedAns: D

Topic: Generalities, leaving group

53. Which of the following is the poorest leaving group?A) H−

B) CH3O−

C) H2OD) OH−

E) NH2−

Ans: E

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Chapter 6


54.

Which of the following is not a nucleophile?

A) H2OB) CH3O-

C) NH3

D) NH4+

E) All are nucleophiles.Ans: D


55. Which ion is the strongest nucleophile in aqueous solution?A) F-

B) Cl-

C) Br-

D) I-

E) All of these are equally strong.Ans: D

Topic: Leaving groups

56. Which of the following is not a good leaving group?A) C2H5O

−

B) Cl−

C) I−

D) CH3CO2−

E) All of these are good leaving groupsAns: A


57. Identify the leaving group in the following reaction:

CH3OH + CH3OH2 CH3OCH3 + H2O

HA) CH3OHB) CH3OH2

+

C) CH3OCH3

D) H2OE) None of these

Ans: D

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Chapter 6

Topic: Nucleophiles

58. Which is the strongest nucleophile?A) OH-

B) CH3CH2O-

C)

D) CH3CH2OHE) H2O

Ans: B

Topic: Nucleophile, solvent effects

59. Which is the most reactive nucleophile in DMF (structure shown below) ?

HCNCH3

CH3

O

A) F-

B) Cl-

C) Br-

D) I-

E) They are all equally reactive.Ans: A


60. Which of these species, acting in a protic solvent, exhibits greater nucleophilic activity than expected on the basis of its basicity?

A) OH-

B) CH3O-

C) SH-

D) Cl-

E) H2OAns: C

184

Chapter 6

Topic: Nucleophilicity

61. The relative nucleophilicities of species do not necessarily parallel the relative basicities of the same species because:

A) not all nucleophiles are bases, and vice versa.B) experimental measurements of sufficient accuracy are not available to make the

comparisons.C) nucleophilicity is a thermodynamic matter; basicity is a matter of kinetics.D) basicity is a thermodynamic matter; nucleophilicity is a matter of kinetics.E) Actually, the relative values do parallel one another.

Ans: D

Topic: Nucleophiles

62. Ambident nucleophiles are ones which can react with a substrate at either of two nucleophilic sites. Which of the following is not an ambident nucleophile?

:CN: :OH

:O:N::OH

H

I II

VIV

N:O:H

:OCH2CH2S:

III

....

.. ......

.. ....

..

......

2

..

..


Ans: B


63. Which ion is the strongest nucleophile in an aprotic solvent such as dimethylsulfoxide?A) I-

B) Br-

C) Cl-

D) F-

E) These are all equal.Ans: D

185

Chapter 6

Topic: Leaving group

64. Which of the following is a feasible substitution reaction?A) CH3CH2Cl + NaBr → CH3CH2I + NaClB) CH3CH3 + NaCN → CH3CH2CN + NaHC) CH3CH2Cl + NaOH → CH2=CH2 + H2O + NaClD) More than one of the aboveE) None of the above

Ans: A

Topic: Predicting mechanism, products

65. Heating tert-butyl chloride with 1.0 M NaOH in a mixture of water and methanol would yield mainly:

A) (CH3)3COH through an SN1 reaction.B) (CH3)3COCH3 through an SN1 reaction.C) (CH3)3COH through an SN2 reaction.D) (CH3)3COCH3 through an SN2 reaction.E) CH2=C(CH3)2 through an E2 reaction.

Ans: E

186

Chapter 6

Topic: Free energy diagrams

66. Select the potential energy diagram that represents an exothermic (exergonic) reaction.


Ans: E

187

Chapter 6


67. Select the potential energy diagram that represents a single-step endothermic (endergonic) reaction.


Ans: A

188

Chapter 6


68. Select the potential energy diagram that represents a two-step endothermic (endergonic) reaction.


Ans: B

Topic: Hammond-Leffler postulate

69. The Hammond-Leffler postulate when applied to nucleophilic substitutions and elimination reactions states that:

A) a negatively-charged nucleophile is stronger than its conjugate acid.B) polar aprotic solvents strongly accelerate the rate of SN2 processes.C) bimolecular nucleophilic substitutions are 2nd order kinetically.D) the transition state for an endergonic reaction step (one accompanied by an increase in

free energy) resembles the product of that step.E) elimination reactions will always compete with nucleophilic substitution reactions.

Ans: D

189

Chapter 6

Topic: Transition state theory

70. Which will be true for any actual or potential nucleophilic substitution reaction?A) ∆ H° is positive.B) ∆ H° is negative.C) ∆ G‡ is positive.D) ∆ G° is positive.E) ∆ G° is negative.

Ans: C

Topic: Entropy changes

71. For the typical SN2 reaction

Y- + RX → RY + X-

it can be predicted that ∆ S‡ will be:A) positive.B) zero.C) negative.D) either positive or negative.E) unpredictable as to algebraic sign.

Ans: C

Topic: Free energy changes

72. The reaction,CH3Cl + OH− CH3OH + Cl−

H2O

has the following thermodynamic values at 27ºC: ∆ H = -75.3 kJ mol-1;∆ S = 54.4 J K-1 mol-1. What is the value of ∆ G for this reaction?

A) -73.8 kJ mol-1

B) -76.8 kJ mol-1

C) -59.0 kJ mol-1

D) +91.6 kJ mol-1

E) -91.6 kJ mol-1

Ans: E

190

Chapter 6

Topic: Activation energy

73. An increase in the temperature at which reaction is carried out increases:A) the collision frequency.B) the fraction of molecules with proper orientation.C) the fraction of molecules with energy greater than Eact.D) More than one of the aboveE) None of the above

Ans: D


74. The difference in the bond energies of reactants and the transition state of a reaction is designated by the notation:

A) ∆ H°B) ∆ H‡

C) ∆ G°D) ∆ G‡

E) ∆ S‡

Ans: B

Topic: Hammond-Leffler postulate

75. Which is a true statement concerning the transition state of the rate-determining step of an SN1 reaction?

A) Structurally, it closely resembles the carbocation intermediate.B) Both covalent bond-breaking and bond-making are occurring.C) Formation of the transition state is an exothermic reaction.D) Necessarily, the transition state has zero charge overall.E) More than one of the above

Ans: A

Topic: Stereochemistry of SN/E reactions

76. Which of the following reactions proceeds with inversion of configuration at the carbon bearing the leaving group?

A) SN2B) SN1C) E2D) E1E) All of these

Ans: A

191

Chapter 6

Topic: Predicting mechanism, stereochemistry of SN reactions

77. Consider the substitution reaction that takes place when (R)-3-bromo-3-methylhexane is treated with methanol. Which of the following would be true?

A) The reaction would take place only with inversion of configuration at the stereogenic center.

B) The reaction would take place only with retention of configuration at the stereogenic center.

C) The reaction would take place with racemization.D) No reaction would take place.E) The alkyl halide does not possess a stereogenic center.

Ans: C


78. Consider the substitution reaction that takes place when (R)-3-bromo-3-methylhexane is treated with sodium methoxide. Which of the following would be true?

A) An SN2 reaction would take place with inversion of configuration at the stereogenic center.

B) An SN1 reaction would take place with retention of configuration at the stereogenic center.

C) An SN1 reaction would take place with racemization of configuration at the stereogenic center.

D) An SN1 reaction would take place, accompanied by an E1 reaction, affording a complex mixture of products.

E) An E2 reaction would take place, during which the stereogenic center is lost.Ans: E


79. Consider the substitution reaction that takes place when (R)-3-iodo-3-methylheptane is treated with sodium acetate (CH3CO2Na). Which of the following would be true?

A) An SN2 reaction would take place with inversion of configuration at the stereogenic center.

B) An SN1 reaction would take place with retention of configuration at the stereogenic center.

C) An SN1 reaction would take place with racemization of configuration at the stereogenic center.

D) An SN1 reaction would take place, accompanied by an E1 reaction, affording a complex mixture of products.

E) An E2 reaction would take place, during which the stereogenic center is lost.Ans: D

192

Chapter 6


80. Which alkyl halide, when treated with sodium ethoxide in ethanol, would afford a product mixture consisting of more than one elimination product?

A) 1-bromo-3,3-dimethylpentaneB) 1-bromo-2,3-dimethylpentaneC) 2-bromo-3,4-dimethylpentaneD) 2-bromo-3,3-dimethylpentaneE) None of the above would yield more than one elimination product

Ans: C

Topic: Leaving groups, nucleophiles

81. Which SN2 reaction will occur most rapidly in a mixture of water and ethanol?A) I- + CH3CH2–Br → CH3CH2–I + Br-

B) I- + CH3CH2–Cl → CH3CH2–I + Cl-

C) I- + CH3CH2–F → CH3CH2–I + F-

D) Br- + CH3CH2–Cl → CH3CH2–Br + Cl-

E) Br- + CH3CH2–F → CH3CH2–Br + F-

Ans: A

Topic: Predicting mechanism, kinetics

82. Which SN2 reaction will occur most rapidly in aqueous acetone solution? Assume concentrations and temperature are the same in each instance.

A) HO- + CH3-Cl → CH3OH + Cl-

B) HO- + CH3CH2-Cl → CH3CH2OH + Cl-

C) HO- + (CH3)2CH-Cl → (CH3)2CHOH + Cl-

D) HO- + (CH3)3C-Cl → (CH3)3COH + Cl-

E) HO- + (CH3)3CCH2-Cl → (CH3)3CCH2OH + Cl-

Ans: A

Topic: Kinetics

83. Which alkyl halide would you expect to undergo an SN2 reaction most slowly?A) 1-bromohexaneB) 1-bromo-2-methylpentaneC) 1-bromo-3-methylpentaneD) 1-bromo-4-methylpentaneE) 1-bromo-2,2-dimethylbutane

Ans: E

193

Chapter 6

Topic: Kinetics, nucleophilicity

84. Which SN2 reaction would take place most rapidly?A)

OH− + CH3Cl CH3OH + Cl−25oC

H2O

B)

H3C O−

OH2O

25oC H3CO CH3

O

+ CH3Cl + Cl−

C)H2O + CH3Cl CH3OH2

+ + Cl−25oC

H2O

D)CH3OH

25oC H3CO

CH3

H

CH3OH + CH3Cl + Cl−

E)SH− + CH3Cl CH3SH + Cl−

25oC

H2O

Ans: E


85. Which nucleophilic substitution reaction would be unlikely to occur?A) HO− + CH3CH2–I → CH3CH2–OH + I−

B) I− + CH3CH2–H → CH3CH2–I + H−

C) CH3S− + CH3–Br → CH3S–CH3 + Br−

D) All of the above would be unlikely to occurE) None of the above would be unlikely to occur

Ans: B


86. Which nucleophilic substitution reaction would be likely to occur (although probably not in excellent yield, due to competing elimination) ?

A) HO− + CH3CH2CH2CHICH3 → CH3 CH2CH2CHOHCH3 + I−

B) Cl− + CH3CH2CH2CH2–OH → CH3CH2CH2CH2–Cl + OH−

C) CH3S− + CH3CH2CH2–OCH3 → CH3S–CH2CH2CH3 + OCH3

−

D) All of the above are likely to occurE) None of the above are likely to occur

Ans: B

194

Chapter 6


87. Which alkyl halide would you expect to react most slowly when heated in aqueous solution?

A) (CH3)3C-FB) (CH3)3C-ClC) (CH3)3C-BrD) (CH3)3C-IE) They would all react at the same rate.

Ans: A

Topic: Nucleophilicity, leaving groups

88. Which SN2 reaction would you expect to take place most rapidly? Assume that the concentrations of the reactants and the temperature are the same in each instance:

A) CH3S− + CH3I CH3SCH3 + I −

B)

H3CS

CH3

H

CH3SH + CH3I + I −

C) CH3O− + CH3I CH3OCH3 + I −

D)

H3CO

CH3

H

CH3OH + CH3I + I −

E) CH3S− + CH3Cl CH3SCH3 + Cl −

Ans: A


89. Which nucleophilic substitution reaction is not likely to occur?A) I− + CH3CH2–Cl → CH3CH2–I + Cl−

B) I− + CH3CH2–Br → CH3CH2–I + Br−

C) I− + CH3CH2–OH → CH3CH2–I + OH−

D) CH3O− + CH3CH2–Br → CH3CH2–OCH3 + Br−

E) OH− + CH3CH2–Cl → CH3CH2–OH + Cl−

Ans: C

195

Chapter 6


90. Which of the following would be most reactive in an SN2 reaction?A) Br

B)

BrC)

BrD) Br

E)

BrAns: D

Topic: Steric hindrance

91. Which alkyl chloride, though primary, is essentially unreactive in SN2 reactions?A) Cl

B)

Cl

C) Cl

D) Cl

E)Cl

Ans: E

196

Chapter 6

Topic: Kinetics

92. Which alkyl halide would be most reactive in an SN1 reaction?

Br Br

BrBr

Br

IV V

IIIIII


Ans: D

Topic: Predicting mechanisms

93. Which of these compounds would give the largest E2/SN2 product ratio on reaction with sodium ethoxide in ethanol at 55°C?

A) Cl

B)

Cl

C) Cl

D)

ClE)

Cl

Ans: D

197

Chapter 6

Topic: Multistep synthesis, acid-base reactions, SN2

94. What final product is likely to be obtained through the following series of reactions?

NaNH2 ABr

NH3(l)B

Ans:

Topic: Multistep synthesis, acid-base reactions, SN2, hydrogenation


NaNH2 ACl

NH3(l)

excess H2B C

Ni

Ans:

Topic: Multistep synthesis, acid-base reactions, SN2


OH NaH

diethyletherA

ClB

Ans: O

SHORT ANSWER QUESTIONS

Topic: Bond length, bond strength, leaving groups

97. As we go down Group 7A of the periodic table, the size of the halogen atom increases; accordingly, the carbon-halogen bond length gets _______________ and the bond strength gets _______________.Ans: longer; weaker

198

Chapter 6

Topic: Transition state theory

98. In the SN2 reaction, the unstable arrangement of atoms in which both the nucleophile and the leaving group are partially bonded to the same carbon atom is called the ______________________.Ans: transition state

Topic: Solvolysis

99. A reaction in which the nucleophile is a molecule of the solvent is referred to as a ______________ reaction.Ans: solvolysis

Topic: Carbocation stability

100. The substitution mechanism whose rate depends primarily on the relative stability of the intermediate carbocation is the _________.Ans: SN1

Topic: Steric hindrance

101. The substitution mechanism whose rate depends primarily on the degree of steric hindrance around the leaving group is the _________.Ans: SN2

Topic: Carbocation stability

102. The stabilizing effect of alkyl substituents on carbocations can be explained through _____________________.Ans: hyperconjugation

Topic: Activation energy, kinetics

103. Increasing the concentration of either of the reactants of an SN2 reaction increases the rate of the reaction. The primary reason for this is that increasing the concentration increases ________________________.

Ans: the frequency of collisions between reactant species, thereby statistically increasing the chance encounters between species having the requisite Eactivation

Topic: Activation energy, transition state theory

104. In order for colliding species to react, they must _____________ and ____________.Ans: have energy equal/greater than the activation energy, approach with the proper

orientation.

199

Chapter 6


105. Draw the potential energy diagram that represents an exothermic reaction between a tertiary alkyl halide and methanol. Briefly explain your rationale.Ans: The reaction is likely to occur via an SN1 mechanism. The first step is the rate-

determining step, leading to the formation of a tertiary carbocation. The second step is a fast step, where the methanol oxygen acts as a nucleophile, attaching to the positively charged carbon, forming a protonated ether; the last step is a fast loss of a proton to form the (uncharged) ether product.

Energy

time

Topic: Predicting reaction mechanisms

106. Give a detailed reaction mechanism for the reaction expected to occur when 2-bromo-2-methylpentane is heated with sodium methoxide. Draw clear structural formulas of all relevant species and use curved arrows to represent electron flow. Also indicate which step is likely to be rate-determining.Ans: The reaction is likely to occur via an E2 mechanism. The first step is the rate-

determining step, leading to the formation of a tertiary carbocation, followed by base promoted elimination to afford 2 different alkene products.

Br H

H

:OCH3

..

..

..: :

slow

fast[ - Br: ]..

..

200

Chapter 6

Topic: SN1 kinetics

107. Typically, increasing the concentration of the nucleophile of an SN1 reaction has no impact on the rate of the reaction. The reason for this is that ________________________.

Ans: An SN1 reaction is a multi-step process; the overall reaction rate is determined by the rate of formation of the carbocation intermediate. Since the nucleophile is not involved in the rate-determining step, changes in its concentration typically have no impact on the rate of reaction.

Topic: Nucleophilicity, solvent effects

108. The relative nucleophilicity of the halide ions in polar aprotic solvents is observed to be markedly different from that in protic solvents. Explain briefly.Ans: The smaller, more highly electronegative fluoride ion is more effectively solvated

in a protic solvent and is therefore a weaker nucleophile compared to the larger, less electronegative and more polarizable iodide ion. The situation is reversed in aprotic solvents, where the nucleophile is essentially unsolvated; thus, the nucleophilicity parallels the basicity of the halide ions, so that the fluoride ion is the strongest nucleophile of the halide ions

Topic: Acid-base reactions, intramolecular SN2

109. When 5-bromo-1-pentanol is treated with sodium hydride in diethyl ether, the product is analyzed to be C5H10O. Propose a likely structure for this product, suggesting a reasonable mechanistic pathway for its formation.Ans: The alcohol is likely to be first deprotonated by the strong base, generating an

alkoxide ion, which can then undergo intramolecular SN2 reaction to afford the observed product, a cyclic ether.

Cl OHNaH

Cl O Na+

−H2

O NaCl....

..

.. :

201

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54968550 Practice Problems for Chapter 6 With Answers 1[1]