chemistry 531 midterm examination i fall 2018

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Chemistry 531 Midterm Examination I

Fall 2018

Name:

The following exam contains 10 pages. Please make sure that they are all accounted for. Answer each question in the spaces provided. Show all work and provide complete explanations. A periodic table is shown below. Additional useful tables of information are attached to the end of this exam. A calculator and molecular model set may be used. The exam will conclude at 8 pm

Page Points Possible Points Earned 2 22 3 20 4 18 5 21 6 13 7 6 8 6 bonus points Total

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1) Provide an approximate pKa value for each of the indicated protons. (1 point each)

2) Determine the hybridization for each of the indicated atoms. (1 point each)

3) In what orbitals do each of the two sulfur lone pairs reside in thiophene (molecule 3, question 2)? (2 points)

4) For each set of molecules shown, circle the one that is lower in energy. Then, provide a brief explanation for your choice. (3 points each)

5) Classify the following cyclization using Baldwin’s rules. (2 points)

OHH

O HNH3

H

N HO

OS N C

O

and

N NF

Fand

N NF F

and

a.

b.

c.

HO O CH3O O

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6) The strain energy for cyclopropane is 27.5 kcal/mol while the strain energy for spirocyclopropane is 65 kcal/mol. Explain the following:

a. What accounts for the strain in cyclopropane? (4 points)

b. Why is the strain energy of spirocyclopropane more than twice the strain energy of cyclopropane? (4 points)

7) Rank the following chair conformations from least stable to most stable relative to their equatorial conformations. (4 points)

8) Consider the following acid/base reaction.

a. Circle the set of equilibrium arrows that best represents the equilibrium. (2 points) b. Estimate the Keq: (2 points)

c. Estimate the DGrxn: (2 points)

d. Explain why the pyridinium ion is a stronger acid than the ammonium ion. (2 points)

27.5 kcal/mol 65 kcal/mol

OOO

CH3 CH3 CH3

X Y Z

< <

LeastStable

<

MostStable

O

CH3

W

+N H HN +N

H2N

pKa 5 pKa 10

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9) Knowing that the A-value for isopropyl is 2.21 kcal/mol and the A-value for OH is 0.6 kcal/mol, determine the strength of the intramolecular hydrogen bond in kcal/mol. The DG for the conformational interconversion is -0.9 kcal/mol. (4 points)

10) Consider the reaction shown below and answer the following questions.

a. Using the attached table of homolytic BDEs, estimate the DHrxn. (3 points)

b. Assuming a negligible entropy (DG = DH), estimate the Keq of the reaction. (2 points)

c. In reality, would you expect a negligible entropy? If not, would you expect DS to be positive or negative? (2 points)

d. What effect would raising the temperature have on the reaction? (2 points)

11) Norbornane has an experimental heat of formation DHf = -12.4 kcal/mol. Use the attached table of group increment values to estimate the heat of formation of norbornane. Then determine the amount of strain energy in this bicyclic molecule. (5 points)

OH

OH

OH

O H

ΔG = -0.9 kcal/mol

+ O O

Experimental ΔHf = -12.4 kcal/mol

Estimated ΔHf =

Strain Energy =

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12) For each set shown below, circle the: (3 points each)

a) compound that is the most basic

NH NH

b) conformation that is lower in energy

HH

FHH

FFH

HHH

F

c) resonance structure that is the major contributor to the resonance hybrid

H2N H2N H2N

d) C-X bond with the highest energy filled HOMO

H3C F H3C Cl H3C Br H3C I

e) reaction that occurs at the faster rate

OTsSiMe3

OTsMe3Si

EtOH

EtOH OEtMe3Si

OEtSiMe3

f) reaction that occurs at the faster rate

OAcNaOH

H2O

OH

OAcNaOH

H2O

OH

g) the more stable species in a polar solvent

NH4H2N

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13) Predict the major product with correct stereochemistry for the following reaction. Use the template provided and complete the structure by adding the appropriate dashed and wedged groups. (4 points)

14) Consider the SN2 reaction shown below and answer the following questions.

a. Draw a frontier molecular orbital diagram that describes the reaction. (3 points)

b. Use stereoelectronic effects to explain why the reaction proceeds with inversion. (3 points)

c. Explain why this reaction would be less favorable if I were replaced with F. (3 points)

15) Explain why water reacts with an enolate at the oxygen while CH3I reacts at the a-carbon. (4 points)

H

OtBu

Me

1. EtLi2. H2O tBu Et

H3C SNa + IH3C

DH H3CS

CH3

DH + NaI

O

H2O

H3C-I

OH O

OCH3

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16) Consider the diastereomeric acetonides shown below. One exists predominately in the chair conformation while the other exists in the twist boat conformation. Which one exists as the chair and which one exists as the twist boat? Explain your reasoning. (6 points)

O O

HH

O O

H H

Compound A Compound B

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Bonus Questions These questions were all taken directly from your homework problems. (2 bonus points each)

17) Discuss the hybridization in the C-C and C-H bonds of cyclopropane given that the H-C-H bond angle is 118 °. [Question 1.7]

18) Provide an explanation for why cyclopentene is less strained than cyclopentane. [Question 2.26]

19) The pKa’s of alkanols are typically in the range of 16-17. However, the pKa’s of vicinal diols are commonly 14-16, and those for geminal diols are in the range of 13-14. Explain this trend. [Question 5.14]

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H3C H

H

H

H

H

H

H

H

H

H3C CH3

H3C F

H3C Cl

H3C Br

H3C I

H3C OH

H3C OCH3

H3C NH2

F F

Cl Cl

Br Br

I I

HO OH

H OH

CC

C C

C O H H

Homolytic Bond Dissociation Energy Values

R R’ R + R’

C-H Bonds

105

98

95

93

110

111132

86

*All values in kcal/mole

88

C-C and C-Z Bonds

90

77

85

92

83

110

85

71

57

C=C and C=Z Bonds

172(2nd Bond 88)

148(2nd Bond 67)

193(3rd Bond 46)

Z-Z Bonds

104

51

H F

H NH2H Cl

H Br

H I

136

103

87

71

38

58

46

36

119

93

H-Z Bonds

H3C SH 74

86

88

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Benson’s Group Increments

chemistry 531 midterm examination i fall 2018

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