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Page 1 of 10
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
Page 2 of 10
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
Page 3 of 10
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
Page 4 of 10
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 =
Page 5 of 10
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
Page 6 of 10
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
Page 7 of 10
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
Page 8 of 10
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]
Page 9 of 10
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
Page 10 of 10
Benson’s Group Increments