S343 Midterm Exam
10/14/14
Name_____________________________________
AI or Section_______________________________
This exam contains 10 questions on 14 pages worth 100 points.
1. _______ (15pts + 2 bonus)
2. _______(15pts)
3. _______ (10pts)
4. _______(12pts)
5. _______(8pts)
6. _______(8pts)
7. _______(6pts)
8. _______(9pts)
9. _______(10pts)
10. _______(10pts)
Total:
1
1. (15pts) Answer the questions on the next page based on these data.
Two students conducted the following experiment, similar to the elimination reaction you
studied.
A team of two students carried out the following two elimination reactions which both yielded a
mixture of 2-methyl-1-hexene (bp 92 oC) and 2-methyl-2-hexene (bp 95 oC.) The mix of alkene
products was then collected by simple distillation. A small drop of the alkene mixture was
dissolved in nonane and then analyzed by gas chromatography. It was determined that 2-methyl2-hexene has a retention time of about 3.21 minutes and 2-methyl-1-hexene has a retention time
of about 2.23 minutes.
GC data for Experiment A:
Peak
Retention time (min)
A
1.89
B
2.25
C
3.20
D
3.76
GC data for Experiment B:
Relative area
2000
1600
400
20000
Peak
A
B
C
D
Relative area
4000
1800
200
40000
Retention time (min)
1.89
2.25
3.20
3.76
Bonus: Two of the peaks are due to the alkenes. What compounds most likely lead to the
other two peaks?
2
A. What is the alkene product distribution for Experiment A?
B. Propose a mechanism for Experiment A, and explain the observed selectivity in the product
distribution based on the mechanism.
C. Propose a mechanism for Experiment B. Is it more or less selective than A? Explain.
D. Would TLC data have been useful to draw these conclusions? Explain.
E. These data could have been obtained by separating the two alkene products by column
chromatography and getting the mass of each alkene product. Why was GC done instead?
3
2. (15pts) Use these data to answer the questions on the next page.
Fran Chemist conducted an extraction experiment just like yours, examining the best solutions to
separate fluorene (target compound) from four impurities:
She also looked up these melting point data: benzoic acid, 122oC; ethyl 4-aminobenzoate, 88-90
o
C; fluorene,113-115 oC ; fluorenone,82-84 oC ; succinimide125-127 oC.
These are the melting point data she collected for each compound after extracting each 50/50
mixture of fluorene and impurity with solution from bottles labeled “dilute aqueous HCl”, “dilute
aqueous NaOH”, and “neutral water.”
Ethyl aminobenzoate
Benzoic acid
fluorenone
Succinimide
Neutral
88-90 oC
78-90 oC
80-84 oC
112-114 oC
Acid
110-113 oC
88-95 oC
79-82 oC
112-114 oC
base
75-92 oC
83-89 oC
80-85 oC
113-115 oC
4
A. Explain how melting point data is useful in determining the success of each extraction.
B. Using Fran’s data, mark the extractions which worked to purify the target compound with an
“X”:
A.
B.
C.
D.
flourenone:
benzoic acid:
Succinimide:
Ethyl 4-aminobenzoate
_____ Aq. HCl
_____ Aq. HCl
_____ Aq. HCl
_____ Aq. HCl
_____ Aq. NaOH _____Water _____None
_____ Aq. NaOH _____Water _____None
_____ Aq. NaOH _____Water _____None
_____ Aq. NaOH _____Water _____None
C. Would recrystallization have worked in the place of melting point to determine the
effectiveness of the extraction? Explain.
D. Fran Chemist recognizes that these data are not what she expected based on solubility
principles. She thinks that the aqueous solutions either had switched labels or that one of the
bottles was mislabeled. Which piece(s) of data caused her to think this?
E. Based on these data, what was the most likely error with the aqueous extraction solutions?
Explain.
5
3. (10pts) Answer the questions below based on these TLC plates. Three authentic compounds,
labeled A, B, and C were developed on Plate #1. The same three compounds were developed
under different conditions on plate #2. Two unknowns, labeled X and Y, were developed on
plate #3. Plate #3 was developed under the same conditions as plate #1.
Plate #1
Plate #2
Plate #3
B
A
C
A
B
C
A. Plate #1 has three lanes with three pure compounds, shown below. Indicate which compound
is most likely in lane A, lane B, and lane C of plate #1.
B. The same three compounds were run on plate #2. If plate #1 was developed in 1:1
hexanes:ethyl acetate, it is likely that plate #2 was run
A. in 2:1 hexanes:ethyl acetate
B. for a shorter time
C. in 1:2 hexanes:ethyl acetate
D. for a longer time.
C. Label the following statements as “True’ or “False.”
________Lane X is streaky because it contains more than one compound.
________Lane Y is was spotted with a solution of at least two compound, most consistent with
compounds A and B.
________If column chromatography were to be performed to separate compounds A, B, and C,
one should use the solvent system used in plate #1 rather than the solvent system of plate #2.
6
4. (12pts) The questions below refer to this Mass Spectrum of 1-phenylpentan-1-one (M+ =162):
A. What causes the small peak at m/z = 163?
B. Provide a mechanism and fragment structure that lead to formation of the base
peak at m/z = 105.
C. Provide a mechanism and fragment structure that lead to formation of the significant peak at
m/z = 120.
7
5. (8pts) What would you look for in the mass spectrum that would indicate each of these:
A. The molecule contains a nitrogen atom
B. The molecule contains a chlorine atom
C. The molecule contains two bromine atoms
D. The molecule contains a benzyl group
6. (8pts) For each of the indicated bonds, give the approximate IR stretch frequency and a
description of the IR stretch as weak (w), medium (m), strong (s) or not observed (no).
8
7. (6pts) A. Does an amide carbonyl have a higher or lower frequency IR stretch than a ketone?
Explain why using physical principles.
B. Does an ester carbonyl have a higher or lower frequency IR stretch than a ketone? Explain
why using physical principles.
8. (6pts) On the following page are three IR spectra. Draw the structure of the compound
below that best matches each IR spectrum.
9
8A Structure:
8B Structure:
8C Structure:
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9. (10pts) Given the following IR and MS data for this C9H10O2 compound, answer the
questions on the following page.
m/z
abundance
27.0
3.8
28.0
1.0
29.0
3.3
39.0
1.2
45.0
1.5
50.0
5.4
51.0
16.0
52.0
1.0
74.0
1.6
75.0
1.2
76.0
2.4
77.0
40.6
78.0
4.0
105.0
100.0
106.0
9.1
122.0
30.7
123.0
2.6
135.0
1.0
149.0
1.7
150.0 (M+) 21.8
151.0
2.3
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Problem 9. Show work for partial credit.
Degrees of unsaturation:
Key pieces of structure based on IR and MS:
Final Structure:
Structure of base peak fragment:
12
10. (10pts) Given the following IR and MS data, propose a structure and answer the questions
on the following page.
m/z
abundance
14.0
2.1
15.0
7.1
17.0
1.6
18.0
7.0
25.0
1.3
26.0
13.0
27.0
5.2
28.0
4.9
29.0
11.2
30.0
1.5
31.0
100.0
32.0
1.1
38.0
2.6
39.0
3.0
40.0
5.9
41.0
66.0
42.0
7.9
43.0
1.7
51.0
5.5
52.0
10.4
53.0
11.7
54.0
1.3
71.0 (M+) 0.1
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Problem 10. Show work for partial credit.
Molecular formula:
Key pieces of structure based on IR and MS:
Final Structure:
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