C HAPTER 13 + 14 HW: S PECTROSCOPY
INFRARED (IR) SPECTROSCOPY
1. A compound containing an NH2 group should have two peaks in its IR spectrum in the 3100-3500 cm-1 range.
Explain why there are two absorbances in this region, and use a drawing to show the type of vibration that
produces each peak.
2. The following IR spectra are of the C5H12O2 isomers below. Match each spectrum to the correct structure.
O
O
O
A
CH3
O
OH
OH
B
C
3. The following IR spectrum is of 1-hexene. Use arrows on the structure to indicate a possible vibration for
each specified wavenumber.
Signal at 3105 cm-1
H
3105
1650
C
H
C
H
H
H
H
H
C
C
C
C
H
H
H
H
Signal at 1650 cm-1
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H
C
H
H
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4. The following IR spectrum is of 3-pentanone. Use arrows on the structure to indicate a possible vibration for
each specified wavenumber.
Signal at 2997 cm-1
H
1712
2997
H
H
O
H
H
C
C
C
C
C
H
H
H
H
Signal at 1712 cm-1
H
H
H
O
H
H
C
C
C
C
C
H
H
H
H
H
H
5. Which of the following structures would produce the IR spectrum below?
O
OH
A
B
C
D
GENERAL CONCEPTS OF NMR
6. Explain in general what happens inside an NMR instrument when a compound absorbs radiation of the radio
frequency.
7. Briefly explain why deuterated solvents are used in preparing samples for NMR analysis.
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CHEMICAL EQUIVALENCE
8. How many signals should ideally be present in the 1H NMR spectrum of each compound (how many different
hydrogen environments are present)? Use labels (a,b,c…) to show which hydrogen atoms should be grouped
together.
O
a.
c.
b.
O
O
NH2
CH3
O
e.
d.
f.
H
H2N
CH3
CHEMICAL SHIFT
9. Use a discussion of shielding to explain why hydrogen “A” absorbs at a higher frequency than hydrogen “B”
in an NMR instrument. Use diagrams with your answer.
Cl
A
H
B
H
C
C
H
Cl H
10. In each pair, which of the hydrogen atoms nearest the asterisk (*) should show up the furthest left (downfield)
in the 1H NMR spectra? Briefly explain each comparison.
a.
*
vs.
O
*
CH3
Br
b.
*
Br
vs.
Br
*
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11. Using an NMR correlation chart, assign the signals in the 1H NMR spectrum (a-c) to hydrogen atoms in the
structure. (Note: signal “a” contains groupings that coincidentally overlap.)
a
O
c
b
O
8
6
4
PPM
2
0
SIGNAL SPLITTING
12. For the carboxylic acid below…
a. What is the expected splitting (singlet, doublet, etc.) for each hydrogen atom?
Signal
Splitting
Ha
Hb
Hc
Br
a
H
b
H
O
C
C
C
c
OH
Br H
b. Using the nuclear orientation of the neighbors, explain the origin of the splitting for hydrogen atoms a +b.
Splitting for Hb:
Splitting for Ha:
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13. For the molecule below….
O
b
e
a
c
d
a. What is the expected splitting (singlet, doublet, etc.) for each hydrogen atom?
Signal
Ha
Hb
Hc
Hd
He
Splitting
b. Using the nuclear orientation of the neighbors, explain the origin of the splitting for hydrogen “d”.
c. Sketch a drawing of the splitting pattern for the following hydrogen atoms as accurately as possible. In
each, state the relative heights of the lines in each signal (for example the lines of a triplet will be in the
ratio of 1:2:1).
Signal
Ha
Hb
Hd
Splitting
drawing
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DRAWING 1 H NMR SPECTRUM FROM STRUCTURE
14. Draw the expected 1H NMR spectrum for each compound, paying attention to chemical shift and splitting.
Mark the integration of each peak, then assign each peak in the NMR to hydrogen atoms in the structure.
a.
O
H3C
C
CH2
CH3
7
8
5
4
3
2
1
0 ppm
6
5
4
3
2
1
0 ppm
6
5
4
3
2
1
0 ppm
6
5
4
3
2
1
0 ppm
6
b.
H3C
H3C
CH3
C
O
CH2
CH3
7
8
c.
Br
H3C
C
CH3
H
7
8
d.
Br
Br
8
CH
CH2
7
CH2
Br
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DEGREES OF UNSATURATION
15. Calculate the degrees of unsaturation associated with each chemical formula. Then circle the structures
beside it that are possible for the formula (which match the degrees of unsaturation).
a. C5H10
OH
b. C6H10O2
O
O
OH
O
O
H3CO
OH
Br
c. C7H6Br2
Br
Br
Br
Br
NH2
OH
d. C5H9NO
Br
CH3
Br
NH2
Br
NH2
O
H2N
O
O
DETERMINING STRUCTURE FROM AN 1 H NMR SPECTRUM
16. Draw the structure that matches the molecular formula and would produce the 1H NMR spectrum for each
problem below. Then assign all peaks in the NMR spectrum (use labels a,b,c) to hydrogen atoms in the
structure.
C9H12
6H, d
5H total
1H, m
8
6
4
PPM
2
0
C3H6O
3H, t
2H, quint.
1H, t
10
8
6
PPM
4
2
0
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17. The following four 1H NMR spectra correspond to four different alcohols with a molecular formula of
C5H12O. Determine which alcohol corresponds to which spectrum, then assign the peaks in the spectrum to
hydrogen atoms in each structure.
2H, t
3H, t
4H, m
2H, m
1H, s
5
4
3
PPM
2
1
0
3H, d
3H, t
4H, m
1H, sext.
1H, s
5
4
3
PPM
2
1
0
6H, s
1H, s
5
2H, q
4
3
PPM
2
3H, t
1
0
6H, d
2H, t
2H, q
1H, m
1H, s
5
4
3
PPM
2
1
0
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18. Draw the structure that matches the molecular formula and would produce the 1H NMR spectrum for each
problem below. Then assign all peaks in the NMR spectrum (use labels a,b,c) to hydrogen atoms in the
structure.
C4H8O2
3H
3H
2H
5
4
3
2
1
0
PPM
C7H7Cl
3H
2H 2H
8
7
6
5
4
PPM
3
2
1
0
C7H13 BrO
9H, s
2H, t
4
2H, t
3
2
PPM
1
0
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18 continued
C9H13 N
3H, s
2H, t
5H, m
8
1H, s
6
2H, t
4
PPM
2
C4H11 N
0
3H
3H
2H
2H
1H
3
2
1
0
PPM
C8H9BrO
3H, d
2H, d 2H, d
1H, q
1H, s
8
6
PPM
4
2
0
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CARBON-13 NMR SPECTROSCOPY
19. List two differences between 13C NMR and 1H NMR spectroscopy, or the spectra they produce.
20. Determine the total number of signals present in the 13C spectrum of each compound. Then use a correlation
chart to indicate the approximate frequency in ppm of each carbon pointed to with an arrow.
Cl
Compound
H3C
O
O
O
Cl
Number of signals in 13C
NMR
Approximate ppm of
indicated carbon
21. An unknown is narrowed down to be one of the three structures below. How can the 13C NMR spectrum of
the unknown be used to definitely determine the compound’s structure?
OH
OH
HO
OH
OH
OH
22. Assign all peaks in the 13C NMR spectrum (use labels A,B,C) to carbon atoms in the structure.
OCH3
O
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COMBINED SPECTROSCOPY PROBLEMS
23. Below are 5 possible structures with a formula of C3H6O2. Identify which of the molecules A-E “fit” each
piece of data. In many cases more than one compound matches each piece of data.
O
O
OH
O
H
A
O
H
OH
O
C
B OH
O
CH3
H
D
O
E
Data
Which match?
The IR spectrum contains a large, broad peak around 3300 cm-1.
The 1H NMR spectrum has a signal at 9.5 ppm.
The IR spectrum contains a strong peak at 1745 cm-1.
(Assume this signal represents exactly what it suggests from the correlation chart.)
All signals in the 1H NMR spectrum are singlets.
The 1H NMR spectrum contains a quartet integrating to 2H at 4.0 ppm.
24. Which compound from the previous problem (A-E) would have the following IR signals and 1H NMR
spectrum?
After identifying the compound, assign the IR signals to bonds in the molecule (fill in the table), and assign
each peak in the 1H NMR spectrum to hydrogen atoms in the structure.
Key IR signals:
Frequency (cm-1)
Intensity
3210 to 3420
strong +broad
2955
strong
1725
strong
Bonds represented
2H, t
1H, t
2H, q
1H
10
9
8
7
6
5
PPM
4
3
2
1
0
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25. Use the chemical formula and given spectra (1H NMR, 13C NMR and/or IR spectra) to determine the structure
of each compound. Then assign each peak in the 1H NMR spectrum to hydrogen atoms in the structure.
a. C8H6O2
1H
NMR
4H, s
2H, s
10
13C
8
6
PPM
4
2
0
NMR
200
150
100
PPM
50
0
IR:
b. C5H12O
13C
NMR
80
1H
60
40
PPM
20
0
NMR
6H, t
4H, quint.
1H, quint.
1H, s
5
4
3
PPM
2
1
0
Page 13
c. C5H8Br2O
d. C10H15N
6H, t
4H, q
2H, d
2H, t
1H, t
8
7
6
5
4
PPM
3
2
1
0
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e. C12H16O2
3450
3050
2980
1740
6H, d
2H, s
2H, d
5H, m
1H, m
8
7
6
5
4
PPM
3
2
1
0
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