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Organic Chemistry - Janice Gorzynski Smith |
Spectroscopy & Structure Elucidation
Practice Exam
150 IR, NMR, MS, DBE, and structure-identification questions with
explanations
QUESTIONS FORMAT COVERAGE
150 A-D + Explanations Spectroscopy
Original practice questions for independent study and exam preparation.
,Spectroscopy & Structure Elucidation Practice Exam
Practice Questions
Choose the best answer for each item. The answer and a detailed explanation follow each question.
IR spectroscopy
1. Which statement best describes a strong IR absorption near 1715 cm-1?
A. only an O-H group is indicated
B. an alkane C-H stretch is indicated
C. a carbonyl group is likely present
D. a nitrile is the only possibility
Answer: C
Explanation: Many aldehydes and ketones show a strong C=O stretching absorption near 1715 cm-1. O-H stretches occur much higher
and are broad, while nitriles absorb near 2200-2260 cm-1. The intensity and position make a carbonyl assignment reasonable.
IR spectroscopy
2. Which statement best describes a very broad IR band from roughly 2500-3300 cm-1 together with a strong
carbonyl absorption?
A. a simple ether is suggested
B. an alkane is suggested
C. a carboxylic acid is strongly suggested
D. a nitrile without O-H is suggested
Answer: C
Explanation: Carboxylic-acid O-H stretches are unusually broad and often extend through the 2500-3300 cm-1 region. The same
molecule also shows a strong C=O stretch. The combination is much more diagnostic than either feature alone.
IR spectroscopy
3. Which statement best describes a sharp IR absorption around 2250 cm-1?
A. a nitrile group is consistent with the spectrum
B. only alkane C-H is indicated
C. an alcohol O-H is indicated
D. a simple ketone carbonyl is indicated
Answer: A
Explanation: The C#N stretching vibration of a nitrile commonly appears near 2200-2260 cm-1. Alcohol O-H absorption is broad and
much higher in frequency. Simple carbonyl absorption appears closer to 1700-1750 cm-1.
IR spectroscopy
4. Which statement best describes a broad IR band near 3200-3600 cm-1 with no strong carbonyl band?
A. an alcohol is consistent with the spectrum
B. a ketone is required
C. an ester is required
D. a nitrile is required
Answer: A
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,Spectroscopy & Structure Elucidation Practice Exam
Explanation: Hydrogen-bonded alcohol O-H stretching gives a broad feature in the 3200-3600 cm-1 region. Absence of a strong C=O
band argues against common ketones, esters, and carboxylic acids. A nitrile would instead show a sharper feature near 2250 cm-1.
IR spectroscopy
5. Which statement best describes a sharp IR band near 3300 cm-1 together with a band near 2100-2200 cm-1?
A. an ester is required
B. a terminal alkyne is plausible
C. a saturated alkane is most likely
D. a ketone is required
Answer: B
Explanation: Terminal alkynes show a sharp sp C-H stretch near 3300 cm-1 and a C#C stretch in the low 2000 cm-1 region. Internal
alkynes lack the terminal sp C-H feature. Carbonyl compounds would instead show a strong band near 1700 cm-1.
IR spectroscopy
6. Which statement best describes a strong IR C=O band near 1735-1750 cm-1 plus strong C-O bands in the
fingerprint region?
A. a nitrile is required
B. an alkane is most likely
C. an ester is plausible
D. an amine without carbonyl is required
Answer: C
Explanation: Esters contain both a carbonyl and C-O single bonds. Their C=O stretch is often somewhat higher in frequency than a
simple ketone. Strong C-O stretching features in the fingerprint region support the ester assignment.
IR spectroscopy
7. Which statement best describes two medium IR N-H stretching bands in the 3300-3500 cm-1 region?
A. a ketone is required
B. a tertiary amine is required
C. a primary amine is consistent
D. an ether is required
Answer: C
Explanation: Primary amines have two N-H bonds and can show two N-H stretching absorptions. Secondary amines often show one
N-H stretch, while tertiary amines have no N-H bond. The exact appearance varies, but the two-band pattern is a useful clue.
IR spectroscopy
8. Which statement best describes one N-H stretch in the 3300-3500 cm-1 region for an amine-like compound?
A. a secondary amine is consistent
B. an alkane is required
C. a tertiary amine with no N-H is required
D. a primary amine must be present
Answer: A
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, Spectroscopy & Structure Elucidation Practice Exam
Explanation: A secondary amine contains one N-H bond and commonly gives one N-H stretching absorption. A primary amine contains
two N-H bonds and often shows two bands. A tertiary amine lacks N-H stretching entirely.
IR spectroscopy
9. Which statement best describes an IR spectrum with no O-H or N-H band and a strong C=O near 1715 cm-1?
A. a ketone is a reasonable possibility
B. a primary amine is required
C. a carboxylic acid is required
D. an alcohol is required
Answer: A
Explanation: A ketone has a strong carbonyl stretch but no O-H or N-H bond. The absence of broad hydrogen-bonding bands is
therefore consistent with a ketone. Additional data would be needed to distinguish it from every other carbonyl class.
IR spectroscopy
10. Which statement best describes an IR spectrum with strong alkyl C-H bands but no major O-H, N-H, C=O, or
C#N absorptions?
A. a carboxylic acid is required
B. an amide is required
C. a simple hydrocarbon is plausible
D. a nitrile is required
Answer: C
Explanation: A hydrocarbon contains primarily C-C and C-H bonds, so its IR spectrum lacks the strong heteroatom-functional-group
signals listed. Alkyl C-H stretches generally appear near 2850-2960 cm-1. The absence of carbonyl, nitrile, and X-H bands supports a
hydrocarbon assignment.
1H NMR spectroscopy
11. Which statement best describes a 1H NMR signal near 9-10 ppm?
A. a typical alkyl methylene is most likely
B. a tert-butyl group is indicated
C. an aldehydic proton is a strong possibility
D. a simple methyl proton is most likely
Answer: C
Explanation: Aldehyde protons are strongly deshielded by the adjacent carbonyl and commonly resonate near 9-10 ppm. Simple alkyl
protons appear much farther upfield. This downfield position is one of the most recognizable proton-NMR clues.
1H NMR spectroscopy
12. Which statement best describes a 1H NMR signal around 10-13 ppm that is broad and exchangeable?
A. an ether methyl is required
B. a carboxylic-acid O-H proton is plausible
C. a nitrile carbon proton is indicated
D. an ordinary alkane proton is likely
Answer: B
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Organic Chemistry - Janice Gorzynski Smith |
Spectroscopy & Structure Elucidation
Practice Exam
150 IR, NMR, MS, DBE, and structure-identification questions with
explanations
QUESTIONS FORMAT COVERAGE
150 A-D + Explanations Spectroscopy
Original practice questions for independent study and exam preparation.
,Spectroscopy & Structure Elucidation Practice Exam
Practice Questions
Choose the best answer for each item. The answer and a detailed explanation follow each question.
IR spectroscopy
1. Which statement best describes a strong IR absorption near 1715 cm-1?
A. only an O-H group is indicated
B. an alkane C-H stretch is indicated
C. a carbonyl group is likely present
D. a nitrile is the only possibility
Answer: C
Explanation: Many aldehydes and ketones show a strong C=O stretching absorption near 1715 cm-1. O-H stretches occur much higher
and are broad, while nitriles absorb near 2200-2260 cm-1. The intensity and position make a carbonyl assignment reasonable.
IR spectroscopy
2. Which statement best describes a very broad IR band from roughly 2500-3300 cm-1 together with a strong
carbonyl absorption?
A. a simple ether is suggested
B. an alkane is suggested
C. a carboxylic acid is strongly suggested
D. a nitrile without O-H is suggested
Answer: C
Explanation: Carboxylic-acid O-H stretches are unusually broad and often extend through the 2500-3300 cm-1 region. The same
molecule also shows a strong C=O stretch. The combination is much more diagnostic than either feature alone.
IR spectroscopy
3. Which statement best describes a sharp IR absorption around 2250 cm-1?
A. a nitrile group is consistent with the spectrum
B. only alkane C-H is indicated
C. an alcohol O-H is indicated
D. a simple ketone carbonyl is indicated
Answer: A
Explanation: The C#N stretching vibration of a nitrile commonly appears near 2200-2260 cm-1. Alcohol O-H absorption is broad and
much higher in frequency. Simple carbonyl absorption appears closer to 1700-1750 cm-1.
IR spectroscopy
4. Which statement best describes a broad IR band near 3200-3600 cm-1 with no strong carbonyl band?
A. an alcohol is consistent with the spectrum
B. a ketone is required
C. an ester is required
D. a nitrile is required
Answer: A
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,Spectroscopy & Structure Elucidation Practice Exam
Explanation: Hydrogen-bonded alcohol O-H stretching gives a broad feature in the 3200-3600 cm-1 region. Absence of a strong C=O
band argues against common ketones, esters, and carboxylic acids. A nitrile would instead show a sharper feature near 2250 cm-1.
IR spectroscopy
5. Which statement best describes a sharp IR band near 3300 cm-1 together with a band near 2100-2200 cm-1?
A. an ester is required
B. a terminal alkyne is plausible
C. a saturated alkane is most likely
D. a ketone is required
Answer: B
Explanation: Terminal alkynes show a sharp sp C-H stretch near 3300 cm-1 and a C#C stretch in the low 2000 cm-1 region. Internal
alkynes lack the terminal sp C-H feature. Carbonyl compounds would instead show a strong band near 1700 cm-1.
IR spectroscopy
6. Which statement best describes a strong IR C=O band near 1735-1750 cm-1 plus strong C-O bands in the
fingerprint region?
A. a nitrile is required
B. an alkane is most likely
C. an ester is plausible
D. an amine without carbonyl is required
Answer: C
Explanation: Esters contain both a carbonyl and C-O single bonds. Their C=O stretch is often somewhat higher in frequency than a
simple ketone. Strong C-O stretching features in the fingerprint region support the ester assignment.
IR spectroscopy
7. Which statement best describes two medium IR N-H stretching bands in the 3300-3500 cm-1 region?
A. a ketone is required
B. a tertiary amine is required
C. a primary amine is consistent
D. an ether is required
Answer: C
Explanation: Primary amines have two N-H bonds and can show two N-H stretching absorptions. Secondary amines often show one
N-H stretch, while tertiary amines have no N-H bond. The exact appearance varies, but the two-band pattern is a useful clue.
IR spectroscopy
8. Which statement best describes one N-H stretch in the 3300-3500 cm-1 region for an amine-like compound?
A. a secondary amine is consistent
B. an alkane is required
C. a tertiary amine with no N-H is required
D. a primary amine must be present
Answer: A
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, Spectroscopy & Structure Elucidation Practice Exam
Explanation: A secondary amine contains one N-H bond and commonly gives one N-H stretching absorption. A primary amine contains
two N-H bonds and often shows two bands. A tertiary amine lacks N-H stretching entirely.
IR spectroscopy
9. Which statement best describes an IR spectrum with no O-H or N-H band and a strong C=O near 1715 cm-1?
A. a ketone is a reasonable possibility
B. a primary amine is required
C. a carboxylic acid is required
D. an alcohol is required
Answer: A
Explanation: A ketone has a strong carbonyl stretch but no O-H or N-H bond. The absence of broad hydrogen-bonding bands is
therefore consistent with a ketone. Additional data would be needed to distinguish it from every other carbonyl class.
IR spectroscopy
10. Which statement best describes an IR spectrum with strong alkyl C-H bands but no major O-H, N-H, C=O, or
C#N absorptions?
A. a carboxylic acid is required
B. an amide is required
C. a simple hydrocarbon is plausible
D. a nitrile is required
Answer: C
Explanation: A hydrocarbon contains primarily C-C and C-H bonds, so its IR spectrum lacks the strong heteroatom-functional-group
signals listed. Alkyl C-H stretches generally appear near 2850-2960 cm-1. The absence of carbonyl, nitrile, and X-H bands supports a
hydrocarbon assignment.
1H NMR spectroscopy
11. Which statement best describes a 1H NMR signal near 9-10 ppm?
A. a typical alkyl methylene is most likely
B. a tert-butyl group is indicated
C. an aldehydic proton is a strong possibility
D. a simple methyl proton is most likely
Answer: C
Explanation: Aldehyde protons are strongly deshielded by the adjacent carbonyl and commonly resonate near 9-10 ppm. Simple alkyl
protons appear much farther upfield. This downfield position is one of the most recognizable proton-NMR clues.
1H NMR spectroscopy
12. Which statement best describes a 1H NMR signal around 10-13 ppm that is broad and exchangeable?
A. an ether methyl is required
B. a carboxylic-acid O-H proton is plausible
C. a nitrile carbon proton is indicated
D. an ordinary alkane proton is likely
Answer: B
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