IR Quiz
Questions: 16 · 10 minutes
1. Two compounds both show a strong C=O absorption. Which additional feature most clearly favors an aldehyde over a ketone?
Two N–H stretching bands around 3300-3500 cm⁻¹
A broad O–H band extending from about 2500 to 3300 cm⁻¹
A sharp C≡N band near 2250 cm⁻¹
One or two weak aldehydic C–H bands near 2720 and 2820 cm⁻¹
2. Which pattern best supports the presence of a terminal alkyne?
A sharp ≡C–H stretch near 3300 cm⁻¹ together with a C≡C absorption around 2100-2260 cm⁻¹
A strong band near 1715 cm⁻¹ with weak bands near 2720 cm⁻¹
A broad band at 3200-3600 cm⁻¹ with a strong band near 1050 cm⁻¹
Two N–H stretches around 3300-3500 cm⁻¹ and no triple-bond-region absorption
3. Which set of absorptions most strongly supports an aromatic ring?
Bands near 1600 and 1500 cm⁻¹ plus aromatic C–H stretching just above 3000 cm⁻¹
A very broad band from 2500 to 3300 cm⁻¹ plus a strong C=O band
A sharp band near 2250 cm⁻¹ plus no C–H stretching bands
Two N–H bands near 3400 cm⁻¹ plus a strong C–O band
4. An unknown spectrum has two distinct N–H stretching absorptions in the 3300-3500 cm⁻¹ region. Which group is the best match?
Tertiary amine
Primary amine
Alcohol
Secondary amine
5. A compound shows a strong, sharp absorption near 1715 cm⁻¹ but no broad O–H band. Which functional group is the best match?
Alcohol
Ketone carbonyl
Nitrile
Primary amine
6. How does conjugation with a C=C bond usually affect a carbonyl stretching frequency?
It eliminates the C=O absorption because the bond becomes nonpolar
It generally shifts the C=O absorption above 2200 cm⁻¹
It splits the C=O absorption into two N–H-like peaks
It generally shifts the C=O absorption to a lower wavenumber
7. Which pair of IR features most strongly supports a carboxylic acid?
A sharp band near 2250 cm⁻¹ and no carbonyl band
Two narrow N–H bands near 3400 cm⁻¹ and a band near 1200 cm⁻¹
A strong C=O band near 1700 cm⁻¹ and a very broad O–H envelope around 2500-3300 cm⁻¹
Only sp³ C–H stretches around 2850-2960 cm⁻¹
8. A hydrocarbon spectrum shows only sp³ C–H stretches around 2850-2960 cm⁻¹ and bending absorptions near 1465 and 1375 cm⁻¹. It has no bands for O–H, C=O, C=C, or C≡C. Which class is most consistent with the evidence?
Alkene
Ketone
Alcohol
Alkane
9. Why might a molecular vibration fail to produce an IR absorption?
The molecule contains more than three atoms
The vibration occurs in the fingerprint region
The vibration does not produce a change in dipole moment
The sample contains covalent bonds rather than ionic bonds
10. A spectrum contains a sharp, medium-strength band at approximately 2240 cm⁻¹. Which group is most likely responsible?
Alkane C–H
Alcohol O–H
Nitrile C≡N
Ketone C=O
11. Which carbonyl-containing compound class typically absorbs at the highest wavenumber among these choices?
Acid chloride
Saturated ketone
Amide
Conjugated ketone
12. Which absorption is most characteristic of an alcohol O–H stretch?
A sharp band around 2210-2260 cm⁻¹
A broad band around 3200-3600 cm⁻¹
A strong, narrow band near 1715 cm⁻¹
A pair of weak bands near 2720 and 2820 cm⁻¹
13. Why is ATR commonly useful when collecting an IR spectrum of a solid or liquid sample?
It determines a complete molecular structure without other evidence
It often permits direct measurement with relatively little sample preparation
It converts all peaks directly into molecular-mass values
It removes every absorption caused by water or carbon dioxide
14. A supposedly dry sample unexpectedly shows a broad O–H absorption and a band near 1640 cm⁻¹. What is a plausible explanation to check before assigning a new functional group?
Water contamination is contributing O–H stretching and H–O–H bending bands
The fingerprint region has shifted above 3000 cm⁻¹
A nitrile group is producing both absorptions
The spectrometer has converted every C–H bond into an O–H bond
15. An unknown has a strong C=O absorption near 1735 cm⁻¹, strong bands in the C–O region around 1050-1300 cm⁻¹, and no broad O–H band. Which compound class fits best?
Carboxylic acid
Nitrile
Primary amine
Ester
16. A student has spectra from an unknown liquid and an authentic reference compound. What is the most reliable use of the fingerprint region below about 1500 cm⁻¹?
Use one peak there to calculate the exact molecular mass
Ignore it because only the functional-group region contains structural information
Compare the overall pattern of the unknown with the reference to assess whether they match
Use it alone to count the exact number of carbon atoms