Chapter 6 — Quiz
Twenty-five questions on IR spectroscopy, mass spectrometry, and integrative structure determination.
1. IR spectroscopy probes: (a) electron transitions (b) bond vibrations (c) nuclear spin transitions (d) molecular rotations
2. Wavenumber is: (a) wavelength in nm (b) inverse of wavelength in cm⁻¹ (c) frequency in Hz (d) energy in Joules
3. A broad peak at 3350 cm⁻¹ suggests: (a) alkyne C-H (b) alcohol O-H (H-bonded) (c) C=O (d) aromatic
4. A strong, sharp peak at 1720 cm⁻¹ suggests: (a) C-O (b) C=O (c) O-H (d) C=C
5. A broad peak 2500-3300 cm⁻¹ is characteristic of: (a) alcohol (b) amine (c) carboxylic acid (d) alkane
6. The diagnostic region of IR: (a) 4000-1500 cm⁻¹ (b) 1500-400 cm⁻¹ (c) below 400 cm⁻¹ (d) above 4000 cm⁻¹
7. The fingerprint region of IR: (a) 4000-1500 cm⁻¹ (b) 1500-400 cm⁻¹ (c) below 400 cm⁻¹ (d) above 4000 cm⁻¹
8. The molecular ion in a mass spectrum is: (a) the lightest fragment (b) the heaviest significant peak (intact molecule ionized) (c) always at m/z = 43 (d) the base peak always
9. A 3:1 isotope pattern at M and M+2: (a) nitrogen (b) bromine (c) chlorine (d) iodine
10. A 1:1 isotope pattern at M and M+2: (a) nitrogen (b) bromine (c) chlorine (d) iodine
11. A loss of 15 mass units suggests: (a) CH₃ group (b) N (c) O (d) H₂O
12. A loss of 29 mass units suggests: (a) CHO or $C_2H_5$ (b) CO (c) CH₃ (d) H₂O
13. Degrees of unsaturation for $C_8H_{10}O$: (a) 2 (b) 3 (c) 4 (d) 5
14. If $M^+$ is odd, the molecule contains: (a) an odd number of carbons (b) an odd number of nitrogens (c) no heteroatoms (d) no hydrogens
15. Ester C=O typically absorbs at: (a) 1630-1690 (b) 1705-1725 (c) 1735-1750 (d) 1700-1725
16. Amide C=O typically absorbs at: (a) 1630-1690 (b) 1705-1725 (c) 1735-1750 (d) 1780-1800
17. A sharp peak at 3300 cm⁻¹: (a) alcohol O-H (b) alkyne C-H (c) N-H (d) water
18. The base peak in a mass spectrum is: (a) the molecular ion (b) the most intense peak (set to 100%) (c) always at m/z = 1 (d) the lowest-mass peak
19. An absorption at 2240 cm⁻¹ is characteristic of: (a) alkyne C≡C (b) nitrile C≡N (c) aldehyde C-H (d) aromatic C=C
20. Which combination uniquely identifies a carboxylic acid? (a) broad 3350 + strong 1715 (b) broad 2500-3300 + strong 1720 (c) sharp 3300 + 2220 (d) 1500 + 1600
21. A primary amine (R-NH₂) shows in IR: (a) one sharp band at 3300 cm⁻¹ (b) two bands around 3300-3500 cm⁻¹ (sym + asym N-H stretches) (c) a broad band at 2500-3000 cm⁻¹ (d) no characteristic absorption
22. Why is a methylester C=O at higher wavenumber than a methyl ketone? (a) The O of the ester withdraws electron density via resonance, strengthening the C=O bond (b) The methyl group is more electronegative (c) Esters have lower mass (d) Resonance reasons increase the bond order
23. McLafferty rearrangement is diagnostic of: (a) alkene-containing molecules (b) carbonyls with γ-H (6-membered transition state for H migration) (c) all aromatic compounds (d) carboxylic acids only
24. High-resolution mass spectrometry (HRMS) gives: (a) the same data as low-resolution MS but printed nicer (b) m/z to 4-5 decimal places, allowing exact molecular formula determination (c) only the molecular ion (d) the IR spectrum simultaneously
25. A useful tropylium cation (C₇H₇⁺) at m/z 91 is diagnostic of: (a) any aromatic compound (b) a benzyl group (Ph-CH₂-) (c) the molecular ion of toluene (no, that's 92) (d) a chlorobenzene fragment
Answer Key
| # | Answer | Explanation |
|---|---|---|
| 1 | b | IR probes vibrations |
| 2 | b | Wavenumber = 1/λ in cm⁻¹ |
| 3 | b | Broad 3350 = H-bonded O-H |
| 4 | b | 1720 = C=O |
| 5 | c | Very broad 2500-3300 + carboxylic acid OH stretch |
| 6 | a | Diagnostic = 4000-1500 |
| 7 | b | Fingerprint = 1500-400 |
| 8 | b | M+ = heaviest significant peak (intact molecule) |
| 9 | c | Cl: ³⁵Cl:³⁷Cl = 3:1 |
| 10 | b | Br: ⁷⁹Br:⁸¹Br = 1:1 |
| 11 | a | 15 = CH₃ |
| 12 | a | 29 = CHO or C₂H₅ |
| 13 | c | DoU = (16+2−10)/2 = 4 |
| 14 | b | Nitrogen rule |
| 15 | c | Ester: 1735-1750 |
| 16 | a | Amide: 1630-1690 |
| 17 | b | Sharp 3300 = terminal alkyne C-H |
| 18 | b | Base peak = most intense |
| 19 | b | 2240 = nitrile (alkyne is 2100-2260 weak) |
| 20 | b | Carboxylic acid: very broad 2500-3300 + sharp 1720 |
| 21 | b | 1° amine has 2 bands (sym/asym N-H) |
| 22 | a | Ester O withdraws by π resonance, raising C=O |
| 23 | b | McLafferty: γ-H transfer in carbonyl, 6-mem TS |
| 24 | b | HRMS gives exact mass |
| 25 | b | Tropylium at 91 = benzyl (Ph-CH₂-) |
Concept connections
- IR probes vibrations → identifies functional groups by characteristic wavenumbers.
- MS measures mass → confirms molecular weight; fragmentations hint at structure.
- Together → IR + MS often nails the structure of a simple molecule.
- HRMS → exact molecular formula.
- NMR (Ch 9) → individual atoms in their environments; the most powerful single tool.
Scoring: 22+ = strong understanding. 17–21 = re-read the diagnostic region table. Below 17 = re-read the chapter and practice with real spectra (SDBS).