Chapter 6 — Exercises
Forty-five problems covering IR spectroscopy, mass spectrometry, and integrative structure determination. ∗ = full solution in Appendix Answers to Selected Exercises.
Section A — Reading IR spectra
6.1∗ (routine) For each IR absorption, state the likely bond type: (a) Broad absorption, 3300 cm⁻¹ (b) Sharp absorption, 3300 cm⁻¹ (c) Broad absorption, 2500-3000 cm⁻¹ (d) Strong, sharp absorption, 1715 cm⁻¹ (e) Medium absorption, 2230 cm⁻¹ (f) Two bands, 1500 and 1600 cm⁻¹ (g) Strong absorption, 1740 cm⁻¹ (h) Strong, two bands, 1500-1570 + 1300-1370 cm⁻¹
6.2 (routine) Distinguish by IR: (a) How would you tell an aldehyde from a ketone? (b) A carboxylic acid from an ester? (c) An alcohol from an ether? (d) A primary amine from a secondary amine?
6.3 (routine) A spectrum shows strong peaks at 2960, 2870, 1720, and 1170 cm⁻¹. Identify the likely functional groups.
6.4 (moderate) An unknown has no broad peak in the 3200-3600 region. This rules out what functional groups?
6.5 (moderate) A spectrum shows broad 2500-3300 + strong 1715 cm⁻¹. This combination is diagnostic for what?
6.6 (moderate) Why does the C=O of cyclobutanone (~1780 cm⁻¹) absorb at higher wavenumber than that of cyclohexanone (~1715 cm⁻¹)? Connect to bond strength and ring strain.
6.7 (challenge) An α,β-unsaturated ketone (e.g., 2-butenone, CH₂=CHCOCH₃) shows its C=O at lower wavenumber (~1675 cm⁻¹) than a saturated ketone (~1715). Explain using resonance.
Section B — IR diagnostic procedures
6.8 (routine) Apply the 5-step IR diagnostic procedure to: (a) An IR with broad O-H, $sp^3$ C-H, no C=O, C-O at 1080 — what is it? (b) An IR with sharp 3300, $sp^3$ C-H, no C=O — what is it? (c) An IR with strong sharp 2230, no C=O, no broad — what is it?
6.9 (moderate) A student says "if there's no C=O, the molecule is not a carbonyl." Always true? Discuss.
6.10 (moderate) A student claims "all O-H is at 3500 cm⁻¹." Identify three counterexamples.
6.11 (challenge) What IR features would distinguish: (a) An ether (R-O-R) from an ester (R-COOR)? (b) An amide (R-CONR₂) from a ketone (R-COR)?
Section C — Mass spectrometry
6.12∗ (routine) An unknown shows $M^+ = 86$ and a large peak at $m/z = 43$. What common loss does this represent? Suggest a possible functional group.
6.13 (routine) An unknown has an $M^+:(M+2)$ ratio of 3:1. What element is present?
6.14 (routine) An unknown has $M^+:(M+2)$ of 1:1 with 2 mass-unit separation. What element?
6.15 (moderate) For a molecule with one Cl and one Br, predict the isotope pattern near $M^+$.
6.16∗ (moderate) A compound has $M^+ = 73$. Apply the nitrogen rule. What does it say about the number of nitrogens?
6.17 (routine) Calculate degrees of unsaturation for: (a) $C_6H_{14}$ (b) $C_6H_{12}$ (c) $C_6H_6$ (d) $C_6H_{12}O$ (e) $C_8H_9NO$ (f) $C_8H_8O_2$ (g) $C_{10}H_{14}N_2$
6.18 (moderate) Estimate the number of carbons in a molecule whose M+1 peak is 6.6% as intense as the M peak.
Section D — Common fragmentations
6.19∗ (routine) Identify the likely loss for each common loss: (a) Loss of 15 (b) Loss of 18 (c) Loss of 28 (d) Loss of 29 (e) Loss of 31 (f) Loss of 43
6.20 (moderate) The base peak at m/z 43 in 2-butanone (CH₃COCH₂CH₃, MW 72) is: (a) the molecular ion (b) loss of CH₃ giving CH₃CO⁺ (c) loss of CH₂CH₃ giving CH₃CO⁺ (d) Both b and c are the same fragment
6.21 (moderate) The base peak at m/z 91 (tropylium cation) is diagnostic of: (a) any aromatic (b) any benzyl group (Ph-CH₂-) (c) the molecular ion of toluene (no, that's 92) (d) loss of CO from a benzaldehyde
6.22 (challenge) Sketch the McLafferty rearrangement for 2-pentanone (MW 86). What is the product fragment? Why is it diagnostic?
Section E — Integrative problems
6.23∗ (moderate) An unknown has molecular formula $C_4H_8O$ and shows an IR peak at 1725 cm⁻¹ (strong, sharp). Suggest a structure.
6.24 (moderate) An unknown $C_6H_{12}O_2$ has a broad IR peak at 2700-3100 and a strong peak at 1710 cm⁻¹. What functional group? Suggest a structure.
6.25 (moderate) An unknown $C_4H_{11}N$ shows an IR broad peak at 3350 cm⁻¹ (two bands). What does this suggest?
6.26 (moderate) An unknown has $M^+ = 150$ with no odd-mass molecular ion. IR shows a strong C=O at 1740 cm⁻¹ and a C-O at 1200 cm⁻¹, but no broad O-H. Propose a functional group.
6.27 (challenge) Aspirin has $M^+ = 180$. Predict the mass of the major fragment losing the acetyl group.
6.28 (challenge) Caffeine has $M^+ = 194$. Apply the nitrogen rule. How many nitrogens?
Section F — Multi-technique problems
6.29∗ (moderate) A compound $C_3H_6O$ has: - IR: no broad peak above 3200, strong peak at 1720 cm⁻¹ - MS: $M^+ = 58$, base peak at 43
Suggest the structure. Distinguish from the other possible $C_3H_6O$ structures.
6.30 (moderate) A compound $C_2H_4O$: - IR: strong C=O at 1725 cm⁻¹, sharp peak at 2720 cm⁻¹ (aldehyde C-H) - MS: $M^+ = 44$, loss of 29
Identify. Which aldehyde?
6.31 (challenge) A compound $C_7H_8O$: - IR: broad absorption 3300 cm⁻¹, aromatic C=C at 1490 and 1600 - MS: $M^+ = 108$, loss of 15
Suggest the structure and explain the fragmentation.
6.32 (challenge) A compound $C_5H_{10}O$: - IR: no broad above 3200; strong C=O 1715 cm⁻¹ - MS: M⁺ 86; base peak m/z 43; another major peak m/z 71
Suggest the structure. (Hint: McLafferty?)
6.33 (challenge) A compound $C_6H_{12}O$ with: - IR: broad O-H at 3380; no C=O; sp³ C-H at 2900 - MS: M⁺ 100; loss of 18; loss of 29
Suggest the structure.
Section G — UV-Vis spectroscopy
6.34 (routine) Why do conjugated dienes absorb at longer wavelengths than non-conjugated alkenes? Connect to π MO energy levels.
6.35 (moderate) A solution of unknown compound has λ_max at 210 nm and ε ≈ 6,000. Is the absorbance from: (a) An isolated C=C (b) A conjugated diene (c) An aromatic ring (n→π*) (d) Cannot tell from this alone
6.36 (challenge) β-carotene has 11 conjugated double bonds and absorbs at λ_max ≈ 450 nm (orange). If we extended the conjugation by adding more C=C units, would the absorption shift to longer or shorter wavelength?
Section H — Computational
6.37 (computational) Build 2-butanone in Avogadro. Note the geometry. Predict its dominant IR stretches (C=O, C-H). If your program supports IR prediction, verify.
6.38 (computational) Look up the IR spectrum of ethanol on SDBS (sdbs.db.aist.go.jp). Label the peaks.
6.39 (computational) Find the IR spectrum of acetone, acetic acid, and ethyl acetate. Compare the C=O wavenumbers. Confirm the order: ester > acetic acid > acetone.
Section I — Multi-step structure determination
6.40 (challenge) An unknown $C_8H_8O_2$: - IR: strong C=O at 1685 cm⁻¹, broad 2500-3200 cm⁻¹ - MS: M⁺ 136, loss of 17 (= OH), giving m/z 119 - DoU: ?
Suggest the structure. Hint: a benzoic acid?
6.41 (challenge) An unknown $C_5H_8O_2$: - IR: strong C=O at 1735 cm⁻¹, no broad O-H - MS: M⁺ 100, base peak 43
Suggest. Hint: methyl ester of butyric acid?
6.42 (challenge) An unknown $C_4H_8O_2$: - IR: very strong, broad O-H at 2700-3300 cm⁻¹; strong C=O at 1715 cm⁻¹ - MS: M⁺ 88, loss of 17 (= OH), loss of 18 (= H₂O)
Suggest. Hint: 2-methylpropanoic acid?
Section J — Open-ended
6.43 (challenge) In Chapter 9, you will learn NMR. A student says "NMR is enough; we don't need IR." How would you respond?
6.44 (challenge) For each of the anchor examples, sketch the key IR features: (a) aspirin (a phenyl ester of acetic acid + a free -COOH) (b) ibuprofen (a benzylic propanoic acid) (c) acetaminophen (an N-aryl acetamide + phenol) (d) thalidomide (two imides)
6.45 (challenge) A research student claims to have made a new compound but has only an IR and an MS, no NMR. The IR shows a broad O-H (3300), strong C=O (1715), and aromatic C=C (1500, 1600). MS shows M⁺ 152, loss of 17 (OH), giving 135. The student claims the compound is "an aromatic hydroxy-ketone." Is the data consistent with this claim? What additional data would be needed?
Notes for instructors: Common stumbling blocks for Chapter 6: (1) confusing the broad O-H with a sharp N-H; (2) forgetting the nitrogen rule; (3) overlooking the small ¹³C M+1 peak; (4) confusing α-cleavage with McLafferty rearrangement; (5) forgetting to compute DoU before guessing structures.