Chapter 20 — Exercises

Forty-five problems on aromaticity, Hückel's rule, and heteroaromatic rings. Drawing required wherever a structure is asked for. ∗ marks problems with full worked solutions in Appendix Answers to Selected Exercises.

Section A — Hückel classification

20.1∗ (routine) Classify each as aromatic, antiaromatic, or non-aromatic, and state the π electron count: (a) benzene (b) cyclobutadiene (c) cyclopentadiene (neutral) (d) cyclopentadienyl anion (Cp⁻) (e) cyclopentadienyl cation (f) cyclooctatetraene (tub-shaped) (g) cycloheptatrienyl cation (tropylium) (h) cyclopropenyl cation

20.2 (routine) How many π electrons are in: (a) naphthalene (b) anthracene (c) phenanthrene (d) coronene (e) ferrocene's two Cp⁻ rings (each is aromatic)

20.3∗ (routine) Why is cyclopentadiene's pKa unusually low (~16)? Connect to the aromatic stability of the conjugate base.

20.4 (moderate) Cyclooctatetraene puckers out of plane to a tub shape. Why? Connect to antiaromaticity at planarity.

20.5 (challenge) Cyclooctatetraene dianion (8 π electrons; Hückel-violation? No — let me re-check: 8 π electrons in the dianion of cyclooctatetraene = 4n where n=2 → antiaromatic? But the dianion is stable as a planar aromatic with 10 π electrons; what's right?). Discuss the cyclooctatetraene dianion's aromaticity.

20.6 (challenge) Why is benzyne (a benzene with one C=C replaced by C≡C, breaking the aromatic ring) so reactive?

Section B — Heteroaromatic rings

20.7∗ (routine) Identify each heteroaromatic ring and count its π electrons: (a) pyridine (b) pyrrole (c) furan (d) thiophene (e) imidazole (f) pyrimidine

20.8 (routine) Compare pyridine's basicity (pKaH 5.2) with pyrrole's basicity (pKaH ~ -4). Why so different?

20.9∗ (moderate) Sketch the lone pairs on: (a) pyridine N (in the plane). (b) pyrrole N (in the π system).

20.10 (moderate) Imidazole has two N atoms with very different pKaH values. Explain.

20.11 (challenge) Histidine's imidazole has pKaH ~6 (close to physiological pH 7.4). Why does this make histidine special among amino acids?

20.12 (challenge) Pyridoxine (vitamin B6) is a pyridine with multiple substituents. Why is it pharmacologically important? Connect to PLP-mediated enzyme catalysis (Ch 27).

Section C — Aromatic ions

20.13∗ (routine) Sketch the cyclopentadienyl anion (Cp⁻). Show: (a) all 5 sp² carbons. (b) the 6 π electrons. (c) why it's aromatic by Hückel.

20.14 (routine) Sketch the tropylium cation. Show the 6 π electrons.

20.15 (moderate) Why is the cyclopentadienyl cation antiaromatic and not just non-aromatic? Connect to the π electron count and Hückel's rule.

20.16 (challenge) Ferrocene (Fe + 2 Cp⁻) is the prototypical metallocene. Why is it stable? Connect to aromaticity of the Cp⁻ ligands.

Section D — Polycyclic aromatics

20.17∗ (routine) Sketch: (a) naphthalene (b) anthracene (c) phenanthrene (d) pyrene

20.18 (routine) Why is phenanthrene more stable than anthracene? Connect to the central ring's aromaticity.

20.19 (moderate) Naphthalene undergoes EAS preferentially at the α-position (C1). Why? Connect to resonance stabilization of the intermediate.

20.20 (challenge) Benzo[a]pyrene is a major carcinogen in cigarette smoke. Sketch its structure. Why is it carcinogenic? Connect to its biological mechanism.

20.21 (challenge) Coronene has 7 fused benzene rings. Predict its physical properties (m.p., solubility, color). Verify against literature.

Section E — Spectroscopy

20.22∗ (routine) Why do aromatic ¹H NMR signals appear at δ 7-8 ppm (downfield)? Connect to the ring current.

20.23 (routine) A compound has ¹H NMR with peaks at δ 7.0-7.5 (5H). What functional group is likely?

20.24 (moderate) Compare the ¹³C NMR shifts of aromatic C (δ 120-150) vs alkene C (δ 100-145). Why are they similar?

20.25 (challenge) A compound has IR peaks at 3050 (vinyl-like CH) and 1500-1600 (aromatic C=C). Combined with ¹H NMR at δ 7-8, identify the functional group.

Section F — Aromaticity tests

20.26∗ (routine) What is the heat of hydrogenation of benzene? Compare to "theoretical cyclohexatriene." What is the resonance energy?

20.27 (moderate) NICS (nucleus-independent chemical shift): a computational test. What does negative NICS indicate? Positive?

20.28 (challenge) Compute NICS for: benzene, cyclobutadiene, naphthalene, pyridine. Predict aromatic, antiaromatic, or non-aromatic for each. (Use literature values.)

Section G — Aromatic biology

20.29∗ (routine) Sketch the four DNA bases: (a) adenine (A) — purine. (b) guanine (G) — purine. (c) cytosine (C) — pyrimidine. (d) thymine (T) — pyrimidine.

Identify which are aromatic and how many π electrons each has.

20.30 (routine) π-stacking of DNA bases contributes ~60% of double-helix stability. Sketch the parallel stacking.

20.31 (moderate) Aromatic amino acids: phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), histidine (His). Sketch each.

20.32 (challenge) Tryptophan's indole ring (benzene + pyrrole fused) is the most distinctive aromatic side chain. Explain its UV absorbance (the basis of protein quantification at 280 nm).

Section H — Aromatic drugs and natural products

20.33∗ (routine) Identify aromatic rings in: (a) aspirin (acetylsalicylic acid) (b) ibuprofen (c) acetaminophen (d) atorvastatin (Lipitor)

20.34 (moderate) Why do most drugs contain at least one aromatic ring? Identify benefits: rigidity, π-stacking with targets, lipophilicity tuning, metabolic stability.

20.35 (challenge) Aromatic chemistry for sustainability: aromatics from biomass (lignin) vs. petroleum. Discuss the renewable feedstock challenge.

Section I — Materials and graphene

20.36 (routine) Graphene is a single sheet of sp² carbons. Why is it conductive? Connect to delocalized π electrons.

20.37 (moderate) Carbon nanotubes are rolled-up graphene sheets. Why are some conductive (metallic) and others semiconductive?

20.38 (challenge) Fullerenes (C₆₀, C₇₀): closed cages of sp² carbons. Are they aromatic? Connect to Hückel.

Section J — Open-ended

20.39 (challenge) Aromatic vs. non-aromatic: identify ring systems and predict their behavior.

20.40 (challenge) Modern aromaticity research: design a hypothetical aromatic system that is unprecedented (e.g., a non-organic aromatic; an antiaromatic that is stable due to substituents).

20.41 (challenge) Compare aromaticity of: (a) the typical 6-member benzene aromatic. (b) the 4n+2 rule extended to larger rings ([10]annulene, [14]annulene, etc.). (c) heteroaromatics where the heteroatom contributes 1 vs 2 π electrons.

20.42 (challenge) Discuss why aromaticity is a "load-bearing" concept in organic chemistry. Use examples from biology, materials, drugs.

20.43 (challenge) Sketch an antiaromatic molecule and predict its properties (instability, distortion to non-planar, paramagnetic, etc.).

20.44 (challenge) Open-ended: choose a complex molecule (e.g., a drug, a vitamin, a dye). Identify all aromatic rings and predict how their aromaticity contributes to function.

20.45 (challenge) Looking forward: aromaticity in 2D materials (graphene), MOFs (metal-organic frameworks), and porous organic polymers. Discuss applications.

Notes for instructors: Common stumbling blocks for Chapter 20: (1) Confusing aromatic and non-aromatic. (2) Mismatching pyridine vs pyrrole basicity. (3) Forgetting cyclopentadienyl anion is aromatic, not the cation. (4) Not recognizing antiaromatic distortion. Computational exercises: compute NICS for various rings; visualize π MOs of benzene.