Chapter 20 — Quiz

Twenty-five questions on aromaticity. ∗ marks questions answered in the answer key.

Multiple choice

1.∗ Aromatic compounds have: (a) 4n π electrons (b) 4n+2 π electrons (Hückel's rule; 2, 6, 10, 14, ...) (c) any number (d) only 6 π electrons

2.∗ Benzene's π electron count is: (a) 4 (b) 6 (n=1; aromatic by Hückel) (c) 8 (d) 10

3.∗ Antiaromatic compounds have: (a) 4n π electrons (4, 8, 12, ...) (b) 4n+2 π electrons (c) zero π electrons (d) random

4.∗ A molecule must be aromatic if it is: (a) planar (b) cyclic (c) fully conjugated (d) all of the above + 4n+2 π electrons

5.∗ The pyridine N contributes to the π system: (a) 1 electron (the unhybridized p orbital; the lone pair is in the plane) (b) 2 electrons (a lone pair) (c) 0 electrons (d) 3 electrons

6.∗ The pyrrole N contributes to the π system: (a) 1 electron (b) 2 electrons (the lone pair is in the perpendicular p orbital, in the π system) (c) 0 electrons (d) 3 electrons

7.∗ Pyridine is basic because: (a) the lone pair is in the plane of the ring (not in the π system); accessible to protonation (b) the lone pair is perpendicular (c) of the C=N bond (d) random

8.∗ Pyrrole is not basic because: (a) the lone pair is in the π system; protonating it would destroy aromaticity (b) of the C-H bond (c) the lone pair is in the plane (d) random

9.∗ Benzene's aromatic stabilization energy is approximately: (a) 0 kcal/mol (no stabilization) (b) 36 kcal/mol (resonance energy from heat of hydrogenation comparison) (c) 100 kcal/mol (d) > 200 kcal/mol

10.∗ Cyclobutadiene: (a) is a stable aromatic compound (b) is antiaromatic and extremely unstable (c) is non-aromatic (d) doesn't exist

11.∗ Cyclopentadienyl anion (Cp⁻) is: (a) aromatic (6 π electrons; 5 sp² C with 1 from each + 2 from the deprotonated C lone pair... wait that's 7? Let me re-do: 4 from 2 C=C + 2 from the CHˉ lone pair = 6 total. Aromatic.) (b) non-aromatic (c) antiaromatic (d) doesn't exist

12.∗ Tropylium cation (cycloheptatrienyl cation) is: (a) aromatic (7-member ring, 3 C=C + 0 from C+, 6 π total) (b) non-aromatic (c) antiaromatic (d) doesn't form

13.∗ Naphthalene is: (a) aromatic with 10 π electrons (n=2) (b) non-aromatic (c) antiaromatic (d) only the bonds at the central junction

14.∗ Why does cyclooctatetraene pucker to a tub shape? (a) to relieve antiaromaticity (would be antiaromatic if planar with 8 π = 4n) (b) random (c) to relieve steric strain only (d) it doesn't pucker

15.∗ Aromatic ¹H NMR signals appear at: (a) δ 0-2 ppm (b) δ 4.5-6.5 ppm (c) δ 6-9 ppm (deshielded by the ring current) (d) δ 9-13 ppm

16.∗ The ring current in aromatic compounds: (a) shifts external H signals downfield (deshielded) and internal H signals upfield (shielded) (b) only affects internal H (c) only affects external H (d) doesn't exist

17.∗ Imidazole has: (a) one pyridine-like N (basic) and one pyrrole-like N (acidic N-H) (b) two equivalent N atoms (c) three N atoms (d) only one N

18.∗ Histidine's imidazole side chain has pKaH: (a) 1 (b) 6 (close to physiological pH; important for enzyme catalysis) (c) 10 (d) 14

19.∗ Polycyclic aromatic hydrocarbons (PAHs): (a) include naphthalene, anthracene, phenanthrene, pyrene, coronene (b) are all non-aromatic (c) only graphene (d) only with heteroatoms

20.∗ Graphene is: (a) an infinite 2D sheet of sp² carbons; fully aromatic (b) only at low temperature (c) only with metals (d) random

Short answer

21. State Hückel's rule and the four conditions for aromaticity. Apply to: benzene, cyclobutadiene, cyclopentadienyl anion.

22. Compare pyridine and pyrrole: (a) Number of π electrons. (b) Position of the lone pair. (c) Basicity.

23. Why is the cyclopentadienyl cation antiaromatic but the cyclopentadienyl anion aromatic?

24. Sketch the molecular orbital diagram of benzene. Identify HOMO, LUMO, and the 6 π electrons.

25. Why does aromaticity matter for biology and pharmaceutical chemistry? Give 3 examples.

Answer key

  1. b — 4n+2 π electrons.
  2. b — Benzene has 6 π electrons.
  3. a — Antiaromatic = 4n.
  4. d — All four conditions.
  5. a — Pyridine N contributes 1 π electron.
  6. b — Pyrrole N contributes 2 π electrons.
  7. a — Pyridine lone pair in plane.
  8. a — Pyrrole lone pair in π system.
  9. b — ~36 kcal/mol.
  10. b — Cyclobutadiene antiaromatic.
  11. a — Cp⁻ aromatic.
  12. a — Tropylium aromatic.
  13. a — Naphthalene aromatic.
  14. a — Cyclooctatetraene puckers to avoid antiaromaticity.
  15. c — Aromatic ¹H δ 6-9.
  16. a — Ring current effects.
  17. a — Imidazole has two N types.
  18. b — Histidine pKaH ~6.
  19. a — PAHs.
  20. a — Graphene description.

21. Hückel's rule: a molecule is aromatic if it is: 1. Cyclic (a ring). 2. Planar (all p orbitals can align perpendicular to the ring). 3. Fully conjugated (every ring atom has a p orbital). 4. Has 4n+2 π electrons (n = 0, 1, 2, ...).

Application: - Benzene (6-member ring; planar; sp² C's; 6 π electrons; n=1). All conditions met → aromatic. - Cyclobutadiene (4-member ring; planar; sp² C's; 4 π electrons = 4n where n=1). Conjugated cyclic but 4n electrons → antiaromatic (destabilized; very unstable). - Cyclopentadienyl anion (5-member ring; planar; sp² C's after deprotonation of cyclopentadiene; 4 from 2 C=C + 2 from the deprotonated C's lone pair = 6 π electrons; n=1). All conditions met → aromatic (very stable; explains low pKa of cyclopentadiene).

22. Pyridine vs. pyrrole: | Feature | Pyridine | Pyrrole | |---|---|---| | Structure | 6-member ring; 5 C + 1 N | 5-member ring; 4 C + 1 NH | | π electrons | 5 from C + 1 from N (in p orbital) = 6 | 4 from C + 2 from N (lone pair in p orbital) = 6 | | N lone pair position | In the plane (sp² hybrid orbital), NOT in π system | Perpendicular (in p orbital), IN the π system | | Basicity | Basic (pKaH 5.2; lone pair available for protonation without disrupting aromaticity) | Not basic (pKaH ~ -4; protonating destroys aromaticity) | | Aromatic? | Yes (6 π) | Yes (6 π) |

The key difference: in pyridine, the basic lone pair is separate from the aromatic π system; protonation doesn't disrupt aromaticity. In pyrrole, the lone pair IS the aromatic π contribution; protonation would destroy aromaticity, so it doesn't happen.

23. Cyclopentadienyl cation vs. anion: - Cation (5C with one C+; 4 π electrons): 5-member ring; planar; conjugated; 4 π electrons = 4n where n=1. Antiaromatic by Hückel. Destabilized. - Anion (5C with one C⁻; 6 π electrons): 4 from 2 C=C + 2 from the C⁻ lone pair = 6 π electrons; n=1. Aromatic by Hückel. Stabilized.

This is one of the clearest examples of how electron count determines aromaticity. The same 5-member ring becomes aromatic when given 2 more π electrons (anion) or antiaromatic when given 2 fewer (cation).

24. Benzene MO diagram: - 6 atomic p orbitals (one from each sp² C, perpendicular to the ring) combine to form 6 π molecular orbitals. - 3 bonding MOs (lower energy): - ψ₁: 0 nodes; all p orbitals in phase (lowest energy). - ψ₂ and ψ₃: 1 node each; degenerate (same energy). - 3 antibonding MOs (higher energy): - ψ₄ and ψ₅: 2 nodes each; degenerate. - ψ₆: 3 nodes; highest energy. - The 6 π electrons fill the 3 bonding MOs (ψ₁, ψ₂, ψ₃) completely. - HOMO = ψ₂/ψ₃ (degenerate). - LUMO = ψ₄/ψ₅ (degenerate). - The HOMO-LUMO gap is large (~6 eV; UV absorption at ~254 nm).

This is the classic Hückel MO picture of benzene's π system. All bonding MOs are filled (closed-shell, stable); all antibonding empty. This electronic structure is the source of benzene's stability.

25. Why aromaticity matters for biology and pharmacy:

Examples: 1. DNA bases: adenine, guanine (purines), cytosine, thymine, uracil (pyrimidines) are all aromatic heterocycles. Their aromatic stability allows reliable hydrogen-bond pairing; their planarity allows π-stacking that contributes ~60% of DNA's stability. 2. Aromatic amino acids: phenylalanine, tyrosine, tryptophan, histidine. These contribute UV absorbance (the basis of A280 protein quantification), π-stacking interactions in protein folding, and specific recognition (e.g., Trp in transcription factors). 3. Drugs: ~80% of FDA-approved drugs contain at least one aromatic ring. Reasons: rigidity (less conformational flexibility = more selective binding); π-stacking (with aromatic residues in the target); bioavailability (intermediate logP). 4. Hormones: estradiol, cortisol, testosterone (steroids; some aromatic rings in estrogens). 5. Neurotransmitters: dopamine, serotonin, epinephrine — all contain catechol (aromatic) rings. 6. Vitamin B6 (pyridoxine): an aromatic pyridine; PLP-dependent enzymes use it for amino acid metabolism (Ch 27). 7. Heme: aromatic porphyrin coordinated to Fe; oxygen carrier in hemoglobin, electron carrier in cytochrome.

The aromaticity concept organizes a huge fraction of biological and pharmaceutical chemistry. Mastery of Chapter 20 is the foundation for many later applications.