# Aromaticity, Antiaromaticity, and Nonaromaticity To determine if a molecule is **aromatic**, **antiaromatic**, or **nonaromatic**, use these criteria: 1. **Aromatic**: - Cyclic - Planar - Fully conjugated (every atom in the ring has a p orbital) - Follows **Hückel’s rule**: \(4n+2\) π electrons (where \(n = , 1, 2, \ldots\)) 2. **Antiaromatic**: - Cyclic - Planar - Fully conjugated - **4n** π electrons 3. **Nonaromatic**: - Not fully conjugated and/or not planar and/or not cyclic --- ## Analysis of Each Compound ### (a) Cyclooctatetraene (C₈H₈) - **Structure:** 8-membered ring, 4 double bonds (8 π electrons) - **Conjugation:** Although it has conjugated double bonds, it's *not planar* (adopts a tub-shaped conformation). - **Conclusion:** **Nonaromatic** - **Reason:** Not planar, not fully conjugated. --- ### (b) [10]-Annulene - **Structure:** 10-membered ring, 5 double bonds (10 π electrons) - **Conjugation:** In principle, it could be aromatic, but steric hindrance from internal hydrogens prevents planarity. - **Conclusion:** **Nonaromatic** - **Reason:** Not planar due to steric strain. --- ### (c) Indene - **Structure:** Fused benzene and cyclopentene (6 + 2 = 8 π electrons, but cyclopentene is not aromatic) - **Conjugation:** Benzene ring is aromatic, but the 5-membered ring is not fully conjugated. - **Conclusion:** **Nonaromatic** - **Reason:** The whole system is not fully conjugated. --- ### (d) Benzocyclobutadiene - **Structure:** Benzene fused to cyclobutadiene - **Conjugation:** Cyclobutadiene is antiaromatic (4 π electrons), benzene is aromatic (6 π electrons) - **Conclusion:** **Nonaromatic** - **Reason:** Fusion disrupts delocalization; system is not fully conjugated over both rings. --- ### (e) Azulene - **Structure:** Fused 5- and 7-membered rings, 10 π electrons - **Conjugation:** Fully conjugated and planar - **Conclusion:** **Aromatic** - **Reason:** Follows Hückel's rule (10 π electrons, n=2) --- ### (f) Cyclopentadienyl cation - **Structure:** 5-membered ring, 2 double bonds, cation (4 π electrons) - **Conjugation:** Planar, fully conjugated - **Conclusion:** **Antiaromatic** - **Reason:** 4n π electrons (n=1), follows antiaromatic rule --- ### (g) Naphthalene - **Structure:** Two fused benzene rings, 10 π electrons - **Conjugation:** Planar, fully conjugated - **Conclusion:** **Aromatic** - **Reason:** Follows Hückel's rule (10 π electrons, n=2) --- ### (h) Biphenylene - **Structure:** Four fused benzene rings (12 π electrons) - **Conjugation:** Planar, but 4n π electrons, destabilized - **Conclusion:** **Antiaromatic** - **Reason:** 4n π electrons (n=3); system is planar and fully conjugated --- ## **Summary Table** | Molecule | Aromatic? | Reason | |------------|------------------|---------------------------------------------| | (a) | Nonaromatic | Not planar | | (b) | Nonaromatic | Not planar | | (c) | Nonaromatic | Not fully conjugated | | (d) | Nonaromatic | Fusion disrupts conjugation | | (e) | Aromatic | 10 π e⁻ (Hückel's rule), planar, conjugated | | (f) | Antiaromatic | 4 π e⁻ (antiaromatic), planar, conjugated | | (g) | Aromatic | 10 π e⁻ (Hückel's rule), planar, conjugated | | (h) | Antiaromatic | 12 π e⁻ (antiaromatic), planar, conjugated | --- **Let me know if you want to go deeper into any of these!**