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Among the following species, which one is the least stabilized by resonance (has the smallest resonance/delocalization energy)?

1. Benzene
2. Furan (the five-membered ring containing one oxygen atom and two C=C double bonds)
3. Pyridine
4. A 4,4-disubstituted cyclohexa-2,5-dien-1-one (a non-aromatic cross-conjugated dienone)

Correct answer: A 4,4-disubstituted cyclohexa-2,5-dien-1-one (a non-aromatic cross-conjugated dienone)

Solution

Benzene (~36 kcal/mol), pyridine and furan are all aromatic ring systems with substantial resonance energies (furan's is lower than benzene's, ~16 kcal/mol, but still significant aromatic stabilization). The 4,4-disubstituted cyclohexa-2,5-dienone has a tetrahedral (sp3) carbon at C-4 that interrupts the cyclic conjugation, so the ring is NOT aromatic; it is only a cross-conjugated dienone whose delocalization energy is much smaller than that of the three aromatic systems. Hence it is the least resonance-stabilized.

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