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The resonance energy of the resonance hybrid of a molecule is greater when:

1. the canonical (contributing) structures are equivalent rather than non-equivalent
2. the molecule is aromatic rather than non-aromatic
3. the canonical structures are non-equivalent rather than equivalent
4. the molecule is non-aromatic rather than aromatic

Correct answer: the canonical (contributing) structures are equivalent rather than non-equivalent

Solution

Resonance energy is largest when the canonical structures are equivalent (degenerate), because they then contribute equally to the hybrid and maximize delocalization. Non-equivalent structures of differing stability contribute unequally and give smaller resonance energy.

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