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In which of the following pairs does the first species have greater resonance (delocalization) energy than the second?

1. a conjugated triene and benzene
2. a conjugated diene anion and cyclopentadienyl cation
3. a conjugated enamine and a conjugated aminoalkene
4. None of these

Correct answer: None of these

Solution

Benzene is aromatic (6 pi electrons, 4n+2) and has much larger resonance energy than an open-chain conjugated triene, so the first < second in pair 1. The cyclopentadienyl cation is anti-aromatic/non-aromatic but the comparison still does not make the open diene anion larger in a clean way; and 'enamine' vs 'aminoalkene' describe essentially the same delocalized N-C=C system, so neither is clearly greater. In none of the pairs is the first species reliably higher in resonance energy, so the answer is 'None of these'.

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