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CH2=CH-CH=CH-CH3 (compound I) is more stable than CH3-CH=C=CH-CH3 (compound II). Why?

1. there is resonance in I but not in II
2. there is tautomerism in I but not in II
3. there is hyperconjugation in I but not in II
4. II has more canonical structures than I

Correct answer: there is resonance in I but not in II

Solution

Compound I (penta-1,3-diene) is a conjugated diene with alternating single and double bonds; its pi electrons delocalise across four carbons via resonance. Compound II (penta-2,3-diene) is a cumulated diene (allene); its two pi systems are mutually perpendicular due to the sp-hybridised central carbon, preventing resonance. Hence I is more stable due to resonance.

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