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The conjugated diene CH2=CH-CH=CH-CH3 (penta-1,3-diene, I) is more stable than the cumulated diene CH3-CH=C=CH-CH3 (penta-2,3-diene/allene, II). The reason is:

1. There is resonance (conjugation) 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 (conjugation) in I but not in II

Solution

In a conjugated diene the alternating single-double bonds allow the p-orbitals to overlap continuously, delocalizing the pi electrons and lowering the energy (resonance stabilization). In an allene (cumulated diene) the two double bonds share the central sp carbon and their pi systems lie in perpendicular planes, so there is no conjugation/resonance. Hence I is more stable due to resonance present in I but absent in II.

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