Exams › JEE Main › Chemistry

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.

Related JEE Main Chemistry questions

• Although the electronegativity gap between N and F is larger than that between N and H, ammonia has a dipole moment of 1.5 D while nitrogen trifluoride has only 0.2 D. The reason is that
• When N₂ is converted into N₂⁺, the dissociation energy of the N–N bond ______, and when O₂ is converted into O₂⁺, the dissociation energy of the O–O bond ______.
• From the ions listed below, which pair has geometries that can be accounted for by the same type of orbital hybridization? NO₂⁻, NO₃⁻, NH₂⁻, NH₄⁺, SCN⁻
• Atoms A and B have electronegativities of 1.20 and 4.0, respectively. What is the percentage ionic character of the A–B bond?
• In the phosphate ion, PO₄³⁻, what is the formal charge on an oxygen atom that is singly bonded to phosphorus in a P–O bond?
• Among the following species, which one has no unpaired electrons?

⚔️ Practice JEE Main Chemistry free + battle 1v1 →