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Which of the following dienes is the most stable?

1. Conjugated diene (CH2=CH-CH=CH2)
2. Isolated diene (CH2=CH-CH2-CH=CH2)
3. Cumulated diene (CH2=C=CH2)
4. All are equally stable

Correct answer: Conjugated diene (CH2=CH-CH=CH2)

Solution

In a conjugated diene the two C=C double bonds are separated by a single bond, allowing the p-orbitals to overlap and pi electrons to delocalize, which lowers the energy and increases stability. Isolated dienes have insulated double bonds (no conjugation), and cumulated dienes (allenes) are strained and least stable. Hence the conjugated diene is most stable.

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