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For the three pentadiene-type isomers below, give the correct increasing order of heat of combustion: (a) a conjugated diene (b) a cumulated (allene-type) diene (c) an isolated (non-conjugated) diene

1. (a) < (c) < (b)
2. (b) < (c) < (a)
3. (c) < (b) < (a)
4. (a) < (b) < (c)

Correct answer: (a) < (c) < (b)

Solution

Conjugated dienes are the most stable (lowest energy, lowest heat of combustion). Cumulated (allene) dienes are the least stable (highest heat of combustion). Isolated dienes lie in between. So increasing heat of combustion is conjugated < isolated < cumulated.

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