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I am unable to find the answer to this question. I was guessing it should be (2) due to symmetry but my book says it is (3) without any explanation.

I know that stability is determined by how low the energy of formation is. Does this have something to do with the different sorts of dienes that are present? Like, one type of diene would have less energy than another?

If nothing else, it would be helpful if someone could indicate the names of these compounds, because I don't even have the faintest idea where to start searching for more information.

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  • $\begingroup$ Lots of conjugation = resonance = stability. There are gradually increasing amounts of orbital involved in conjugation, so they are in order of increasing stability. $\endgroup$ – Nij May 8 '17 at 9:51
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You must be aware about the concept of resonance. Similar ly over here as you can see in option number 3 there is longer conjugation between double bonds then compared with any other option because Three double bonds are in conjugation and this conjugation supplies stability to the structure

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    $\begingroup$ This is not completely true, the rings are not planar; therefore there are only two bonds in resonance in 3. $\endgroup$ – Martin - マーチン♦ Dec 2 '18 at 0:15
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Structure $3$ is stabilized through resonance.

Double bond - Single bond - Double bond, can perform resonance.

More the number of resonating (or canonical) structures, more the stability.

Structure (2) also exhibits resonance, but it has less number of resonating structures than structure $(3)$.

Therefore, structure $(3)$ is the most stable one.

If you are not familiar with the concept of resonance, read it. You will encounter many situations where resonance will have a direct link with the stability of certain structures.

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3) option has an extended resonance than 2) and 1) which is major cause for its extra stability.

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    $\begingroup$ You make a good point, but should elaborate. $\endgroup$ – Mithoron May 8 '17 at 12:24

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