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Problem

Compare stability of 1 and 2:

1: 5‐methylcyclohexa‐1,3‐diene; 2: 3‐methylenecyclohexene

Answer

1 is more stable than 2.

My approach

  • Both have the same number of π-bonds.
  • Both have one resonance structure.
  • Both are non-aromatic.
  • In 1 there are three α-hydrogens, whereas in 2 there are four α-hydrogens.

As extent of hyper-conjugation is more in 2 than in 1, making 2 more stable, which contradicts the answer.

Addressing the comment:

A factor you haven't considered is ring strain. I'm not sure which of the two structures would have more ring strain, though.

I think that 2 has more strain because there are three $\mathrm{sp^3}$ carbon atoms which have been forced to be in a 120° angle, while their ideal angle is 109°28′. So, there is more strain in 2 compared to 1 because in 1 only two carbon atoms are forced to do so.

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    $\begingroup$ A factor you haven't considered is ring strain. I'm not sure which of the two structures would have more ring strain, though. $\endgroup$
    – zwol
    Apr 15, 2021 at 17:53
  • $\begingroup$ AFAIK, given that short of running an (expensive) numerical simulation, we only got some heuristics to work with, it'd be worth knowing to what extent do those really differ in terms of stability. If it's a small difference (say <1%), then I'd imagine that the heuristics may not be accurate enough to be trustworthy. It'd really help to know whether the authors of the textbook that posed this question have actually made any measurements or used published data, otherwise any answers you get may be misleading (garbage in = garbage out). $\endgroup$
    – Kuba hasn't forgotten Monica
    Apr 15, 2021 at 19:05
  • $\begingroup$ If you have access to the two compounds, determine their heats of hydrogenation or combustion. The less exothermic compound is more stable $\endgroup$
    – user55119
    Dec 6, 2021 at 15:41

1 Answer 1

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As far as I have read, in general, exocyclic double bonds (which are outside the ring) are stable for 3 or 4 numbered rings while endocyclic double bonds (which are inside the ring) are stable in 5 or 6 numbered rings.

As you note all the factors, more or less all factors are same in both cases, so it becomes a little handwavy thing.

Additional Reading:

  1. Goumans, T.; Ehlers, A.; Lammertsma, K.; Würthwein, E. endo/exo Preferences for Double Bonds in Three-Membered Rings Including Phosphorus Compounds. Eur. J. Org. Chem. 2003, 2003 (15), 2941–2946. DOI: 10.1002/ejoc.200300120.

  2. Fleck, B. Notes - Stability and Ease of Formation of Endo and Exo Double Bonds in 5- and 6- Ring Systems. J. Org. Chem. 1957, 22 (4), 439–439. DOI: 10.1021/jo01355a023.

  3. Exocyclic vs. endocyclic double bonds in E1 elimination (although this one is not much relevant here).

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