8
$\begingroup$

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.

Answer 2.0:

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 the second structure has more strain because in the second structure there are 3 $\ce{sp^{3}}$ 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 second structure as compared to first structure because in 1st, only 2 carbon atoms are forced to do so.

$\endgroup$
2
  • $\begingroup$ 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 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). $\endgroup$ Apr 15 at 19:05
7
$\begingroup$

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).

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.