20
$\begingroup$

When we tell students about the formation of alkenes (by elimination for example), we often tell them that reactions will favour the thermodynamically favourable most substituted alkene. Zaistev's rule empirically described this:

"The alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the β-carbon having the fewest hydrogen substituents"

The stability of the alkene with increasing substitution mono>di>tri is easily explained by hyperconjugation/qualitative MO theory (Phillip wrote a concise answer about that here). As an example to clarify, if theres a choice between eliminating to form a trisubstituted double bond over a mono substituted, the tri will predominate in most situations.

What I often struggle to explain in any detailed way is why tetra-substituted alkenes don't entirely follow the trend. They are generally quite difficult to form (metathesis aside, but that isn't an elimination in the classical sense), and if formed, are generally quite reactive (on a similar order of magnitude to di-substituted alkenes if some studies on hydrogenation and some calculations are correct).

My general answer is 'sterics', based on the fact that 4 substituents around the same alkene is quite crowded, but I'm the first to acknowledge that this 1)doesn't really address all of the deviations from expected properties and 2)isn't really an explanation.

$\endgroup$
  • 1
    $\begingroup$ Again hyperconjugation- it increases the nucleophlicity of the alkene. (P.S. like the photo on your profile site) $\endgroup$ – RBW Dec 22 '16 at 21:31
  • $\begingroup$ Perhaps, I'm not convinced by that argument in itself, since the 'phenomenon' actually holds even when there aren't groups capable of hyperconjugation, it also wouldnt explain why tri>di>mono but tetra isn't > tri. $\endgroup$ – NotEvans. Dec 22 '16 at 21:34
  • 1
    $\begingroup$ One very important question to ask is what exactly are you comparing? $\endgroup$ – Zhe Dec 27 '16 at 16:36
  • 2
    $\begingroup$ That's not what I mean. You're talking about "stability," so you are comparing relative energies. Which differences are you comparing? From some cation for elimination (i.e., where Zaitsev's rule would apply), or from constituent elements (i.e., $\Delta G_{f}^{\varnothing}$)? $\endgroup$ – Zhe Dec 27 '16 at 18:41
  • 1
    $\begingroup$ Applauds @Zhe. I also have a bit trouble understanding what you are getting at. Are you asking why tetra-substituted double bonds are hardly formed by elimination? Or are you asking why tetra-substituted alkenes are less stable than tri-substituted alkenes for some -to be specified- set of conditions? (which I doubt) But then I don't understand where Zaitsev's rule comes into play. Stability is a relative concept, you have to state what you are comparing... $\endgroup$ – Martin - マーチン Dec 28 '16 at 11:17
4
$\begingroup$

I agree with the OP and have seen numerous examples where tetrasubstituted olefins are indeed quite difficult to make, both in the literature and in my own experience, leading to the formation of Anti-Zaitzev (Hoffman) elimination products. Many question writers are reluctant to make a solution where the tetra-substituted option is the correct answer. It is not as simple as the trend in carbenium ion stability where hyperconjugation and conjugation dominate around a trigonal planar (less sterically encumbered) center. A tetrasubstituted olefin is too sterically encumbered due to 1) two A(1,2) interactions and 2) two exo substituents (gem-dimethyl) interactions.

Such proof for this failure in the stability trend is included in the original question as the kinetic stability of such an olefin is unusually and unexpectedly high. As far as the the thermodynamic stability, torsional strain makes effective hyperconjugation less possible in the transition state which varies only marginally upon the strength of base and the lability of the bond to the leaving group.

This trend: mono is less stable than exo-di is less stable than cis-di is less stable than trans-di is less stable than tri is....that is about all you can correctly say about the trend. Consider why cis-disubstituted is less stable than a trans-disubstituted olefin. Now consider a tetra-substituted alkene.

Zaitsev's "rule" like Markownikoff's "rule" is a just fast and dirty guideline for undergraduates and which both have numerous exceptions. It does not follow that tetrasubstituted must be more thermodynamically stable than tri-substituted and product ratios indicate this repeatedly in the literature. Stereoselectivity of E over Z is a non-problem compared to the difficulty of and abysmal yields when creating a tetrasubstituted olefin.

Simply put an ellipsis ("...") after the trisubstituted alkene as that is all that can be stated to an undergraduate class without presenting a falsehood.

$\endgroup$

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.