# Stability of alkenes tri>di>mono, but how to explain tetra

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.

• Again hyperconjugation- it increases the nucleophlicity of the alkene. (P.S. like the photo on your profile site) – RBW Dec 22 '16 at 21:31
• 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. – NotEvans. Dec 22 '16 at 21:34
• One very important question to ask is what exactly are you comparing? – Zhe Dec 27 '16 at 16:36
• 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., $\Delta G_{f}^{\varnothing}$)? – Zhe Dec 27 '16 at 18:41
• 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... – Martin - マーチン Dec 28 '16 at 11:17