Why Tetrasubstituted Alkenes More Stable Than Tri/Bi/Mono

[Queequeg]

I understand it when you have a mechanism and the tertiary carbocation is more stable than a secondary and primary carbocation, but is that the reason? Cis-isomers are less stable than trans isomers because of steric strain on that side of the double bond, but a tetrasubstituted alkene will have twice the steric strain and more so with bulkier substituents.

If I had to guess, it's because the stability of the tertiary carbocation outweighs the significance of the steric strain.

 

[Yanick]

You are correct about about the cis- and trans-alkene case being readily explained by the increased steric strain for the cis-alkene case. When it comes to alkene stability in regard to substitution, the theory is meant to explain the trend that a more substituted alkene will have a smaller enthalpy change upon hydrogenation. Since the enthalpy change is inversely proportional to "stability" of the alkene, we can simply take the stability trend as an experimentally determined trend. The theory which explains the trend, from memory, is that highly substituted alkenes have a greater extent of hyperconjugation interactions.