# Stability of branched alkenes

I'm having difficulty understand the relative stabilities of the compounds

2,4-dimethyl-3-methylidenepentane < (3⁠Z)-2,5-dimethylhex-3-ene < (3⁠E)-2,5-dimethylhex-3-ene

which, for simplicity’s sake, can be described as

1,1-diisopropylethene < (Z)-1,2-diisopropylethene < (E)-1,2-diisopropylethene

The compounds are listed in order of increasing stability.

Also, what does the $\ce{C=C-C}$ bond angle have do to with stability?

Additionally, the degree of substitution of an alkene has a dramatic effect on stability. The presence of alkyl groups on the carbons of an alkene is generally stabilizing (as can be empirically confirmed by examining thermodynamic data, i.e., heats of hydrogenation and heats of combustion). Evidently, calculations grounded in molecular orbital theory indicate that the mixing of $\sigma$ type molecular orbitals of alkanes with the $\pi$ orbitals of less-substituted alkenes is typically energy-lowering overall, and that the $\pi^{*}$ antibonding orbitals are sufficiently close in energy to the $\sigma$ orbitals of alkanes that hyperconjugative stabilization occurs (albeit relatively small in magnitude, since the energies are not very close). In the 1,1- isomer, only one carbon of the alkene can be stabilized by hyperconjugation, while both carbons should be able to benefit from it in the 1,2- isomers. (Speculatively, I would also think that the extent of orbital overlap should be poorer in the 1,1- isomer, since the geminal isopropyl groups would likely be staggered with respect to one another, which might prevent proper alignment of the alkyl group orbitals with those of the alkene $\pi$ system.)