Which of the following compounds is the most stable?...

Which of the following compounds is the most stable?
3,4-dimethyl-2-hexene; 2,3-dimethyl-2-hexene; 4,5-dimethyl-2-hexene
Which would you expect to have the largest heat of hydrogenation? Which would you expect to have the lowest heat of hydrogenation?

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Key Concepts

Alkene Substitution and Stability

The degree of substitution on the carbon–carbon double bond strongly influences alkene stability. Generally, more substituted alkenes (those bearing more alkyl groups around the double bond) are more stable than less substituted alkenes because alkyl groups donate electron density through inductive and hyperconjugative effects, reducing the double bond's reactivity.

Hyperconjugation

Hyperconjugation is a stabilizing interaction that results from the delocalization of electrons in ?-bonds (typically C–H or C–C bonds) adjacent to an empty or partially filled p-orbital or a ?-system. This electron delocalization helps to stabilize alkenes by distributing electron density around the double bond, and its effect increases with greater alkyl substitution.

Heat of Hydrogenation

Heat of hydrogenation is the measure of the energy released when an alkene is converted into an alkane through the addition of hydrogen. It is used to assess the stability of alkenes; a lower heat of hydrogenation indicates a more stable alkene because less energy is released when hydrogenation occurs, while a larger heat of hydrogenation suggests that the alkene is less stable.

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