Question

How hyperconjugation decreases the heat of hydrogenation and how heat of hydrogeation is to do with the stability of alkenes. Please give a detailed explanation.

Answer 1

If an alkene is stable, its in a "low energy state", so when it becomes more stable it will be in a "lower energy state". So, given that, if something goes from a high energy state to a low energy state it will have a higher heat of hydrogenation because its energy released was large as it went from high to low. And, if something was in a low energy state and goes into a lower energy state, the heat of hydrogenation will be smaller because it went from low to lower.

I believe heat of hydrogenation is an enthalpy value, so think of it in terms of enthalpy and I think it will make more sense.

ΔH = ΔE + PΔV - if we assume that there is no change in volume.
ΔH = ΔE = E_final - E_initial


So, when something is unstable it has a "high internal energy" and when it is stable it has a "low energy". So, the former case I presented in the first paragraph is indicative of a large, negative ΔE (Efinal << Einitial), and the latter case is indicative of a small, negative ΔE (Efinal < Einitial, but not "much"), so their heats of hydrogenation are large, and small, respectively.

hence hyper conjugation also consist of formation of double bond which is ultimatly formed in order to provide stability to compound hence which results in formation of low energy state
which results in decrement of heat of hydrogenation due to above states reasons