However, the carbonyl C=O group heavily diminishes the electron density of the double C=C bond that can thus be attacked by nucleophiles, just the opposite to what happens with a "normal" double C=C bond.
In alpha,beta-unsaturated carbonyl compounds, both the conjugation and the electronegativity of the oxygen atom induce a stabilization of the "pi" molecular orbitals relative to those of a "normal" alkene.
Consequently, alpha,beta-unsaturated carbonyl compounds react through their electron-empty LUMO with an electron-full nucleophile's HOMO that gives up electrons.
On the contrary, a "normal" alkene holds its LUMO too high in energy and then reacts through its HOMO, ceding electrons towards an electrophile's LUMO that receives the electrons.
The hydrogenation of 1,3-pentadiene (conjugated diene) happens to be less exothermic than that of its 1,4-isomer (isolated diene) meaning that the conjugated diene is more stable than anticipated. 
When 2-butenal (conjugated double bonds) is hydrogenated the reaction cedes less energy than its isomer 3-butenal (isolated double bonds).
