The chemical reduction of 1,3-butadiene with sodium in alcohol gives mainly the 1,4-addition product that is but-2-ene. Under these condition the isolated double bond are not reduced. This suggest that dienes are much more reactive than simple alkenes.

Which diene is most reactive?

The most reactive dienophile is the aldehyde — propenal.

What makes a dienophile less reactive?

The quinone dienophile in reaction F has two dienophilic double bonds. However, the double bond with two methyl substituents is less reactive than the unsubstituted dienophile due in part to the electron donating properties of the methyl groups and in part to steric hindrance.