dienophile

In the Diels-Alder reaction it takes two to tango: the four-carbon diene supplies four pi electrons, and a smaller two-carbon partner supplies two. That smaller partner is the dienophile — literally the 'diene-loving' molecule. It is the alkene (or alkyne) that the diene wraps around to close the new six-membered ring.

On its own a dienophile is just a double or triple bond, but the good ones are not plain alkenes — they carry electron-withdrawing groups such as a carbonyl, a nitrile, an ester, or a nitro group right on the reacting double bond. Why? Those groups pull electron density away and lower the energy of the dienophile's empty pi orbital (its LUMO). The Diels-Alder works best when the diene's filled orbital (HOMO) and the dienophile's empty orbital (LUMO) are close in energy so they overlap strongly; an electron-poor dienophile brings its LUMO down to meet the diene's HOMO, and the reaction speeds up dramatically. A bare ethylene reacts only sluggishly; maleic anhydride or acrolein react readily.

Recognizing the dienophile is half of planning a Diels-Alder. In synthesis you deliberately choose a dienophile bearing an electron-withdrawing group both to make the reaction go and because that group becomes a useful handle on the product ring. The electron-withdrawing substituent also helps explain the endo preference, since it is the dienophile's substituent that tucks under the diene in the favoured transition state. Diene plus dienophile is the simplest way to remember what a Diels-Alder needs.