 Many kinds of diketones are known, some with unusual properties.The simplest is diacetyl (CH3C(O)C(O)CH3), once used as butter-flavoring in popcorn.Acetylacetone (pentane-2,4-dione) is virtually a misnomer (inappropriate name) because this species exists mainly as the monoenol CH3C(O)CH=C(OH)CH3.