 A variety of hydroxylic compounds can add as nucleophiles, forming either enol or ester products.As examples, a water molecule easily adds to ketene to give 1,1-dihydroxyethene and acetic anhydride is produced by the reaction of acetic acid with ketene.Reactions between diols (HO−R−OH) and bis-ketenes (O=C=CH−R′−CH=C=O) yield polyesters with a repeat unit of (−O−R−O−CO−R′−CO).