 For the most important ketones, however, traditional nonsystematic names are still generally used, for example acetone and benzophenone.These nonsystematic names are considered retained IUPAC names, although some introductory chemistry textbooks use systematic names such as "2-propanone" or "propan-2-one" for the simplest ketone (CH3−CO−CH3) instead of "acetone".The derived names of ketones are obtained by writing separately the names of the two alkyl groups attached to the carbonyl group, followed by "ketone" as a separate word.