According to IUPAC rules, am I right in assuming that butanone is sufficient, because there's only one possible molecule? I keep noticing butan-2-one, so I want to make sure.
The preferred IUPAC name is butan-2-one. This is explicitly mentioned in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book).
Generally, the complete information about a structure is explicitly given by the name and does not rely on any implied information. Nevertheless, the practice of omitting locants when there is no ambiguity is widespread in general nomenclature. For preferred IUPAC names, however, locants are omitted only in a few exceptional cases described in Subsection P-14.3.4 of the Blue Book:
- P-126.96.36.199 terminal locants
- P-188.8.131.52 the locant ‘1’
- P-184.108.40.206 where there is only one kind of substitutable hydrogen
- P-220.127.116.11 when no isomer can be generated by moving suffixes or prefixes from their position to another or by interchanging them between two different positions
- P-18.104.22.168 when all substitutable positions are completely substituted
- P-22.214.171.124 when all substitutable hydrogen atoms have the same locant.
None of these cases applies to butan-2-one. Therefore, the locant cannot be omitted in the preferred IUPAC name.