Thioacetic acid. Ambiguity by IUPAC design?

Issue #11 resolved
Steve Chapman
created an issue

Opsin parses thioacetic acid as thioacetic O-acid, i.e., the hydroxy moiety is undisturbed. ACD/Labs Name parses thioacetic acid as thioacetic S-acid, i.e. the carbonyl moiety is undisturbed.Wikipedia also favours the S-acid.

Opsin rejects the qualifier for the acid, e.g. thioacetic S-acid, thioacetic O-acid, but accepts it in an ester, e.g. Thioacetic acid S-methyl ester, Thioacetic acid O-methyl ester

I don't know that there is a right or wrong answer, just an ill-conceived overload of thio in this context with no clear guidance from IUPAC that I can find.

Comments (6)

  1. Daniel Lowe repo owner

    In IUPAC nomenclature if unspecified it is formally ambiguous as to which is replaced

    P- states "Replacement of oxygen atom(s) of a carboxylic acid group by another chalcogen is indicated by the affixes ‘thio’, ‘seleno’, and ‘telluro’. These names do not differentiate between tautomeric forms of mixed chalcogen acids"

    The choice to replace the carbonyl simplifies interpretation of names like "thiocarbonyl" (which internally is actually handled as a derivative of carbonic acid!). It also simplifies handling of anhydride names e.g. "thioacetic thiopropionic anhydride" which is O rather than S linked. When I originally made the arbitrary choice to prefer the carbonyl I think the fact that this was ChemDraw's default interpretation was also a strong influence.

    Admittedly if as Wikipedia implied the thiol tautomer is heavily preferred this is clearly not an optimal interpretation. As it's not strictly wrong I'm not certain I will change it though due to complications it introduces elsewhere.

    The lack of support for S-acid and O-acid is something I've been meaning to do for a while. It's slightly annoying to implement as the suffix "ic S-acid" is a mix of the specification for a carboxylic acid and the indicating of the correct tautomer for a sulfur atom which is not strictly part of the suffix [you can easily get incorrect names like acetic S-acid]. Probably internally it would be split out into these two meanings to allow things to be done correctly in sequence: Add carboxylic acid suffix Replace one oxygen with sulfur (record degeneracy between atoms; this is already done to handle the ester case) If not the specified tautomer, change the group to give the specified tautomer.

  2. Steve Chapman reporter

    Thanks Daniel, I appreciate your elaboration. Opsin is like experimental science: unexpected results so often lead to deeper understanding. I had never confronted thio acid nomenclature before this chance encounter.


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