Organic molecules are often thought of as comprising a skeleton, for example a chain or a ring, adorned by functional groups. Having just read about skeletal replacement, you might think that functional replacement has something to do with those functional groups. It’s only logical. But you’d be mistaken.
Let’s name the structure (a):
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| (a) |
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Of course, its formula [CS(SH)2] gives us a clue: we can call such an entity additively sulfidodisulfanidocarbon. But there is another way to do it.
Compare the structure (a) with that of carbonic acid (b).
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| (b) |
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All we need to do is to replace the oxygen atoms with sulfurs. And functional replacement allows us to do exactly that with parent oxoacids.
| Replacement operation | Prefix | Infix |
|---|---|---|
| –OH → –Br | bromo | bromid(o) |
| –OH → –Cl | chloro | chlorid(o) |
| –OH → –CN | cyano | cyanid(o) |
| –OH → –F | fluoro | fluorid(o) |
| –OH → –I | iodo | iodid(o) |
| –OH → –NH2 | amid(o) | amid(o) |
| O → OO | peroxy | peroxo |
| O → S | thio | thio |
| O → Se | seleno | seleno |
| O → Te | telluro | telluro |
Thus modified parent carbonic acid becomes trithiocarbonic acid if we use the prefix method, or carbonotrithioic acid if we use the infix method. By some reason IUPAC seems to favour “infix names”. Could it be because the “prefix names” can be confused with the substitutive names? I doubt it, for inorganic oxo acids are neither parent hydrides nor functional parents. Personally, I prefer the “prefix names”: they are shorter and easier to pronounce*.
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| (c) | (d) |
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Likewise, the structure of anion (c) can be seen as a derivative of phosphate (d) where one of the oxygens is replaced by peroxo group, which allows us to give it a rather simple “prefix name” peroxyphosphate — much shorter than the mouthful of trioxidoperoxidophosphate(3−). The Red Book refers to peroxyphosphate as to “acceptable common name” while recommends the functional replacement name phosphoroperoxoate [1, p. 139]. This latter name is derived from phosphoroperoxoic acid, which is a peroxo derivative of phosphoric acid. The concern that “prefix names” for anions such as trithiocarbonate, the fully deprotonated anion of (a), could be mistaken for additive names is voiced — and immediately dismissed — in the Red Book: an additive name would employ ‘sulfido’ or ‘sulfanediido’ prefix, not ‘thio’ [1, p. 138]. So no confusion here.
Note that ‘peroxo’, ‘thio’, ‘seleno’ and ‘telluro’ replacement operations are not confined to –OH group as is the case with the rest of functional replacement operations. For example, ‘thio’ can mean –OH → –SH, =O → =S, or –O− → –S−. This is bound to lead to ambiguity.
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| (e) | (f) | (g) |
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For instance, the common name thiosulfurous acid refers to both (e) and (f), as each of these tautomers is a result of replacement of one oxygen atom in sulfurous acid [SO(OH)2]. To disambiguate, we call the structure (e) sulfurothious O-acid and the structure (f) sulfurothious S-acid, which could be important to name the corresponding esters. In case of fully deprotonated anion (g), we don’t care whether it originates from (e) or (f): we can call it just sulfurothioite or, easier, thiosulfite.
To sum up:
- Functional replacement nomenclature uses the common names of oxoacids to create the names for their –OH or =O substitution derivatives;
- This substitution is indicated by either prefixes or infixes;
- The “infix names” cannot be confused with substitutive or additive names but are longer than the more commonly used “prefix names”.
| * | But see my post discussing whether “prefixes” in chemical nomenclature are really prefixes. |
Reference
- Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005.
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