There is a small number of relatively simple organic entities containing at least one characteristic group that also could be treated as parent structures. These entities are known as functional parents.
For example, the preferred IUPAC name of the structure (a) is 2-hydroxybenzoic acid. Why not 2-hydroxybenzenecarboxylic acid? Because (a) is thought of as a derivative of benzoic acid (b). Now (b) contains a characteristic group –COOH and so cannot be called “parent hydride”, while benzene (c), indeed, is a parent hydride of both (a) and (b). However (b) is a functional parent and thus the name “benzoic acid” can be used to construct the substitutive name “2-hydroxybenzoic acid”.
 |
 |
 |
| (a) | (b) | (c) |
|---|
|
As far as naming is concerned, the functional parent names are used like parent hydride names, with one important constraint: if the most senior characteristic group of the functional parent is cited as a suffix (e.g. -ol, -one, -oic acid), so it should remain in the name under construction. As a corollary, the functional parent names are allowed to be be modified by prefixes only.
 |
 |
| (d) | (e) |
|---|
|
For example, (d) could be considered a derivative of either benzoic acid (b) or aniline (e). However the preferred name for (d) is 4-aminobenzoic acid, not 4-carboxyaniline (because –COOH is more senior than –NH2), nor aniline-4-carboxylic acid (because the functional parent name “aniline” cannot accept suffixes)*.
The list of IUPAC-approved functional parents includes several structures which do not contain any characteristic group, e.g. acetylene, toluene and styrene.
 |
 |
 |
| (f) | (g) | (h) |
|---|
|
For instance, both (f) and (g) can be considered to be derived from toluene (h). In case of (f), the preferred name is 2,4,6-trinitrotoluene. We know that nitro group can be only cited as a prefix. This is not the case with amino group: it could be used as either suffix (if it is a principal group) or prefix (if otherwise). In case of (g), the amino group is the only (and thus the most senior) type of characteristic group here, so the preferred name will be 2-methylbenzene-1,3,5-triamine, not 2,4,6-triaminotoluene (the principal group should be cited as suffix, not prefix) and not toluene-2,4,6-triamine (because the functional parent name “toluene” cannot accept suffixes). These two latter names are neither wrong nor ambiguous — their interpretation would yield the same structure (g) —, just not what is recommended according to IUPAC rules.
| * | Note that the ChEBI ontology uses the concept of functional parent broader than that of IUPAC. The ChEBI relationship “has functional parent” is
used to denote the relationship between two molecular entities (or classes of entities), one of which possesses one or more characteristic groups from which the other can be derived by functional modification.Thus, any “functionalised” molecular entity can be a functional parent. The purpose of IUPAC’s functional parents is naming, therefore only one functional parent has to be chosen for a structure to be named. On the other hand, in ChEBI a molecular entity can have more than one functional parent. To take our example (d):
4-aminobenzoic acid (d) has functional parent aniline (e) |
No comments:
Post a Comment