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, 
, and 
, but many exceptions are recognized, such as 
and 
. It
seems appropriate at this time to retain the general view of functionality as implying the presence of heteroatoms and/or unsaturation, but it would not be helpful to attempt to define
precisely the limits of application of the term.
Carbon-carbon unsaturation in acyclic species is regarded as a special type of functionality and it is therefore treated here in Section R-3 rather then in Section R-2 (Parent Hydrides); however, its presence here (and that of hydrogenation of parent hydrides containing the maximum number of noncumulative double bonds) is anomalous in that for some purposes, for example, choice of parent, it can be regarded as part of the parent; but for others, such as numbering, it is treated like a substituent.
Section R-3 also deals with functional parents, i.e., structures which are treated as parent structures, having substitutable hydrogen atoms, but which possess the characteristics normally
associated with functionality [e.g., phosphonic acid 
].
Although, strictly speaking, ions and radicals do not fall within the concept of functionality, as described above, an ionic centre or a radical centre is treated like a function, and this treatment is also included, therefore, here in Section R-3.
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