Section IV of the Liege Rules was headed "Simple Functions", and Section V "Complex Functions". Those rules defined compounds of simple function as those containing one kind of function only, and compounds of complex function as those possessing different functions (that is, functions of more than one kind). But the rules did not define or explain the meaning to be attached to the word "function" itself; nor can its precise meaning be inferred uniformly from the usages in the rules themselves. Usage of this term by chemical authors has varied greatly.

To minimize confusion the following terms are used in the present rules:

Functional class name: a word such as ketone, chloride, or alcohol, used in radicofunctional nomenclature (according to the language) as an ending or as a separate word (see Rules C-21 to C-24).

Substituent: any atom or group replacing hydrogen of a parent compound.

Characteristic group: an atom or group that is incorporated into a parent compound otherwise than by a direct carbon-carbon linkage, but including groups and >C=X where X is O, S, Se, Te, NH, or substituted NH. (N.BThe phrase "characteristic group" includes both groups such as OH, NH2, COOH, and single atoms such as halogen, =O, and It does not apply to substituents such as methyl, phenyl, 2-pyridyl, but does include, for example, piperidino and acetyl.)

Principal group: the characteristic group chosen for expression as suffix in a particular name. (This is equivalent to the "principal function" of the Liege rules.)

See Recommendations'93 R-0.2


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