This restriction does not apply to suffixes denoting unsaturation, such as "-ene" and "-yne" (see R-3.1), which are not regarded as principal characteristic groups; nor to the ionic and
radical suffixes, such as "-ium", "-ide", and "-yl" (see R-126.96.36.199), which may be combined and may also be added to certain principal characteristic group suffixes, for example,
ethanaminium and pyridin-1-ium-3-yl.
This HTML reproduction is as close as possible to the published version [see IUPAC, Commission on Nomenclature of Organic Chemistry. A Guide to IUPAC Nomenclature of Organic Compounds
(Recommendations 1993), 1993, Blackwell Scientific publications, Copyright 1993 IUPAC]. If you need to cite these rules please quote this reference as their source.
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