In a few cases its use is widely recognized: unsaturation in aliphatic compounds and cycloalkanes, bicycloalkanes, etc., is specified by endings "-ene" and "-yne" denoting loss of hydrogen (Rules A-3 and A-11.3); loss of water from two molecules of a monobasic acid or internal loss of water from a dibasic acid is described by the functional class name "anhydride" (Subsection C-4.9); other uses are for naming lactones and related compounds (Subsection C-4.7) and their sulfur analogues.
Three further methods of subtractive nomenclature are in use, namely, the use of the prefixes "de-" (Rule C-41), "nor-" (Rules C-42 and C-43) and "anhydro-" (Rule C-44).
See Recommendations'93 R-1.2.5
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