Hantzsch-Widman names indicated hydrogen

In one method, the non-detachable prefixes hydro- and dehydro- (in the case of mancudes only) can be used to indicate the addition or subtraction of one hydrogen atom. As even numbers of hydrogen atoms are involved when a carbon-carbon single bond becomes a double or triple bond, the multiplicative prefixes di-, tetra-, etc., as well as the appropriate locants, are used. The prefix dehydro- is always used to indicate the subtraction of hydrogen atoms from saturated heterocycles having Hantzsch-Widman names. 

Hantzsch-Widman names are used for one-ring heterocyclic systems that do not have trivial names. The names are applied to monocyclic compounds containing one or more heteroatoms in three- to ten-membered rings. The names are derived by combining the appropriate prefix or prefixes for the heteroatoms with a stem denoting the size of the ring (see below). The state of hydrogenation is indicated either in the stem or by the prefixes dihydro-, tetrahydro-, etc. 

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