Principles of Organic Nomenclature
R-1.3 Indicated Hydrogen Under certain circumstances it is necessary to indicate in the name of a ring, or ring system, containing the maximum number of noncumulative double bonds, one or more positions where
no multiple bond is attached. This is done by specifying the presence of an "extra" hydrogen atom at such positions by citation of the appropriate numerical locant followed by an italicized capital
In the above example, the "indicated hydrogen" locates two hydrogen atoms at position 3 (an "extra" hydrogen over the number present if there were a double bond in the ring at that position), thus
specifying a particular pyrrole tautomer, as illustrated. Indicated hydrogen of this type normally precedes the name of a parent hydride.
Example to R-1.3
A second type of indicated hydrogen (sometimes referred to as "added hydrogen") describes hydrogen atoms added to a specified structure as a consequence of the addition of a suffix or a prefix
describing a structural modification. This type of indicated hydrogen is normally cited in parentheses after the locant of the additional structural feature.
The detailed procedures for using indicated hydrogen, including differences in approach used in index nomenclature by Chemical Abstracts Service and in Beilstein, will be described in a
Example to R-1.3
- See Also:
R-1.1 Bonding Number
R-1.2 Nomenclature Operations
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(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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