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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 H.

Example to R-1.3


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.

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.

Example to R-1.3


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 separate publication.

See Also:
R-1.0 Introduction
R-1.1 Bonding Number
R-1.2 Nomenclature Operations

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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