Project Details A Comparison of Assignment of Hydro Prefixes, Added and Indicated Hydrogens in IUPAC, CAS and Beilstein Nomenclature Systems

Project No.:
Start Date:
01 November 2012
End Date:


  • To explain currently recommended IUPAC nomenclature procedures to assign indicated and added hydrogens and hydro prefixes.
  • To explain differences between IUPAC, Chemical Abstracts Service and Beilstein procedures and their influence on chemical names.


Chemical names of compounds with partially hydrogenated ring systems, derived from parent structures containing the maximum number of noncumulative double bonds (mancude), need specification of positions without double bonds. This is achieved by specifying the presence of an “extra” hydrogen atom by indicated and added hydrogens and hydro prefixes.

Currently published IUPAC recommendations on nomenclature of organic compounds allow different ways to specify the saturated positions. Other variations of the procedures are represented by Chemical Abstracts Service and Beilstein nomenclature systems. Several different names may be created for the same compound. These multiple conventions do not help chemists to understand such names and as a result make this procedure probably one of the least understood in organic nomenclature.

The aim of the project is description of currently recommended IUPAC procedure to specify saturated positions for mancude parent systems and comparison of alternative procedures applied in IUPAC, CAS and Beilstein names.


Oct 2020 – A draft is nearly ready for review.

Last update 15 Oct 2020