I was looking up protoporphyrin IX and was surprised to find that no two websites gave the same IUPAC name for the compound. Here are some examples:

  1. 3,3'-(7,12-Diethenyl-3,8,13,17-tetramethylporphyrin-2,18-diyl)dipropanoic acid (EPA)
  2. 3,3'-(3,7,12,17-Tetramethyl-8,13-divinyl-2,18-porphyrindiyl)dipropanoic acid (CHEMSPIDER)
  3. 3-[18-(2-carboxyethyl)-8,13-bis(ethenyl)-3,7,12,17-tetramethyl-22,23-dihydroporphyrin-2-yl]propanoic acid (PUBCHEM)

Anyone know if an IUPAC name is agreed upon among scientists or if there is a preferred IUPAC name?

  • $\begingroup$ I'd assume that the name from pubchem is right. I've not kept up all the naming rules, but they must have changed a number of times since organic chemists are clever fellows and create all sorts of weird molecules that no one considered that would have to be named. Would you have thought that a buckyball could actually be created? $\endgroup$ – MaxW May 2 '20 at 4:49
  • $\begingroup$ At a first glance, the Chemspider name with "vinyl" and "2,18-porphyrindiyl" cannot be the preferred IUPAC name. $\endgroup$ – user7951 May 2 '20 at 9:13

I think both EPA and ChemSpider names are acceptable. However, I don't agree with dihydroporphyrin part in PubChem name. This is because the Gold Book gives following framework for porphyrin nucleus:

Protoporphyrin IX

According to the numbering the IUPAC name of protoporphyrin IX is 3,3'-(7,12-Diethenyl-3,8,13,17-tetramethylporphyrin-2,18-diyl)dipropanoic acid. This name based on least locant numbers for ethynyl group, which have higher priority before methyl groups (3,3'- is based on positions of propanoic acid). The same name is given to protoporphyrin IX in Ref.2, without 3,3'- locants.


  1. IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.
  2. IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN): Nomenclature of tetrapyrroles (Recommendations 1986); Prepared for publication by Gerard P. Moss, Queen Mary College, London, Eur. J. Biochem. 1988, 178(2), 277-328 (https://doi.org/10.1111/j.1432-1033.1988.tb14453.x) (PDF).

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