Every entity that exists in the nature has a name and a name is how we identify that entity. Semilarly, each chemical compound is identified by it’s name or names. Naming chemical compounds by IUPAC is long process. However, it does not mater which name or names the compound is assigned, it will be 100% that compound’s name or names.

But in practice (in databases, Excel spreadsheets, books, reviews, peer-reviewed articles, etc.), you’d find some disagreement about the structure that a particular identifier should be linked with, and vice-versa.

The disagreements would range from the baffling (completely wrong structure) to the annoying (wrong stereochemistry) to the amusing (ionized carboxylate vs. protonated).

For databases that aggregate content from diverse sources, the best practice may be to model this situation with a many-to-many relationship, rather than a one-to-one or even one-to-many.

In other words, CAS numbers, trade names, and IUPAC names may be better modeled as social networking-style tags than as unique identifiers. I’m not saying this is the way things should be – just that this is how situation appears to have evolved.

See this article, which discusses the problem as it applies to CAS numbers used in the wild and how Chempedia addresses it:

http://depth-first.com/articles/2008/05/26/simple-cas-number-lookup-and-more-with-chempedia