Discussion

My name is Angela. I'm a chemistry major at Campbell University. I am taking this course as a seminar credit for the curriculum.

My name is Nathan. I am a senior at Campbell University and I am looking to expand my knowledge in all areas of chemistry. I"m looking forward to learning and growing in the area of Cheminformatics.

Phuc

I think Leah and Sunghwan have answered your question very well. You may also be interested in comments Anja/I made about CAS RNs in the document: Reaxys_SubstancesSearch_270117 that appears on the Exploring Reaxys module. Specifically:

A note about CAS Registry Numbers
CAS Registry Numbers were developed by CAS to use as a single identifier for the systematic indexing of (precise) substances in CAS databases. Because many substances can be identified easily in this way, CAS Registry Numbers are quite commonly used worldwide.
However, nearly all database producers have alternative ways to (systematically) index substances. Many of these index terms are text-based and are often the most commonly used name for the substance. Some database producers also index substances such as salts and mixtures under the name of the parent substance, so, unlike with the CAS registry system, a single index name can be used to include many closely related substances.
Algorithm-based correlations between CAS Registry Numbers and the systematic substance terms used by other databases are quite difficult to establish. Commonly, intellectual correlations need to be undertaken. In practice, this means that CAS Registry Numbers should be used only as an initial step to find the corresponding Substance Records in Reaxys. They should not be considered as precise or comprehensive terms for use in Reaxys, as you may find the answer with an alternative search.

Over the years I have often heard chemists say: "The CAS Registry System is crazy", and one example they refer to is the registration of sodium acetate in Registry, specifically the molecular formula: C2H4O2.Na . In taking this course you will realise that such a registration (and the various other registrations used by other databases) has logic - and use (e.g., it allows related salts to be retrieved easily). You will further understand that Sunghwan's comment that the way we talk about substances in lectures is not necessarily the way computers think about substances - for good reason.

Damon

Hi, Phuc.

Yes, a compound can have multiple CAS numbers for *many* reasons. to help you understand this topic, please visit the PubChem Compound Summary page for "benzene" (CID 241).

https://pubchem.ncbi.nlm.nih.gov/compound/241#section=CAS

You will see four different CAS numbers for benzene:

71-43-2 (for benzene; from 8 sources)
8030-30-6 (for naphtha, which a primary source of benzene; from 3 sources)
26181-88-4 (for benzene with Carbon-14 & Trinitum (hydrogen-3); from 1 source)
27271-55-2 (for benzene with Carbon-14; from 1 source)

You can check who provided the CAS numbers to PubChem, by *clicking* the "from XXX, YYY, ZZZ, ......." string below each CAS number. After you expand the source information, carefully look at the "Record Name" part for each CAS number. You will see that different CAS numbers were assigned to each of four cases, including benzene, naphtha (the primary natural source of benzene), benzene with C-14 isotopes, and benzene with C-14 and tritinium.

Similarly, you can check what CAS numbers phenol have *in what context* from the PubChem Compound Summary page for phenol (CID 996):

https://pubchem.ncbi.nlm.nih.gov/compound/996#section=CAS

108-95-2 (phenol)
65996-83-0 (Extracts, coal tar oil alk.)
61788-41-8 (Phenol, sulfurated)
84650-60-2 (Tea, extrats)
27073-41-2 (Phenol, homopolymer)
63496-48-0 (phenosmolin)
73607-76-8 (phenol, labeled with C-14 isotope)

Well, while chemists tend to think the one-to-one correspondence between structures and identifiers (or names), here you can see that there are a lot of contexts about "phenol", in terms of isotopic labels, polymeric states, "natural" sources (e.g., whether extracted from coal tar or tea), or whether it's mixed or modified (e.g., sulfurated).

Well, as a side note, in cheminformatics (or at least in this course), it is critical to unteach yourself about what you've learned from traditional chemistry classes. These courses are designed to teach you knowledge that is relevant to a particular context, often leading you to use a wrong assumption/context when looking at the data generated in different assumptions/contexts.