Chemistry

Chemical Abstracts Service has registered millions of chemical substances since 1965. CAS Registry Numbers (RNs) are very useful when searching for information about a specific chemical structure, as well as polymers, mixtures, alloys, and substances whose exact formula is unknown or variable.

A Registry Number (RN) looks like this:

123456-78-9

where the first segment can be from two to seven digits long, followed by two digits, then a single check-digit. It is a sequential accession number from the CAS Registry database. The RN carries no chemical or structural meaning in itself. It is simply an identifier for a specific substance that CAS has registered during the process of indexing the literature (or added from another source). The shorter the first segment, the older the registration and the more common (and probably better described) the compound is.

Using Registry Numbers as Search Terms

A Registry Number allows you to avoid using chemical names when searching for information about a compound. If you have a RN in hand, use it as a search term in SciFinder, in place of a chemical name. Most non-bibliographic chemical databases also allow searching by Registry Number.

Caveats

Registry numbers are useful substitutes for names, but they are not perfect.

• CAS registration policies are complex and highly specific. This is especially true for substances with undefined structures, commercial formulations, polymers, mixtures, inorganic substances (separate RNs are assigned for ions, charge states, hydration states, etc.), drugs (separate RNs for the free base and HCl forms), and for organic salts and stereoisomers.
• RNs used in non-CAS resources, such as supplier catalogs, SDS and commercial databases, may not always be correctly assigned, and often are not checked for accuracy or updated to reflect changes. When in doubt, verify a Registry Number's exact name and structure in SciFinder.
• While a de facto standard for chemical identification, CAS RNs are proprietary. Any systematic use of RNs in a third-party database requires a license.
• For intellectual property (IP) questions:  The actual date of a substance's registration can't be determined from CAS products. Contact Chemical Abstracts Service for more information.

Where to Find Registry Numbers

Many chemical databases and commercial catalogs use CAS Registry Numbers as a standard identifier.

Nuclides and Isotopes

NMR

X-Ray Properties

Fun & Educational...

Every discipline, and sometimes individual publishers and journals, have their rules about how bibliographic references and footnotes are to be formatted:  these are called styles.  Specific styles are often adopted by graduate schools for dissertation writers, and by instructors for student assignments as well.  In chemistry, ACS style is the standard; in biochemistry the CSE style is standard, but you should check author instructions for every journal you submit to.  The easiest way to cope with the many styles available is to use a reference manager tool like EndNote or Zotero, which can make formatting your references fairly easy.  For more detail and explanation about writing for publication, you can turn to style guides.