Finding and Using Chemical Abstracts Registry Numbers

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Chemical Abstracts Service has assigned over 48 million registry numbers for chemical compounds 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 variable substances whose exact formula is unknown or variable. CAS has also assigned RNs to around 61 million biological sequences.

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, and it carries no chemical or structural meaning in itself. It is simply an identification number for a specific substance that CAS has registered during the process of indexing the literature. The shorter the first segment, the older the registration and the more common (and probably better described) the compound is.

Where to Find Registry Numbers

Many printed and online reference sources about chemicals use CAS Registry Numbers as a standard identifier. (See Caveats below.)

• Registry Database (SciFinder)
• Sigma-Aldrich Catalog (or virtually any chemical supplier's catalog)
• NIST Chemistry WebBook
• Combined Chemical Dictionary
• Common Chemistry (CAS)
• ChemFinder
• Beilstein Crossfire
  (Note: Beilstein has not included CAS RNs for compounds added to the database since 1994, due to legal issues.)
• CRC Handbook of Chemistry and Physics (Organic and Inorganic compound tables)
• Kirk Othmer Encyclopedia of Chemical Technology
• Merck Index

Chemical Names and Structures

Using Registry Numbers as Literature Search Terms

A registry number allows you to avoid using chemical names when searching for information about that compound. If you have a Registry Number in hand, use it as a search term in place of the chemical name. Bibliographic indexes that allow RN searching include:

• SciFinder - Explore Substances - Substance Identifier; or include the RN in a research topic query
• CA Student Edition - type RN into a search box and use pulldown menu to select the Registry Number field.
• Medline (Ebsco and ISI) - type RN into search box and select RN field.

Most non-bibliographic chemical databases also allow searching by Registry Number, including:

• Beilstein Crossfire
• Combined Chemical Dictionary
• CRC Handbook
• NIST Chemistry WebBook

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Caveats

Registry numbers are very useful shortcuts, but they are not perfect.

• CAS' complex registration policies, plus registrations from non-CAS (i.e., non-literature) sources, can cause ambiguity about proper assignment and structure definition. This is especially true for commercial formulations, polymers, inorganic substances (separate RNs are assigned for ions, charge states, hydration states, etc.), and for organic salts and stereoisomers.
• RNs used in non-CAS resources, such as supplier catalogs, MSDS, handbooks and databases, may sometimes be incorrectly assigned, and often are not checked for accuracy or updated to reflect changes. Any doubtful RN should be verified in the Registry database. In 2008 CAS began a cooperative project to verify RNs in Wikipedia articles on chemicals.
• While a de facto standard for chemical identification, RNs are proprietary. Any systematic use in a third-party database requires a license.

More Information

See the CAS Registry System to learn more about Registry Numbers and their importance.