CAS registry numbers are unique numerical identifiers for chemical compounds. They are also referred to as CAS numbers or CAS RNs. The Chemical Abstract Service, a division of the American Chemical Society, assigns these identifiers to every chemical that has been described in the literature. About 20 million compounds have received a CAS number so far, with about 4,000 new ones being added each day. The intention is to make database searches more convenient, as chemicals often have many names. Almost all molecule databases today allow searching by CAS number.
A CAS registry number is separated by hyphens into three parts, the first consisting of up to 6 digits, the second consisting of two digits, and the third consisting of a single digit serving as a checksum. The numbers are assigned in increasing order and do not have any inherent meaning. The checksum is calculated by taking the last digit times 1, the next digit times 2, the next digit times 3 etc., adding all these up and computing the sum modulo 10. For example, the CAS number of water is 7732-18-5, and the checksum is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) mod 10 = 105 mod 10 = 5.
Different isomers of a molecule receive different CAS numbers: D-glucose has 50-99-7 and L-glucose has 921-60-8. Occasionally, whole classes of molecules receive a single CAS number: all alcohol dehydrogenases have 9031-72-5.
To find the CAS number of a compound given its name, formula or structure, the following free resources can be used:
- NIST's Chemistry WebBook at http://webbook.nist.gov/chemistry/
- The NCI Database Browser at http://cactvs.cit.nih.gov/
- The Dutch Dictionary on Organic Chemistry at http://www-woc.sci.kun.nl/index.en.html