A single chemical compound can have many different names: the generic name, trade name, colloquial name, or systematic name. The number of synonyms for any one particular substance can lead to confusion about which "name" is the correct one to use when researching a specific chemical compound.The best way to solve the problem of the synonyms is to use the Chemical Abstracts Service Registry Number (also known as CAS RN, CAS number, or the Registry Number).

CAS RNs are unique identifiers for chemical substances. Registry Numbers are assigned by the Chemical Abstracts Service and consists of 5 - 9 digits divided into 3 parts (XX-XX-X through XXXXXX-XX-X). Think of a CAS RN as a Social Security Number for chemical substances: just as each person is assigned a unique SSN, each chemical substance is assigned a unique identifying CAS Registry Number. Remember, substances often go by many different names, but the CAS-RN will remain the same no matter what name is used to refer to the substance.

CAS Registry Numbers are used to identify and index substances in reference works, databases, and articles. The CAS RN generally appears in brackets in the chemical literature. For example [58-08-2] is the CAS RN for caffeine.

Quickest ways to find the CAS RN for a specific substance:

ChemId Plus
Type in name or molecular formula into search box. The CAS RN will be displayed immediately under the name using the abbreviation RN.
http://chem.sis.nlm.nih.gov/chemidplus/

ChemFinder
Type chemical name, molecular formula or weight into search box. The CAS RN will be displayed on the results screen in brackets below the chemical name. Allows for five free searches before registration (which is free) is required.
http://www.chemfinder.com