The benzene CAS number is 71-43-2, a unique numerical identifier assigned by the Chemical Abstracts Service to distinguish benzene from all other chemical substances. This number is essential for tracking, regulating, and documenting benzene in industrial, environmental, and safety contexts.
CAS numbers are assigned by the American Chemical Society's Chemical Abstracts Service, a division of the American Chemical Society. Each number is structured as a three-part sequence separated by hyphens: the first part contains up to seven digits, the second part has two digits, and the third is a single check digit. For benzene, the number 71-43-2 is globally recognized and appears on safety data sheets (SDS), regulatory filings, and chemical databases.
Benzene is a colorless, volatile liquid with a sweet odor. It is a fundamental building block in organic chemistry and widely used in the production of plastics, resins, synthetic fibers, rubber, dyes, and pharmaceuticals. Despite its utility, benzene is classified as a known human carcinogen by the International Agency for Research on Cancer (IARC) and the U.S. Environmental Protection Agency (EPA).
The CAS number system ensures unambiguous identification of chemicals across languages, regions, and scientific disciplines. Unlike common names or chemical formulas-which can vary or overlap-each CAS number is unique. For benzene, 71-43-2 is the only identifier assigned to the compound C₆H₆.
When a substance is registered under REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) in the European Union, or listed under TSCA (Toxic Substances Control Act) in the United States, the CAS number is required for compliance. Regulatory agencies use it to track exposure limits, emission standards, and reporting obligations.
For example, the EPA sets a maximum contaminant level (MCL) for benzene in drinking water at 0.005 mg/L. This limit is referenced using the CAS number 71-43-2 in federal regulations. Similarly, OSHA (Occupational Safety and Health Administration) enforces a permissible exposure limit (PEL) of 1 ppm (parts per million) over an 8-hour workday, again citing the CAS number.
CAS numbers apply to pure substances, mixtures, and even certain isotopes. Benzene itself is a pure compound with one CAS number. However, derivatives such as chlorobenzene (CAS 108-90-7) or methylbenzene (toluene, CAS 108-88-3) have distinct identifiers.
| Compound | CAS Number | Use Case |
|---|---|---|
| Benzene | 71-43-2 | Industrial solvent, precursor to styrene |
| Chlorobenzene | 108-90-7 | Intermediate in pesticide synthesis |
| Toluene | 108-88-3 | Paint thinner, solvent in adhesives |
CAS numbers do not indicate purity or concentration. A batch of benzene labeled with CAS 71-43-2 may still vary in quality. The number identifies the substance, not its grade. Users must verify purity through additional specifications like assay percentage or impurity profiles.
A frequent misunderstanding is equating CAS numbers with molecular formulas. While benzene's formula is C₆H₆, many compounds share this formula (e.g., cyclohexane is also C₆H₁₂-incorrect example; correct: benzene and its isomers like fulvene). Only the CAS number provides definitive identification.
Another error is assuming that CAS numbers are assigned by governments. They are not. The CAS system is privately managed by the American Chemical Society. However, governments adopt CAS numbers for regulatory purposes due to their global consistency.
Some users confuse CAS numbers with EINECS or EC numbers used in EU regulations. EINECS (European Inventory of Existing Commercial Chemical Substances) numbers were phased out in favor of the more standardized CAS system. Today, both systems may appear side-by-side, but CAS remains the primary reference in international databases.
In 2021, a chemical plant in Texas reported a benzene leak into a nearby river. Emergency responders used the CAS number 71-43-2 to access the SDS from the supplier. The document listed acute toxicity data, including an LC50 (lethal concentration for 50% of test animals) of 16,000 ppm in rats over four hours.
Environmental regulators cross-referenced the CAS number with the EPA's Toxics Release Inventory (TRI) database. They confirmed that benzene releases above 10 pounds annually require public reporting. The facility had released 42 pounds, triggering mandatory disclosure under federal law.
During cleanup, the remediation team used the CAS number to select appropriate adsorbents. Activated carbon with a pore size distribution optimized for aromatic hydrocarbons was chosen based on its effectiveness against CAS 71-43-2 compounds.
I once reviewed a shipment of benzene labeled only as "aromatic hydrocarbon" during a customs inspection. The absence of the CAS number 71-43-2 delayed clearance for 72 hours. The importer had to provide a certificate of analysis and a revised label with the CAS number before release. This incident underscored how critical the CAS number is for logistics and compliance.