1,4-Dichlorobenzene

1,4-Dichlorobenzene (1,4-DCB, p-DCB, or para-dichlorobenzene, sometimes abbreviated as PDB or para) is an organic compound with the formula C6H4Cl2. This colorless solid has a strong odor. The molecule consists of a benzene ring with two chlorine atoms (replacing hydrogen atoms) on opposing sites of the ring.
It is used as a disinfectant, pesticide, and deodorant, most familiarly in mothballs in which it is a replacement for the more traditional naphthalene because of naphthalene's greater flammability (though both chemicals have the same NFPA 704 rating). It is also used as a precursor in the production of the chemically and thermally resistant polymer poly("p"-phenylene sulfide).
Production.
"p"-DCB is produced by chlorination of benzene using ferric chloride as a catalyst:
The chief impurity is the 1,2 isomer. The compound can be purified by fractional crystallization, taking advantage of its relatively high melting point of 53.5 °C; the isomeric dichlorobenzenes and chlorobenzene melt well below room temperature.
Uses.
Disinfectant, deodorant, and pesticide.
"p"-DCB is used to control moths, molds, and mildew. It also finds use as a disinfectant in waste containers and restrooms and is the characteristic smell associated with urinal cakes. Its usefulness for these applications arises from "p"-DCB's low solubility in water and its relatively high volatility: it sublimes readily near room temperature.
Precursor to other chemicals.
Nitration gives 1,4-dichloronitrobenzene, a precursor to commercial dyes and pigments. The chloride sites on "p"-DCB can be substituted with hydroxylamine and sulfide groups. In a growing application, "p"-DCB is the precursor to the high performance polymer poly("p"-phenylene sulfide):
Environmental and health effects.
"p"-DCB is poorly soluble in water and is not easily broken down by soil organisms. Like many hydrocarbons, "p"-DCB is lipophilic and will accumulate in fatty tissues if consumed by a person or animal.
The United States Department of Health and Human Services (DHHS) and the International Agency for Research on Cancer (IARC) have determined that "p"-DCB may reasonably be anticipated to be a carcinogen. This has been indicated by animal studies, although a full-scale human study has not been done.
The United States Environmental Protection Agency (EPA) has set a target maximum contaminant level of 75 micrograms of "p"-DCB per liter of drinking water (75 μg/L), but publishes no information on the cancer risk. "p"-DCB is also an EPA-registered pesticide. The United States Occupational Safety and Health Administration (OSHA) has set a maximum level of 75 parts of "p"-DCB per million parts air in the workplace (75 ppm) for an 8-hour day, 40-hour workweek.
Under California's Proposition 65, "p"-DCB is listed as "known to the State to cause cancer". A mechanism for the carcinogenic effects of mothballs and some types of air fresheners containing "p"-DCB has been identified in roundworms.