1,4-Dichlorobenzene
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1,4-Dichlorobenzene (1,4-DCB, p-DCB, or para-dichlorobenzene, sometimes abbreviated as PDCB or para) is an aryl chloride and isomer of dichlorobenzene 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).<ref name=Ross>Template:Ullmann</ref>
Production
p-DCB is produced by chlorination of benzene using ferric chloride as a catalyst:
- C6H6 + 2 Cl2 → C6H4Cl2 + 2 HCl
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.<ref name=Ross/>
Uses
Disinfectant, deodorant, and pesticide
p-DCB is used to control moths, molds, and mildew.<ref name="orst">Template:Cite web</ref> It also finds use as a disinfectant<ref name=Ross/> 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.<ref name=Ross/>
Precursor to other chemicals
Nitration gives 1,4-dichloronitrobenzene, a precursor to commercial dyes and pigments.<ref name=Ullmann1>K. Hunger. W. Herbst "Pigments, Organic" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2012. Template:Doi</ref> 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):<ref>Template:Cite journal</ref>
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As ceruminolytic agent
2% Paradichlorobenzene used as a ceruminolytic agent for the treatment of impacted earwax.<ref>Template:Cite journal</ref>
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.<ref>Preamble to the IARC Monographs Template:Webarchive definition of "Group 2B: Possibly carcinogenic to humans", the International Agency for Research on Cancer classification of this chemical</ref> This has been indicated by animal studies, although a full-scale human study has not been done.<ref name=ToxFAQ-Dichlor>Template:Cite web</ref>
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),<ref name="epa-water">Template:Cite web</ref> but publishes no information on the cancer risk.<ref>Template:Cite web</ref> p-DCB is also an EPA-registered pesticide.<ref name="epa-rereg">Template:Cite web</ref> 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.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>
A mechanism for the carcinogenic effects of mothballs and some types of air fresheners containing p-DCB has been identified in roundworms.<ref name="EHANS">Template:Cite journal</ref>
Due to its carcinogenic nature, use of paradichlorobenzene in the European Union is forbidden as an air freshener (since 2005) and in mothballs (since 2008).
Biodegradation
Rhodococcus phenolicus is a bacterium species able to degrade dichlorobenzene as its sole carbon source.<ref>Template:Cite journal</ref>
See also
References
External links
- Template:ICSC
- Mothball sniffing warning issued, BBC News, 27 July 2006
- NIOSH Pocket Guide to Chemical Hazards, Centers for Disease Control and Prevention