Chemistry: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).[1]

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.[1]

Uses

Disinfectant, deodorant, and pesticide

1,4-dichlorobenzene balls sold as urinal disinfectant

p-DCB is used to control moths, molds, and mildew.[2] It also finds use as a disinfectant[1] 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.[1]

Precursor to other chemicals

Nitration gives 1,4-dichloronitrobenzene, a precursor to commercial dyes and pigments.[3] 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):[4]

Synthesis of polyphenylensulfide

As ceruminolytic agent

2% Paradichlorobenzene used as a ceruminolytic agent for the treatment of impacted earwax.[5]

Recreational Use

Recreational abuse of PDCB [6] has been described, usually through use of urinal cakes. It is typically huffed. Deaths have been recorded from abuse.

Environmental and health effects

Shipping container of the substance, showing several hazard labels confirming the environmental hazard posed by the substance.

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.[7] This has been indicated by animal studies, although a full-scale human study has not been done.[8]

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),[9] but publishes no information on the cancer risk.[10] p-DCB is also an EPA-registered pesticide.[11] 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.[12][13]

A mechanism for the carcinogenic effects of mothballs and some types of air fresheners containing p-DCB has been identified in roundworms.[14]

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.[15]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 Rossberg, M.; Lendle, W.; Pfleiderer, G.; Tögel, A.; Dreher, E. L.; Langer, E.; Rassaerts, H.; Kleinschmidt, P. et al. (2006). "Ullmann's Encyclopedia of Industrial Chemistry". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a06_233.pub2. 
  2. ↑ "National Pesticide Information Center – Mothballs Case Profile". http://npic.orst.edu/capro/Mothballs1.pdf. 
  3. ↑ K. Hunger. W. Herbst "Pigments, Organic" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2012. doi:10.1002/14356007.a20_371
  4. ↑ Fahey, D. R.; Ash, C. E. (1991). "Mechanism of poly(p-phenylene sulfide) growth from p-dichlorobenzene and sodium sulfide". Macromolecules 24 (15): 4242. doi:10.1021/ma00015a003. Bibcode: 1991MaMol..24.4242F. 
  5. ↑ Nair, P.; Golhar, S.; Baisakhiya, N.; Deshmukh, P. T. (2009). "A comparative study of ceruminolytic agents". Indian Journal of Otolaryngology and Head and Neck Surgery 61 (3): 185–192. doi:10.1007/s12070-009-0063-z. PMID 23120632. 
  6. ↑ "Looking for Light: Paradichlorobenzene Toxicity in a 30 Year-Old Woman Following Long Term Ingestion of Mothballs (1955)". Neurology 94. 2020-04-14. doi:10.1212/WNL.94.15_supplement.1955. https://www.neurology.org/doi/10.1212/WNL.94.15_supplement.1955. Retrieved 2026-03-20. 
  7. ↑ Preamble to the IARC Monographs definition of "Group 2B: Possibly carcinogenic to humans", the International Agency for Research on Cancer classification of this chemical
  8. ↑ "ToxFAQs for Dichlorobenzenes". Toxic Substances Portal. Agency for Toxic Substances and Disease Registry. https://wwwn.cdc.gov/TSP/ToxFAQs/ToxFAQsLanding.aspx?id=703&tid=126. 
  9. ↑ "Consumer Factsheet on: PARA-DICHLOROBENZENE (p-DCB)". 28 November 2006. http://www.epa.gov/ogwdw000/contaminants/dw_contamfs/p-dichlo.html. 
  10. ↑ "1,4-Dichlorobenzene (para-Dichlorobenzene)" (in en). https://www3.epa.gov/airtoxics/hlthef/dich-ben.html. 
  11. ↑ "Reregistration Eligibility Decision for Para-dichlorobenzene". December 2008. http://www.epa.gov/oppsrrd1/REDs/para-dichlorobenzene-red-revised.pdf. 
  12. ↑ "Chemical Sampling – p-Diclorobenzine". Occupational Safety & Health Administration. https://www.osha.gov/dts/chemicalsampling/data/CH_232900.html. 
  13. ↑ "Common Name: 1,4-DICHLOROBENZENE". New Jersey Department of Health and Senior Services. December 2005. http://nj.gov/health/eoh/rtkweb/documents/fs/0643.pdf. 
  14. ↑ Kokel, David (14 May 2006). "The nongenotoxic carcinogens naphthalene and para-dichlorobenzene suppress apoptosis in Caenorhabditis elegans". Nature Chemical Biology 2 (6): 338–345. doi:10.1038/nchembio791. PMID 16699520. 
  15. ↑ Rehfuss, M.; Urban, J. (2005). "Rhodococcus phenolicus sp. nov., a novel bioprocessor isolated actinomycete with the ability to degrade chlorobenzene, dichlorobenzene and phenol as sole carbon sources". Systematic and Applied Microbiology 28 (8): 695–701. doi:10.1016/j.syapm.2005.05.011. PMID 16261859. Bibcode: 2005SyApM..28..695R. 



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