1,4-Dichlorobenzene

1,4-Dichlorobenzene

1,4-Dichlorobenzene

Chemical compound

Quick Facts Names, Identifiers ...

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).[3]

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.[3]

Uses

Disinfectant, deodorant, and pesticide

1,4-dichlorobenzene balls sold as urinal disinfectant

p-DCB is used to control moths, molds, and mildew.[4] It also finds use as a disinfectant[3] 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.[3]

Precursor to other chemicals

Nitration gives 1,4-dichloronitrobenzene, a precursor to commercial dyes and pigments.[5] 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):[6]

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.[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]
    NIOSH Pocket Guide to Chemical Hazards. "#0190". National Institute for Occupational Safety and Health (NIOSH).
  2. [2]
    "p-Dichlorobenzene". National Institute for Occupational Safety and Health (NIOSH). 4 December 2014. Retrieved 6 March 2015.
  3. [3]
    Rossberg, M.; Lendle, W.; Pfleiderer, G.; Tögel, A.; Dreher, E. L.; Langer, E.; Rassaerts, H.; Kleinschmidt, P.; Strack (2006). "Chlorinated Hydrocarbons". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a06_233.pub2. ISBN 978-3527306732.
  4. [4]
    "National Pesticide Information Center – Mothballs Case Profile" (PDF). Archived from the original (PDF) on 22 June 2010. Retrieved 10 August 2009.
  5. [5]
    K. Hunger. W. Herbst "Pigments, Organic" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2012. doi:10.1002/14356007.a20_371
  6. [6]
    Fahey, D. R.; Ash, C. E. (1991). "Mechanism of poly(p-phenylene sulfide) growth from p-dichlorobenzene and sodium sulfide". Macromolecules. 24 (15): 4242. Bibcode:1991MaMol..24.4242F. doi:10.1021/ma00015a003.
  7. [7]
    Preamble to the IARC Monographs Archived 9 August 2016 at the Wayback Machine definition of "Group 2B: Possibly carcinogenic to humans", the International Agency for Research on Cancer classification of this chemical
  8. [8]
    "ToxFAQs for Dichlorobenzenes". Toxic Substances Portal. Agency for Toxic Substances and Disease Registry. Retrieved 24 May 2013.
  9. [9]
    "Consumer Factsheet on: PARA-DICHLOROBENZENE (p-DCB)". 28 November 2006. Archived from the original on 6 October 2009. Retrieved 10 August 2009.
  10. [10]
    "1,4-Dichlorobenzene (para-Dichlorobenzene)". US Environmental Protection Agency. Archived from the original on 4 April 2016. Retrieved 24 March 2016.
  11. [11]
    "Reregistration Eligibility Decision for Para-dichlorobenzene" (PDF). December 2008. Archived from the original (PDF) on 26 September 2009. Retrieved 10 August 2009.
  12. [12]
    "Chemical Sampling – p-Diclorobenzine". United States Department of Labor. Occupational Safety & Health Administration. Archived from the original on 31 July 2017. Retrieved 23 March 2016.
  13. [13]
    "Common Name: 1,4-DICHLOROBENZENE" (PDF). New Jersey Department of Health and Senior Services. December 2005. Retrieved 24 March 2016.
  14. [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. S2CID 18402091.
  15. [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.
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