Pentachloronitrobenzene, typically abbreviated PCNB, is a registered fungicide formally derived from
nitrobenzene
Nitrobenzene is an organic compound with the chemical formula C6H5 NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale from benzene as a precursor t ...
. It is either an off-white or yellow solid, depending on its purity, with a musty odor.
Preparation
PCNB was originally synthesized in the laboratory in 1868. It was introduced to the agricultural world in the 1930s in
Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
by
Bayer AG
Bayer AG (, commonly pronounced ; ) is a German multinational pharmaceutical and biotechnology company and one of the largest pharmaceutical companies in the world. Headquartered in Leverkusen, Bayer's areas of business include pharmaceutica ...
as a substitute to
mercurial
Mercurial is a distributed revision control tool for software developers. It is supported on Microsoft Windows and Unix-like systems, such as FreeBSD, macOS, and Linux.
Mercurial's major design goals include high performance and scalability, d ...
pesticide
Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampri ...
s. PCNB is prepared by chlorination of nitrobenzene at 60–70 °C in
chlorosulfuric acid
Chlorosulfuric acid (IUPAC name: sulfurochloridic acid) is the inorganic compound with the formula HSO3Cl. It is also known as chlorosulfonic acid, being the sulfonic acid of chlorine. It is a distillable, colorless liquid which is hygroscopic and ...
, with
iodine
Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
as a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
. It can also be produced by the nitration of chlorinated benzenes. A side product of the synthesis of PCNB is
hexachlorobenzene
Hexachlorobenzene, or perchlorobenzene, is an organochloride with the molecular formula C6Cl6. It is a fungicide formerly used as a seed treatment, especially on wheat to control the fungal disease bunt. It has been banned globally under the Sto ...
(HCB), which is considered as hazardous as PCNB.
:5 Cl
2 + C
2H
5NO
2 → C
6Cl
5NO
2 + 5 HCl
Main reactions
Reaction with
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
and
potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash.
Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...
yields
pentachlorophenetole, indicating its high reactivity:
:C
6Cl
5NO
2 + KOCH
2CH
3 → C
6Cl
5OCH
2CH
3 + KNO
2
Although PCNB has a long shelflife, it is labile in soil, with a half life of 1.8 days. It degrades to other metabolites, mainly reducing to
pentachloroaniline (PCA), but also to
pentachlorophenol
Pentachlorophenol (PCP) is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names. It can be found as pure PCP, or as the sodium salt of PCP, the latter of which diss ...
(PCP) through
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
and
pentachlorothioanisole (PCTA). Another metabolite is methyl pentachlorophenyl sulfide (MPCPS). Little information is available about the degradation mechanisms.
:C
6Cl
5NO
2 + 6
→ C
6Cl
5NH
2 + 2 H
2O
:C
6Cl
5NO
2 + 6 H
2O → C
6Cl
5OH + HNO
2
Applications
PCNB is used as a fungicide to suppress the growth of fungi in various crops, such as cotton, rice, and seed grains. It is also used to prevent the formation of slime in industrial waters. Residual amounts of the compound and its metabolites can be found in crops. The degradation products, PCA and PCTA have been found in farming soils and in river sediments.
Regulation
In April 1993, PCNB was declared a hazardous air pollutant in the U.S.
[Howard, P. H. ''Handbook of Environmental Fate and Exposure Data for Organic Chemicals: Pesticides '', CRC Press, 1991; pp. 551-553. , ] PCNB was reexamined for re-registration eligibility by the U.S. EPA in 2006 as part of the 1996 Food Protection Quality Act (FPQA) and as a result, use on a number of crops were ended or limited. In August 2010, in response to the discovery of a potentially toxic metabolite in technical grade PCNB to be used in fungicide formulations, the sale of PCNB was halted by the U.S. EPA until the issue could be resolved. In November 2011, the EPA approved certain registrations for PCNB, allowing it back on the market for golf course turf, potato, cotton, ornamental bulb and cole crop uses in the United States. PCNB is used widely as a fungicide in other countries, such as China and Japan,
however, "it is no longer approved for use within the European Union".
[{{cite web , title=Pentachloronitrobenzene , url=https://pubchem.ncbi.nlm.nih.gov/compound/Pentachloronitrobenzene , website=Pubchem , publisher=National Library of Medicine , access-date=28 November 2022]
References
Bibliography
* Aschbacher PW, Feil VJ, ''Metabolism of pentachloronitrobenzene by goats and sheep''; J Agric Food Chem. 1983 Nov-Dec; 31(6):1150-8.
Chloroarenes
Nitrobenzenes
Fungicides
Endocrine disruptors