Pentachloronitrobenzene, typically abbreviated PCNB, is a registered fungicide formally derived from

nitrobenzene Nitrobenzene is an aromatic nitro compound and the simplest of the nitrobenzenes, 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 ...

. It is a off-white to yellow crystalline solid 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 lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

Bayer AG Bayer AG (English: , commonly pronounced ; ) is a German multinational pharmaceutical and biotechnology company and is one of the largest pharmaceutical companies and biomedical companies in the world. Headquartered in Leverkusen, Bayer's ...

as a substitute to

mercurial Mercurial is a distributed revision control tool for software developers. It is supported on Microsoft Windows, Linux, and other Unix-like systems, such as FreeBSD and macOS. Mercurial's major design goals include high performance and scalabi ...

pesticide Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all p ...

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; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

as a

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

. 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 aryl chloride and a six-substituted chlorobenzene with the molecular formula C6Cl6. It is a fungicide formerly used as a seed treatment, especially on wheat to control the fungal disease bunt. Its u ...

(HCB), which is considered as hazardous as PCNB. :5 Cl₂ + C₆H₅NO₂ → C₆Cl₅NO₂ + 5 HCl

Main reactions

Reaction with

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

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 utili ...

yields pentachlorophenetole, indicating its high reactivity: :C₆Cl₅NO₂ + KOCH₂CH₃ → C₆Cl₅OCH₂CH₃ + KNO₂ 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 disso ...

(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. :C₆Cl₅NO₂ + 6 → C₆Cl₅NH₂ + 2 H₂O :C₆Cl₅NO₂ + 6 H₂O → C₆Cl₅OH + HNO₂

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. Chlorobenzene derivatives Nitrobenzene derivatives Fungicides Endocrine disruptors