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Introduction
Pentachloronitrobenzene is a registered fungicide formally derived from nitrobenzene. It is either an off-white or yellow crystal, depending on its purity, with a musty odor.

Preparation
Pentachloronitrobenzene was originally synthesized in a laboratory in 1868. It was introduced to the agricultural world in the 1930s in Germany by Bayer AG as a substitute to mercurial pesticides. Pentachloronitrobenzene is prepared by chlorination of nitrobenzene at 60 – 70 ◦C in chlorosulfuric acid, with iodine as a catalyst. It can also be produced by the nitration of chlorinated benzenes.1 A side product of the synthesis of PCNB is hexachlorobenzene (HCB), which is considered equally as hazardous as PCNB


 * 1. 5 Cl2 + C2H5NO2 → C6Cl5NO2 + 5 HCl

Main Reactions
Reaction with ethanol – potassium hydroxide yields pentachlorophenetole, exemplifying an easy route to pentachlorophenyl ethers.


 * 2. C6Cl5NO2 + KOCH2CCH3 → C6Cl5OCH2CH3 + KNO2

Although PCNB has a very stable shelf life, 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 (PCP) through hydrolysis and pentachlorothioanisole (PCTA). There is little information known about the degradation mechanisms.


 * 3. C6Cl5NO2 + 6 [H] → C6Cl5NH2 + 2 H2O


 * 4. 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. In 1989, usage of the compound was banned in the United States because of its persistence in the environment. In April 1993, PCNB became a federal hazardous air pollutant and was identified as a toxic air contaminant. PCNB is still used widely in other countries as a fungicide, such as China and Japan. Residual amounts of the compound and its metabolites are still found in crops today. The degradation products, PCA and PCTA have been found in farming soils and in river sediments.