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

Nitrobenzene

Nitrobenzene is an organic compound with the chemical formula C6H5NO2. 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 as a precursor to aniline. Although occasionally used as a flavoring or perfume...

. 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 by Bayer AG as a substitute to mercurial pesticide

Pesticide

Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...

s. PCNB is prepared by chlorination of nitrobenzene at 60–70 °C in chlorosulfuric acid

Chlorosulfuric acid

Chlorosulfuric acid is the inorganic compound with the formula HSO3Cl. It is also known as chlorosulfonic acid. It is a distillable, colorless liquid that should be handled with care. It is a hygroscopic and a powerful lachrymator.-Structure and properties:Chlorosulfuric acid is a tetrahedral...

, with iodine

Iodine

Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 as a catalyst. It can also be produced by the nitration of chlorinated benzenes. A side product of the synthesis of PCNB is hexachlorobenzene

Hexachlorobenzene

Hexachlorobenzene, or perchlorobenzene, is a chlorocarbon with the molecular formula C6Cl6. It is a fungicide formerly used as a seed treatment, especially on wheat to control the fungal disease bunt...

 (HCB), which is considered as hazardous as PCNB.

5 Cl₂ + C₂H₅NO₂ → C₆Cl₅NO₂ + 5 HCl

Main reactions

Reaction with ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 and potassium hydroxide

Potassium hydroxide

Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...

 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

Pentachlorophenol is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names...

 (PCP) through hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 and pentachlorothioanisole (PCTA). Little information is available about the degradation mechanisms.

C₆Cl₅NO₂ + 6 [H] → 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. In April 1993, PCNB was declared a hazardous air pollutant in the U.S. 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 of 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 . PCNB is still used widely as a fungicide in other countries, 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.