Pentachlorobenzene
Encyclopedia
Pentachlorobenzene is a chemical compound with the molecular formula C6HCl5 which is a chlorinated
aromatic
hydrocarbon
. It consists of a benzene
ring substituted with five chlorine atoms. PeCB was once used industrially for a variety of uses, but because of environmental concerns there are currently no large scale uses of PeCB.
and benzene. It is extracted by distillation
and crystallization
. The direct production of pure PeCB is not practical because of the simultaneous production of other chlorinated compounds. Since PeCB is generally produced in small quantities in the chlorination of benzene, it is also contained in other chlorobenzenes (dichlorobenzene
s, trichlorobenzene
s, etc.)
Today, a majority of the PeCB released into the environment is a result of backyard trash burning and municipal waste incineration.
pentachloronitrobenzene
. Pentachloronitrobenzene is now made by
the chlorination of nitrobenzene
in order to avoid the use of PeCB.
PeCB was a component of a mixture of chlorobenzenes added to products containing polychlorinated biphenyl
s in order to reduce viscosity
. PeCB has also been used as a fire retardant.
, allowing an accumulation in the food chain. Consequently, pentachlorobenzene was added in 2009 to the list of chemical compounds covered by the Stockholm Convention
, an international treaty which restricts the production and use of persistent organic pollutants. PeCB has been banned in the European Union since 2002.
PeCB is very toxic to aquatic organisms, and decomposes on heating or on burning with the formation of toxic, corrosive fumes including hydrogen chloride
. Combustion of PeCB may also result in the formation of polychlorinated dibenzodioxins ("dioxins") and polychlorinated dibenzofurans.
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
aromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
. It consists of a benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
ring substituted with five chlorine atoms. PeCB was once used industrially for a variety of uses, but because of environmental concerns there are currently no large scale uses of PeCB.
Production
PeCB can be produced as a byproduct of the manufacture of carbon tetrachlorideCarbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
and benzene. It is extracted by distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
and crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
. The direct production of pure PeCB is not practical because of the simultaneous production of other chlorinated compounds. Since PeCB is generally produced in small quantities in the chlorination of benzene, it is also contained in other chlorobenzenes (dichlorobenzene
Dichlorobenzene
There are three distinct chemical compounds which are dichlorobenzenes:*1,2-Dichlorobenzene or ortho-dichlorobenzene;*1,3-Dichlorobenzene or meta-dichlorobenzene;*1,4-Dichlorobenzene or para-dichlorobenzene....
s, trichlorobenzene
Trichlorobenzene
Trichlorobenzene may refer to any of three isomeric chlorinated derivatives of benzene with the molecular formula C6H3Cl3:* 1,2,3-Trichlorobenzene* 1,2,4-Trichlorobenzene* 1,3,5-Trichlorobenzene...
s, etc.)
Today, a majority of the PeCB released into the environment is a result of backyard trash burning and municipal waste incineration.
Uses
PeCB was used as an intermediate in the manufacture of pesticides, particularly the fungicideFungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
pentachloronitrobenzene
Pentachloronitrobenzene
Pentachloronitrobenzene, typically abbreviated PCNB, is a registered fungicide formally derived from nitrobenzene. It is either an off-white or yellow solid, depending on its purity, with a musty odor.-Preparation:...
. Pentachloronitrobenzene is now made by
the chlorination of 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...
in order to avoid the use of PeCB.
PeCB was a component of a mixture of chlorobenzenes added to products containing polychlorinated biphenyl
Polychlorinated biphenyl
Polychlorinated biphenyls are a class of organic compounds with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx...
s in order to reduce viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
. PeCB has also been used as a fire retardant.
Safety and regulation
PeCB is a persistent organic pollutantPersistent organic pollutant
thumb|right|275px|State parties to the Stockholm Convention on Persistent Organic PollutantsPersistent organic pollutants are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes...
, allowing an accumulation in the food chain. Consequently, pentachlorobenzene was added in 2009 to the list of chemical compounds covered by the Stockholm Convention
Stockholm Convention
Stockholm Convention on Persistent Organic Pollutants is an international environmental treaty, signed in 2001 and effective from May 2004, that aims to eliminate or restrict the production and use of persistent organic pollutants .- History :...
, an international treaty which restricts the production and use of persistent organic pollutants. PeCB has been banned in the European Union since 2002.
PeCB is very toxic to aquatic organisms, and decomposes on heating or on burning with the formation of toxic, corrosive fumes including hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
. Combustion of PeCB may also result in the formation of polychlorinated dibenzodioxins ("dioxins") and polychlorinated dibenzofurans.